Biol. J. Linn. SOC., 6: 65-87
March 1974
Xerophytes, xeromorphs and sclerophylls: the
history of some concepts in ecology
G . SEDDON
Centre for Environmental Studies, The University o f Melbourne,
Parkville, Victoria, Australia 3052
Accepted f o r publication September 1973
Research on xeromorphic and sclerophyllous (the literal meanings of which are “dry-form” and
“hard-leaved”) plants offers a case-history illustrating the nature of “progress” in one branch of
science. The story runs from about 1890-1970, beginning with the birth of ecological concepts,
including Warming’s 1895 classification of plants into hydrophytes, xerophytes and mesophyta, Schimper’s pioneer work on the sclerophylls, and with the conceptions that lay behind
this work; and so on through the main lines of research, concluding with an account of work on
the “anomalous” distribution of the sclerophylls in Australia. This case-history shows how the
problems of classification and categorization may be linked to conceptual and empirical
problems of substance, and hence are not “merely” classificatory. Indeed, the hypotheses under
test are not formulated explicitly, but are encapsulated in the terminology, as is so often the
case in the biological sciences.
CONTENTS
. . . . . . . . . . . . . . . . . . . . . .
Introduction
Thegrowth of ecological terminology in the 19th century . . . . . . . . .
The structure of Warming’s ciassificatory system
. . . . . . . . . . .
“Psammophyte” as a unit of classification
. . . . . . . . . . . . .
Warming’s classification of plants according to their water-relations
. . . . . .
Schimper and “sclerophyll”
. . . . . . . . . . . . . . . . .
The impact of experimental physiology
. . . . . . . . . . . . . .
Changes in the meaning of “xeromorph” and “xerophyte”
. . . . . . . .
Changes in the meaning of “sclerophyll”
. . . . . . . . . . . . . .
The anomalies of distribution in Australia
. . . . . . . . . . . . .
The role of soil nutrients, light and fire in the distribution of sclerophyll forest and rain
forest of Australia . . . . . . . . . . . . . . . . . . . .
Conclusion
. . . . . . . . . . . . . . . . . . . . . .
Acknowledgements
. . . . . . . . . . . . . . . . . . . .
References
. . . . . . . . . . . . . . . . . . . . . .
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INTRODUCTION
There is often a gap in our descriptive account of the sciences. Historians of
science rarely come within, fifty years of the present, whilst scientists
themselves generally regard any paper that is more than five years old as “of
historic interest only” (a very good paper that is more than ten years old is a
“classic”). This paper is meant to be gap-filling, in that the work discussed
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spans the last 70 years, and in that Ecology is a branch of science that has not
attracted much philosophical analysis.
The story has three features of general interest. First, the research activity
described here might be seen as characteristic of an “immature science” (for a
detailed analysis of “immaturity” in science, see Ravetz, 197 1: 364-402). For
nearly 70 years of research activity, controversy has tended to break out at
three levels simultaneously, at the level of “facts”, and of methodology, and of
theory. But the concept of “immaturity” itself needs scrutiny. It is, of course,
a metaphor of progressive growth, and carries with it the further concepts of
maturity and senility. I t may be that Ecology will indeed “grow up”, but not
by getting t o be more like Physics or Chemistry, the customary models of
“mature” science. Or it may be that it will be forever “immature”, in which
case the metaphor is misleading.
A second feature of interest is the central place that debate about
terminology takes in the story. Terminological wrangles are not peculiar to
Ecology, and they can be found throughout the history of both what used to
be called Natural Philosophy and Natural History, but they last longer in the
latter; there is good reason for this, which I hope to illustrate. The natural
philosophers tend to see such debates as a further sign of immaturity, because
for them the establishing of an agreed terminology is preliminary to the
framing of general laws, but for many natural historians, classification is an end
in itself, because the theory is embedded in the system of classification. This
might equally be said of Mineralogy in Werner’s day or of the new
nomenclature of Chemistry proposed by Lavoisier and others in the 1780s. The
debates that took place while they were being established in their present form
bear some resemblance to the example given in this paper, although the
theories on which mineralogical and chemical classification are based have since
been fully articulated. In the present case there is little overt discussion of
theory, and there are few explicit “hypotheses” to be tested. The propriety and
application of a classificatory scheme is that which is being tested, and so
terminological debate is central.
A third feature of interest arises from the nature of the subject matter with
which ecologists work. Lord Bowden (1956: 48) once said that “an art may
become a science if it is concerned with less than about seven variables”. If we
take this with a second aphorism, “Science is the art of the soluble” (Medawar,
1967) Ecology might seem to be neither art nor science. Like most biologists,
the ecologist is often dealing with more than seven variables, and worse than
that, their mutual interaction is the very core of his concern. Traditionally,
scientists are supposed to be good at the art of solving problems by breaking
large problems down into separable components, by solving the small problems,
and by restricting themselves to problems that they can in fact solve. But I
suspect that biologists quite often attempt to solve problems that turn out to
be insoluble, but learn something else of interest on the way. Certainly it is not
easy to identify a series of neatly solved problems in the example I am about to
give.
THE GROWTH OF ECOLOGICAL TERMINOLOGY IN THE 19TH CENTURY
My concern is with the history of the concepts “xerophyte”, “xeromorph”
and “sclerophyll”, all terms coined in the 19th century. This was a great age of
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
67
botanical exploration. New plants from all over the world were brought back to
herbaria, described, named, and classified. Taxonomists sorted them into higher
groups, and with the help of evolutionary theory, plant fossils, and later genetic
studies, botanists and palaeobotanists attempted to chart their distribution
through time, while biogeographers mapped their distribution through space,
around the globe. Both distributions called for explanation. The Darwinian
theory of Evolution through Natural Selection threw emphasis on adaptations
of plants to their environment, and their responses to environmental change.
This focus on the relation between organism and environment gave birth to the
new science of Ecology. To draw new distinctions, a rash of new terms
appeared, one of the first being “Ecology”. Ernst Haeckel (1868) gave the
word currency, and defined it as “the study of reciprocal relations between
organisms and their environment” (for a discussion of its origins, see
Kormondy, 1969:viii).
The first text-book of Plant Ecology appeared a few years later, the work of
Eugenius Warming,* Professor of Botany at the University of Copenhagen. A
substantially revised version in English appeared in 1909.In the preface to this
edition, Warming (1909:41) says that, “the ecology of plants is still in its
infancy”; Warming’s book is nevertheless a remarkable achievement, in that its
structure, reflected in the chapter divisions, is essentially that of most later
texts of Plant Ecology (e.g. Daubenmire, 1959) until the concept of the
“ecosystem” had gained general recognition, and thus began to influence the
structure of text-books in the late ‘fifties. A number of other major works
appeared in rapid succession after Warming’s book (Schimper, 1898, 1903;
Solms-Laubach, 1905;Clements, 1904, 1905),and it is clear from this sudden
and initially very rapid progress that “time was ripe”; Warming and others
provided a structure which organized in a significant way a vast mass of
available observations about plant morphology and distribution. Subsequent
work up to about 1955 consisted primarily in attempts to assess for specific
cases the relative significance of the ecological factors outlined by Warming and
his contemporaries. This proved highly intractable, and to the outsider,
progress may seem to have been slow.
THE STRUCTURE OF WARMING’S CLASSIFICATORY SYSTEM
Warming’s classification begins with a clear distinction between the
growth-form and the systematic form of plants, following in the steps of
von Humboldt (1807).Systematic form consists in those characters by virtue
of which a group of plants is recognized as belonging to a given taxonomic unit:
for instance, all, and only, members of the family Papilionaceae have
papilionate flowers, the familiar, bilaterally symmetrical pea-flower with keel
and standard. There are other family characters which all or most Papilionaceae
share, and these are the systematic form (Systematics-the science or theory of
Warming’s book first appeared in Danish as Plantesurmfund. Crundnak of den Okologische
Phnregeografi (Copenhagen: 1895). A German edition appeared in 1896, E. Knoblauch (Ed.) (Berlin:
Borntrager, 1896), and a revised version in English in 1909, Ecology of plants, an innoduction to the
study of plant communities, Percy Groom & Isaac Bayley Balfour (Eds) (Oxford: Clarendon Press, 1909).
Subsequent references are to the English edition. Warming claims in the Preface that “mine was the first
attempt to write a work in Oecological Plantgeography”, but this is an overstatement. An ecological
classification of plants is attempted in Theophrastus Historia Plantarum, (1754, Andreas Hedenberg,
respondent);the great plantgeographer of the 19th century was de Candolle (1820).
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C. SEDDON
classification), which reflects common ancestry. But plants that are not related
systematically may nevertheless be very similar in growth-form. (A good
example is afforded by the Cactaceae and the Euphorbiaceae. The true cactus is
confined to the Americas, but Africa has many euphorbias and other succulents
that are very cactus-like in growth form, although a comparison of floral
structure shows them to be quite unrelated systematically.)
So Warming classified plants into growth-forms, which he judged (1909: 5 )
to be the units of ecological botany, just as the species is said to be the unit of
systematic botany. He began with a primary division into plants that produced
flowers and fruit (or spores) once, and then die (monocarpic), and those that
flower and fruit repeatedly (polycarpic). He then subdivided further, for
example into rosette-plants, cushion-plants, creepers and so on. Before moving
on to consider plant communities, Warming also classified plants in relation to
a series of environmental factors taken separately, the main ones being: light,
temperature, humidity and precipitation, wind, and a variety of soil factors. In
doing so, he deployed an elaborate vocabulary with a vast array of new
coinages, almost all of which saw light in the last two decades of the 19th
century. For example, plants are grouped according to their preference for sun
or shade, and are thus heliophytes or heliophobes. Halophytes can survive in
salty soils; calciphytes like lime; Psammophytes live on sand or gravel. Some of
these terms were already in use, but Warming introduced three terms in
classifying plants according to their water-relations: the three classes are
hydrophytes, xerophytes and mesophytes referring to plants of wet, dry and
moist habitats respectively. (Warming coined the term mesophyta (PEUO
middle) by analogy with the terms xerophyta (tqpo dry; $UTOU, plant) and
hydrophytu, which he took over from Schouw (1823): see Warming, 1909:
96-101. Such classifications are endemic in Biology, usually dichotomies and
trichotomies on the “endo-”, “meso-”, “ecto-” plan; cf. “poikilothermic”,
“stenohaline”, “eutrophic” etc.) Many ecologists have since passed comment
on this classification in words like the following: “In common with all other
ecologic classifications, the groups delimited on this basis include species of
very diverse taxonomic affinities. As with most biologic classifications, the
limits between the groups are ill defined” (Daubenmire, 1959: 138). My
concern in this paper is with the class of xerophytes, but I am also interested to
ask why it is that the limits are ill defined in “most biologic classifications”,
and before turning to the difficult case of the xerophytes, we might begin with
a simpler case, that of the psammophytes.
“PSAMMOPHYTE” AS A UNIT OF CLASSIFICATION
What is a psammophyte? It is a plant that grows on sand, usually having a
group of characters that are common to most plants that live on coastal sands.
Such plants are usually deep-rooted. They commonly have creeping rhizomes
that bind the sand and allow rapid regrowth after burial. Some are succulent;
many, are hairy, woolly or wax-coated (pubescent, tomentose or glaucous);
they are often pale grey or almost white in colour; they are generally compact
in form, growing either in dense mats or low cushions.
But what are the relations between these characters and the sandy
environment? Irrespective of rainfall, sand drains so quickly that it is usually
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
69
physically dry, and so most of these plants are xerophytes. Warming (1909:
262) excludes plants that grow on permanently moist sand, where the water
table is high, from the class “psammophyte”-so it might seem that it is not the
“sandiness”, but the dryness, that is relevant. Many plants that grow in sand are
also exposed t o salt spray and high salt concentrations in the soil solution.
They are therefore also halophytes, and many of their distinctive characters
may be a response to this, rather than “sandiness”. They are exposed to the
wind and to sand-blast, which is a sanddependent factor and also to much sun
and glare. These are variable factors. Plants growing in white sand are subject to
much more glare than those in dark sands. Some sands are calcareous
(supporting calciphytes), while others are siliceous (supporting silicophytes).
Sands may be coastal or inland, where temperature ranges and humidity are
very different. Some plants, such as coastal spinifex and marram, can only live
in sand, and are thus obligatory psammophytes, whereas other plants can also
survive in other types of substrate. Even within a specific dune habitat, plants
are markedly specialized-some grow only on the open strand and foredune,
while others make their first appearance on the more sheltered, lee-side of the
first primary dune.
With all this qualification, what has happened to the general class
“psammophyte”? Has it been splintered beyond repair? Only if we misunderstand the function of such terms. The scientist tries to order phenomena,
but a biologist does not take such categories as rigid. For him they are but preliminary, and sooner or later he has to get down to individual cases. Despite the
interdependence of environmental factors in a holistic discipline, all environmental factors cannot be considered simultaneously, so they are taken one at a
time, remembering always they do not in fact work that way. A biologist is
always aware of the need for different levels and kinds of synthesis. The
individual environmental factors must be integrated and weighted to give an
account of the whole plant in relation to its total environment. In so doing a
group of factors often separated out as “biotic factors” must be taken into
account. These include the effects of soil organisms, grazing, and browsing
animals, insect attack, pollination and dissemination by insects and other
organisms-and also the profound effects plants have on one another. In short,
plants do not grow as individuals, but in communities, and many ecologists
would therefore now contest Warming’s claim that growth-forms are the units
of ecological botany, and insist that plant communities are the significant unit.
A division has arisen since Warming’s day between autecology, the study of the
relation between the individual and its environment, and synecology, the study
of plant communities. For example, some would argue that we should not talk
about psammophytes, because this term has no significant content, but about
the dune habitat, and communities of dune plants, or about the dune as an
ecosystem, which consists of both the plants and their environment. Notwithstanding, one cannot talk about plant communities without talking about
the individual members of the communities, so that autecology and synecology
may be seen as complementary, although they differ in their methodological
assumptions. *
Synecologists often echo the phraseology of Warming, but with a different intent: e.g. Evans (1956:
1127): “The ecosystem thus stands as a basic unit of ecology, a unit that is as important to this field of
natural science as the species is to taxonomy and systematics”. The change in basic unit is significant, in
that it promotes a quite different kind of research, focussed on energy flow through ecosystems.
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WARMING’S CLASSIFICATION O F PLANTS ACCORDING
TO THEIR WATER-RELATIONS
Warming is responsible for the three-fold division into hydrophytes,
mesophytes and xerophytes. Hydrophytes are plants that live in water, either
partly or wholly submerged. This is a relatively small, rather distinct group of
plants, with a common set of characteristic adaptations to their specialized
habitat; for example, abundant air-spaces within the tissue “to decrease their
specific gravity (as a flotation device), and to facilitate gaseous interchange and
especially respiration”.
When water-plants are grouped in the class “hydrophyte”, the land-plants
remain, and Warming divided these into two classes. “Those species that are
adapted to meet the conditions of strongest transpiration and most precarious
water-supply are termed xerophytes: the remainder are termed mesophytes:
between these two classes there is of course no strict boundary” (Warming,
1909: 101). The three-fold division is thus very unequal, with a very large
middle class, which is negatively defined, and with divisions that differ in
sharpness-the hydrophyte-mesophyte boundary is fairly strict, but the
mesophyte-xerophyte boundary is not, in that the moistdry contrast is drawn
from a continuum.
A xerophyte is then a class which could be defined either by listing all its
members, or by providing a set of criteria for determining class membership.
Today it is generally understood to be a purely descriptive-locatory term: a
xerophyte is a plant from a dry habitat. This is sometimes given in
semiquantitative form: xerophytes are plants that grow on substrates that
usually become depleted of water to a depth of 20 cm during a normal season
(Daubenmire, 1959: 142). But Warming (1909: 101) defined the class in part
by references to adaptation, “as those species that are adapted to meet the
conditions of strongest transpiration and most precarious water supply are
called xerophytes: the remainder are termed rnesophy tes” (Warming, 1909:
101). He was too sophisticated to give a definition of a class of plants in terms
of their habitat because it leaves the plants out of the story. A habitat is dry for
a plant if the water uptake of that plant is seriously restricted Rainfall is only
one factor: available water is what matters. Excessively drained, sandy soils
may be physically dry much of the time, even where rainfall is reliable, heavy,
and evenly distributed throughout the year. Warming (1909: 134) also
introduced the notion of physiological drought. “Soil is physiologically dry
when it contains a considerable amount of water which, nevertheless, is
available to the plant only to a slight extent or can be absorbed only with
difficulty, either because the soil holds firmly to a large quantity of water or
because the osmotic force of the root is inadequate to overcome that of the
concentrated salt solution in the soil”. Thus a halophyte, a plant growing in
salty soil, has a restricted water uptake, and is therefore a special forni of
xerophyte. So is the conifer of the northern pine forests, where the soil is too
cold for free uptake of water for more than half the year. But the mosses,
lichens, heaths and scrub of peaty, acid soil also have problems of water uptake
similar to those of the halophytes, and are thus xerophytes, according to
Warming. He actually classifies them as xerophilous (1909: 136) but it is clear
from the context that xerophyte is the noun and xerophilous the corresponding adjectival form. He does not use xerophytic as an adjective.
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
71
In describing the formation of sour or acid soils, Warming casually
introduces a new term xeromorphy : “These communities” (for example the
heath and heather communities of northern Europe) “all exhibit xeromorphy,
that is to say, they are protected from desiccation by certain devices” (1909:
193). He then lists these devices: a welldeveloped coating of hairs; protected
stomates; wax; thick cuticle; sclerophylly (used here in its literal sense “with
thick and semi-rigid leaves”); mucilage; ericoid leaves like the heathers, or
leaves reduced to stiff spines, or leafless (aphyllous) stems; bilateral leaves that
are held vertically and expose their edges, as distinct from dorsiventral leaves,
that have a distinct upper and lower side; closure of leaves (as will be seen later,
Grieve (1955: 3 1 4 5 ) gives an almost identical list of characters for Australian
sclerophylls). In short, xerophytes are not to be recognized from the dryness of
the habitat, because they may be found in wet habitats. I t is the dryness or
wetness to the plant that matters, and this will be indicated by the adaptations
that the plant has made to the environment. Thus xerophytes are defined in
terms of their adaptations, and these are given as the set of morphological traits
listed above. Xerophytes can be recognized by their xeromorphy, and their
xeromorphic characters serve to reduce water-loss, primarily by reducing
transpirat ion.
SCHIMPER AND “SCLEROPHYLL”
Warming has been discussed at length because he introduced this terminology, but he was not a great originator. His work is impressive, but it is in large
part compilatory, and should be seen as reviewing the large body of work that
appeared at the turn of the century. Another major work, one of many, that
appeared at this time is Plant geography upon a physiological basis by A. F. W.
Schimper,* a professor at the University of Bonn. Schimper’s work (1898)is
only three years younger than that of Warming, but it shows the benefits of
rapid progress: it is much better documented, with more quantitative data,
including many tables; it is better illustrated, with many detailed drawings of
critical structures (e.g., sunken stomates); and Schimper’s interest in physiology
adds a new dimension to the work, although the title indicates a wish rather
than an achievement. However, the structure of the two books is very similar,
and Schimper uses Warming’s xerophyte-xeromorph categories in the same
way, and the interest of his work here is mainly that he introduced (1903:8)a
new term, sclerophylly. “The plants of all these stations” (for example, deserts,
sandy soil, peat bogs, alpine high-lands, and sea-shores, where there is a high
salt content) “are provided with devices for the safeguarding of their
transpiration; they are xerophytes. Reduced surface is very general in their
case. With increasing physiological dryness, the leaves become smaller in surface
but proportionally thicker, more leathery (sclerophylly),fleshy (chylophylly or
leaf-succulence) or rudimentary and caducous (aphylly)”. Thus sclerophylls are
plants with leathery leaves, contrasted with succulent and leafless xeromorphs.
Schimper (1903:2) is in agreement with Warming on two critical issues:
xerophytes are to be recognized by their morphology and physiology rather
Schimper’s work appeared in German in 1898, and in English in 1903 (Oxford: Clarendon Press,
1903), and thus appeared in English before Warming (1909). Schimper quotes Warming in 1898 and
1903; Warming quotes Schimper in the English version, which was a revision.
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than their habitat (“it is usual to designate the plants . . . of dry localities as
xerophytes, but in this due attention is not paid t o the fact that the
characteristics of organisms are physiological, those of habitats are physical,
and that there is no necessary connection between these two groups of
characteristics”). And the function of the xeromorphic characters is to reduce
water-loss, primarily through transpiration.
However, Schimper’s insistence on the importance of physiological
experiment leads directly to the next phase of investigation: “The ecology of
plantdistribution will succeed in opening out new paths on condition only that
it leans closely on experimental physiology, for it presupposes an accurate
knowledge of the conditions of the life of plants which experiment only will
bestow. Thus only will it be possible to sever the study of adaptations from
dilettantism which revels in them, and to free it from anthropomorphic trifling,
which has threatened to bring it into complete discredit” (Schimper, 1903:
VI). This theme was to be repeated in later ecological literature (for example,
see Daubenmire, 1959: 154-155).
THE IMPACT OF EXPERIMENTAL PHYSIOLOGY
Turning to experimental physiology, we find that the first results provoked
consternation rather than resolved problems. If the function of the xeromorphic characters is to conserve water, it should be possible t o demonstrate
this experimentally, either for individual characters or for typical xeromorphs.
Transpiration rates have attracted the most attention. But they are not easy to
measure or compare. In most research papers on this topic, there is a discussion
of the experimental methods used, and the results of other workers are treated
with reserve. For an outline of some of the difficulties in comparing
transpiration rates, see Daubenmire, 1959: 154-155, who concludes that “the
concept of relative transpiration has little real value unless applied comparatively under conditions where wind movement is prevented, light is
uniform, and the same type of evaporimeter is used throughout.”
This degree of control indicates laboratory conditions, yet laboratory results
are sometimes discounted as atypical, and the necessity for experiment under
field conditions is stressed. For example
“Wilson’s results indicate a very high rate of water loss for sclerophylls under the
conditions of his experiments. In this connection, it may be noted that Henrici
(1937),in South Africa, found that potted xerophytic plants could display a very
high transpiration rate, but in plants tested in the field the rate was much lower.
Wilson’s results therefore can not be considered as being typical for field conditions in
summer.’’(Grieve, 1955: 3 1 4 5 ) .
It seems that the control that can be achieved in laboratory conditions
introduces a degree of artificiality that may vitiate the experiment. This is a
recurring problem in all the fields sciences, and in this way they differ in an
obvious way from a science such as chemistry.
Uncertainties about methods and procedures persist to the present day;
competent research reports are accompanied by a detailed account of the
methods employed (e.g., Specht, 1969: 297-299). In many areas of science,
standardized methods and procedures are taken for granted.
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
73
In 1916, Maximov, a Russian physiologist, showed that xerophytic plants in
the neardesert country at Tiflis transpired at a higher rate than mesophytes
growing nearby in a shady irrigated garden. In his book The Plant in Relation
to Water Maximov (1929) strongly advocated the view that xerophytes as a
class transpired faster than mesophytes, and made exception only of the
succulents of the cactus-type and those plants with continually blocked or
deeply sunken stomata. Maquis sclerophylls were considered as belonging to
the group which transpired faster than mesophytes under favourable
conditions. However, none of the xerophytes studied by Maximov near Tiflis or
those investigated by other Russian workers such as Vasiljev (193 1 ) appear to
be sclerophylls. Seybold (1929), using evergreen plants including at least one
typical Mediterranean sclerophyll, the laurel (Laurus nobilis), demonstrated
that their transpiration was below that of the mesophytic plants, and criticized
the general conclusions of Maximov. Maximov (193 1 ) replied that these
evergreens were not true xerophytes, and reiterated his view that true
xerophytes of desert regions had a higher transpiration rate than mesophytes.
Maximov had thus redefined ‘‘xerophyte”. Xerophytes were to be distinguished
not by reduced transpiration, but by their capacity to survive drought and
dehydration of tissues with little or no injury. The revised Schimper definition
according to Maximov (1931) should read:- Xerophytes are plants of dry
habitats which are able to decrease the transpiration rate to a minimum when
under conditions of water deficiency. It appears that sclerophylls are excluded
from Maximov’s concept of “true xerophytes”, since they are not restricted by
any means to dry areas. (Part of the trouble here is the imprecision in the use
of the word “dry”-and to that extent, “xerophyte”. Tiflis and Rome are both
dry, but in quite different ways. Most of the Mediterranean lands have a
reliable and substantial winter-rainfall, with a very dry summer. Transcaucasia
has a low, unreliable, and irregularly distributed rainfall. Thus they are “dry” in
very different ways, and one would expect this to be reflected in the
vegetation.)
Research in South Africa (Henrici: 1937, 1941) showed that the Karroo
types of xerophyte, including large Karroo sclerophylls, were different from
Maximov’s Tiflis type, because only rarely did they transpire highly, even in the
presence of adequate water. They not only restricted their water loss with
declining water supply of the soil, but also restricted transpiration even with
adequate water during the hot part of the day. Thus Maximov’s findings on the
“high transpiration of xerophytes’’ apply to Karroo bushes only when the soil
is about half saturated, a condition seldom met on the veld. Although plants in
drier regions undoubtedly have the ability under some conditions to transpire
freely, the South African work suggests that it is more important to know what
the plants do under the adverse conditions which probably prevail for a great
part of their lifetime. Work in Australia yields a different conclusion again, that
“southern Australian sclerophylls, growing in areas subject to summer drought
(due to either climate or soil or both) have as a class lower average rates of
transpiration than mesomorphs growing under comparable conditions in
summer and that they reduce their water loss before extreme water deficiency
conditions develop” (Grieve, 1955).
The main outlines of the debate may be summarized: Schimper’s argument is
quasideductive in form; according to the Theory of Evolution plants are
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broadly adapted to their environment. Plants from arid and semi-arid
environments have a group of common characters. Therefore these characters
are adaptations which enable those plants to survive in such environments. But
in describing these environments as arid and semi-arid we have already said
that their common feature is waterdeficit. Therefore the xeromorphic
characters are an adaptation to water-stress, and allow plants to reduce
water-loss, primarily through low transpiration rates.
Maximov followed a similar line of reasoning, but he found that water-loss of
the xerophytes he studied was greater than that of mesophytes in the same
area, and so he concluded that these characters must therefore be of protective
value when the plant is wilting. Research in South Africa, the Mediterranean
and south-western Australia suggests that the average water-loss in the
sclerophylls of those regions is lower than that in mesophytes, and that
stornatal control, with the assistance of the xeromorphic features, may reduce
transpiration long before the plant is at the wilting stage. Thus there has been a
sustained programme of physiological research, but it has not led to firm,
general conclusions about the adaptive functions of xeromorphic characters.
“That the xeromorphs are xerophytes adapted to a short, dry spell during the
summer, a view widely held 30 years ago and still persisting with some, has
never been substantiated” (Beadle, 1966: 1006).
CHANGES IN THE MEANING OF “XEROMORPH” AND “XEROPHYTE”
Before asking why this question seems not to have been settled by more than
fifty years of research, we should consider the changes in the way the key
terms have been used: “xeromorph” and “xerophyte” are no longer coextensive. Since it is not clear what function the so-called xeromorphic
characters serve, then they cannot be used to identify xerophytic plants.
Moreover, if it cannot be shown that all the so-called xrromorphic characters
are adaptations to conserve water, then the term becomes divorced from its
etymology. This is just what has happened. The divorce between etymology
and current usage is not rare in the history of science-an “a-tom” is an
indivisible unit, and “oxygen” is the sour-maker or acidifying principle-but
there is usually an interval of confusion while the new meaning is being
established. “Xeromorph” is now used to describe any plant that has any of the
characteristics listed by Warming as xeromorphic characters, irrespective of
their function. “Xerophyte”, on the other hand, is now generally used in its
literal sense, despite the warnings of Schimper and Warming that this leaves the
plant out of account. A xerophyte is any plant that grows in a dry habitat.
When redefined in this way, we soon find that there are xerophytes that are not
xeromorphs, and vice versa. For example, Phyllunthus culycinus is a common
small shrub in the south-west of Western Australia. Along with the rest of the
flora, it is subjected to prolonged summer drought, and is therefore a
xerophyte, but it lacks the typical xeromorphic characters: it has small, soft,
thin, bright green leaves that show a high rate of water-loss in spring and early
summer. With increasing dryness, it maintains its water balance by shedding
most of its leaves. It has been used in research on transpiration as a contrast
with its sclerophyll neighbours (by Grieve, 1955, 1957; Grieve & Helmuth,
1970). Similarly, many sclerophylls, which are xeromorphs by definition, are
XEROPHYTES, XEROMORPHS .4ND SCLEROPHYLLS
75
found in habitats that are anything but dry, especially in Australia, where
xeromorphic species are as abundant in the wetter regions of Australia as they
are in the desert. The mountain ash, Eiiculyptus regnuns, grows to its grqatest
height in permanently moist gullies in well-watered Tasmania and Victoria, and
is thus a xeromorph but no xerophyte.
But the changes in meaning have not been universally accepted, and
“xerophyte”, “xeromorph” and “sclerophyll” are all unstable terms. For
example, there has been much discussion of “true” and “false” xerophytes, and
of “true” and “false” xeromorphs, a discussion that could not take place if a
xerophyte were simply any plant from a dry habitat. Maximov (193 1 : 282) had
his own definition: “Xerophytes are plants of dry habitats which are able to
decrease the transpiration rate to a minimum when under conditions of water
deficiency”. The emphasis is thus still on plant behaviour rather than o n
habitat. For Maximov, therefore, our example of a non-xeromorphic xerophyte, Phyllunthus culycinus, would be a xerophyte not because of the
seasonally dry habitat alone, but also because it reduced transpiration by losing
its leaves. In fact he argues that the deciduous habit is an adaptation to
seasonally adverse conditions, primarily drought: “it is known from botanical
geography that evergreen trees are quite definitely adapted to the more humid
maritime climate, . . . and the transition to the drier continental climate results
in their disappearance and the substitution of deciduous trees” (Maximov,
1931: 275). He rejects conclusions based on work on the laurel (Luurus
nobilis), yew (Tuxus baccata), ivy (Hederu helix), and ilex (Ilex aquifolium),
the typical sclerophylls of the Mediterranean, because they are evergreen, and
thus “false xerophytes”, none of which penetrate into the drier forests and
steppes of the eastern parts of Transcaucasia. (“It is to be regretted that the
experiments of Seybold, although carried out very accurately, cannot be
considered as answering the question, since he never used a true xerophyte in
his experiments; as examples of xerophytic plants he mostly used the leaves of
evergreen trees”; Maximov, 1931: 275.) On this line of reasoning, there would
be very few “true xerophytes” in Australia.
But in fact the right to the title “xerophyte” has been disputed for all of the
major plant groups customarily assigned to that category. Maximov, Beadle,
and many others would exclude the sclerophylls from automatic membership.
Sclerophylls may be xerophytes, but are not so by virtue of being sclerophylls
(Beadle), and are rarely so as defined by habitat and transpiration-behaviour
(Maximov). Ephemeral annuals are common in arid regions. They spring up
after rain, flower, set seed and die as drought returns, and their ability to
complete their life cycle in a few weeks is clearly an adaptation to a primarily
arid regime-they pass through the dry season as well-protected seeds. If
xerophily is defined by habitat, desert annuals are clearly xerophytes, but
many botanists are reluctant to describe them as “true” xerophytes, because
they avoid rather than withstand critically dry seasons (Daubenmire, 1959:
143). There may be an implicit requirement here that xerophytes not just be
xerophytes, but that they also behave like xerophytes. On the other hand,
there is also room for the legalistic quibble that the desert annuals, at least in
their vegetative phase, d o not in fact live in a dry environment-they live in a
moist one for all of their short lives. The ensuing three-year drought is
irrelevant-the little annual won’t be there. But its seed will: why define
“plant” so narrowly?
76
G . SEDDON
This argument can be refined further, with intricacies and convolutions that
mirror the larger argument. Everyone knows that seeds are plants. But if one
excludes specialized dormancy and germination requirements, the physiological
variability in seeds is not great, and it is possible for seeds of a very large
number of species (xero-, meso- and even hydro-morphs) to withstand a
three-year drought, as this would represent quite good storage conditions. But
there is little point in calling all such seeds “xeromorphic”, because the word is
useful only if it points to differences. However, desert annuals usually have
sophisticated dormancy/germination requirements that make possible a lifecycle adjusted to occasional rain and recurrent drought, which could be seen as
the extreme case of droughtdeciduous behaviour. Eucalyptus alba loses its
leaves in a drought, and retreats to trunk and root; the desert annual loses the
“plant”, and retreats to seed.
A similar debate has arisen over the third major group of xerophytes, the
succulents, which store large reserves of water in the tissue of their stems and
leaves during rainy periods, and then slowly expend it. Thus, to be effective,
succulence must be accompanied by low transpiration rates during the ensuing
drought. A thick cuticle restricts cuticular water loss, as with the sclerophylls,
but this is, as a rule, their only conventionally xeromorphic structure. The
tomentose succulents are an exception-everyone allows them to be xeromorphs, but because they are tomentose rather than succulent. In other ways,
the succulents are structurally similar to the hydrophytes, with very large cells,
internal air spaces, and few stomates. In contrast with most other plants, the
stomates in one large group of succulents are closed during the day (when
transpiration and water loss would be greatest) and open at night. This sets a
special problem for these plants, which they solve in a special way. The
stomates in all plants have a double function. Most water-loss is through the
stomates, but these structures also have a vital role in the exchange of gases
other than water vapour; they function in the uptake of carbondioxide from
which the leaf is able to synthesize its basic food supply; and from them
diffuses oxygen, the by-product of this process. The problem for t$e plant in
an arid environment is one of balance: evaporation of water from the stomates
tends to keep the leaf cool. Photosynthesis is also vital to the plant, and
optimal photosynthesis (at least in Calvin Cycle or C 3 plants) would be
achieved with a large leaf area and many, wide-open stomates. Successful
xerophytes must balance leaf temperature control, photosynthetic activity, and
water-loss. It is clear that different groups o f xerophytes do this in quite
different ways. In restricting transpiration, some succulents also restrict their
uptake of carbon dioxide during the day, and they are unique among
xerophytes in that under certain conditions, the carbon source used in
photosynthesis is derived from organic acids produced by the dark fixation
(i.e., night fixation) of carbondioxide. But because they lack the typical
xeromorphic characters, and evade rather than withstand drought by drawing
on their water reserve, they have, like the annuals, been described as “false”
xerophytes (e.g., by Kamerling, 1914: 106).
The complex requirements of living organisms and their diverse ways of
meeting them occasion some of the apparent absurdities of this debate. Desert
plants that have few stomates and low transpiration rates are well adapted to
desert conditions. Plants such as Artemisia and Centaurea which have many
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
77
stomates and high transpiration rates are also well adapted to desert conditions.
The high transpiration helps to keep the leaf cool, thus avoiding protoplasmic
damage in extreme heat, and the wide open stomates permit maximum uptake
of carbondioxide for photosynthesis. It is only at wilting point (according to
Maximov) that the stomates close in these plants, and the hairy or waxy leaf
covering begins to play a significant role in restricting cuticular water loss. The
plant behaves as a xerophyte only in an emergency.
The heart of the problem is that the distinction between “plant” and
“environment” is one that is convenient at some levels of analysis, but
confusing at others. We generally describe the environment in physical terms.
For instance, an infertile soil is one that is low in phosphates and other
minerals necessary to plant growth. But our standards are usually taken from
the mineral requirements of crop plants, and are irrelevant to plants that grow
naturally in soils that are infertile by such standards. Soils in Australia are in
general notably infertile, but they are not infertile to the plants of the
bushland, for which an increase in soil fertility by the addition of phosphorus
and nitrogen may be fatal (Beadle, 1966: 1006). The same can be said of
aridity. An exceptionally long drought may kill off the plants of the desert, as
natural catastrophe may kill off the plants of any environment. Under all but
exceptional conditions, however, the plants of an arid environment are able to
satisfy their water requirements-they could not otherwise survive. I t is not arid
for them, and in this sense, there are no true xerophytes.
The dangers of drawing too sharp a distinction between organisms and
environment were given formal recognition by Tansley (1935) in coining the
term “ecosystem” (see Evans (1956) for the history of the concept), as a name
for the interaction system comprising living things together with their
non-living habitat: “not only the organism-complex, but also the whole
complex of physical factors forming what we call the environment”. This is a
useful word because it reminds us of interactions. But it does not seem to solve
the problem in hand. Suppose a biologist decides to investigate an arid
ecosystem. His primary concern will be “with the quantities of matter and
energy that pass through . . . [the] ecosystem and with the rates at which they
do so. Of almost equal importance, however, are the kinds of organisms that
are present in . . . [the] ecosystem and the roles that they occupy in its
structure and organization’’ (Evans, 1956: 1127). The syncretic term is useful,
but a biologist can hardly practise his science without analysis, lower level
synthesis, and comparisons within and across the ecosystem at various levels.
He will certainly note that most of his plants get along with little water, and
will want to know how they do it. Sooner or later he will need a word such as
“xerophyte” to refer to them.
The fortunes of “xeromorph” as a term are similar to those of “xerophyte”.
Schimper used “xeromorph” to refer to plants with a set of gross morphological characters with a specific function, to reduce transpiration. There is a
quaint adaptation of this original use in Australian and New Zealand Botany
(McLuckie and McKee, 1954), a standard text in Australia for many years:
“The term xerophyte is used of all plants which grow in dry localities. It should
be distinguished carefully from xerornorph, applied to plants showing
structural features which may be expected to reduce the transpiration of
water”. Expectation is a curious basis for recognition; hope has been so long
78
G . SEDDON
denied that a xeromorph is now, in effect, any plant that has some of the
characters listed by Schimper as xeromorphic, function being ignored. The
term is thus at least relatively unambiguous in its application, although the
precision is paid for by the arbitrary nature of the current usage. It deals in
gross morphology. Many drought-resistant plants show no gross xeromorphic
characters, and it is sometimes said that their resistance to drought depends on
physiological characters. This means only that the relevant structures are on a
finer scale, at the intercellular or cytoplasmic level. It would be useful to have a
word for all such structures at whatever level, but the word “xeromorphic”
cannot now be used in this sense. Its sense is in fact very restricted, and it is
now, effectively, a synonym of “sclerophyll”, because neither the succulents
nor the ephemeral annuals display xeromorphic characters in any marked
degree. This synonymy has been achieved partly by a shrinkage in the range of
application of “xeromorph”, and partly by an expansion of that of
“sclerophyll”, which has grown to meet it.
CHANGES IN THE MEANING OF “SCLEROPHYLL”
The history of “sclerophyll” and its derivatives in the last 70 years is
complex and confusing, but it illustrates well the problems, and hence the
research-styles, that face the ecologist. Some of the confusion stems from the
original usage of Schimper and Warming. They used the word in three senses.
They assumed that these three senses coincide, but they do not, although they
overlap. “Sclerophyll” means hard leaved, and was used, in part, by Schimper
as a relatively straight-forward descriptive term for leathery, rigid, heavily
cutinized leaves, and by extension, to the trees and shrubs that bear them. This
usage is imprecise-because there are degrees of leatheriness-but unambiguous.
As we have seen, Schimper also built a functional interpretation into the
descriptive term. Sclerophylls are xeromorphs, and xeromorphs, xerophytes:
and thus sclerophylly is an adaptation by which plants survive drought. As this
is still a matter for empirical investigation it is now usual to snap the
terminological link between form and function sensu Schimper. For example,
Beadle (1966: 999) distinguished the terms as follows: “Sclerophylly” is used
to indicate leaf hardness or harshness, “xeromorphy” to describe those
morphological characters usually associated with xerophytes (drought-resistant
plants). “The two terms are used for morphological description only and do not
connote either habitat or physiological characters ” (my italics).
Thus sense one persists to the present; sense two must be excised and set
aside for separate consideration. Sense three is biogeographical, in that
Schimper discussed the “sclerophylls” as distinctive and wide-ranging plant
communities; “The mild temperature districts with winter-rain and prolonged
summer-drought are the home of evergreen xerophilous woody plants, which ,
owing to the stiffness of their thick, leathery leaves, may be termed
sclerophyllous woody plants. The climate districts belonging to this group are
the littoral countries of the Mediterranean Sea, the south-west extremity of
Africa, South-West Australia and the greater part of South Australia, Central
Chile, and the greater part of the coastland of California. In all these widely
separated countries the.vegetation bears essentially the same stamp, in spite of
deep-seated difference in the composition of the flora. It is dominated by
sclerophyllous plants, . . .” (Schimper, 1903 : 506).
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
79
The similarities in the morphology and physiognomy of the vegetation of t h e
“Mediterranean” lands in both hemispheres is striking, and further research has
increased the known parallels rather than reduced them, or shown them to b e
trivial. For example, the structure of the plant community changes as one
moves from fertile to infertile soils in much the same way in Provence,
California and South Australia, with a climax vegetation dominated by
evergreen sclerophyllous trees in a woodland formation, but with an understorey of grass and herbs on fertile soils, giving way to an understorey of
sclerophyllous shrubs on less fertile soils. These communities also all tend to
react to heavy grazing and repeated burning in much the same way,
degenerating first to a dense thicket of rather tall shrubs (maquis, chaparral,
espinal, scrub) and then to a more open heath of low shrubs, on the less fertile
soils; and on fertile soils, from grassy woodland, to perennial grassland, to
semi-arid grassland with many short-lived annuals. These plant-communities,
made up of entirely different species of plants separated by thousands of miles,
are also comparable in their life-form spectra and in their growth rates (Specht,
1969).
“Evergreen sclerophyllous woody plants with semierect rigid dull green
leaves are characteristic of these mild temperate regions with winter rain and
summer drought” (Specht, 1969: 277). When “sclerophyll” is used as a noun it
usually refers to this “Mediterranean” vegetation, and this usage goes back to
Schimper. But if such plants are “the sclerophylls”, then the characteristics of
the sclerophylls become all the characteristics such plants have in common, and
this list extends well beyond “hard leaves”. Even the “dullgreen” and
“semicrect” of the quotation above is an extension, but it is also common to
say that lignotubers (an enlarged, woody rootstock) are common among the
sclerophylls. So are woody fruits, especially in Australia. Grieve (1955)
compiled a list of “morphological and structural modifications characteristic of
Australian sclerophylls”, (Table 1); “aphylly” is one of these characteristics.
Taken at face value, this seems to say that one group of the plants
characterized by leathery leaves has no leaves. Schimper distinguished three
classes of xeromorphy: that of plants with leaves that are leathery (sclerophylly); fleshy (chylophylly);and rudimentary or lacking (aphylly). But Grieve
is not referring to “sclerophylly” but “the sclerophylls” and he attributes to
them all of the characters, except succulence, originally ascribed to xeromorphic plants. This usage is now general, particularly in Australia, although
not universal (see Beadle, 1966). The adjective, “sclerophyllous” may thus
mean “characteristic of the sclerophylls”, and so “the common occurrence of
a woody rootstock” is characteristic of “the sclerophyllous communities of
southern Australia”. “Sclerophyllous” is therefore both imprecise and
ambiguous, although the sense intended is usually apparent from the context.
For example, consider the following: “Sclerophyllous leaves are typically
lignified, with a lower water content than mesomorphic species . . . leaves of
true Rain forest species are often leathery and lignified . . . Rain forest species
are conveniently regarded as possessing “mesic” broad leaves. But in the drier
or colder subformations, or on the exposed surface of the canopy of wet
forests, leaf texture is typically coriaceous and even sclerophyllous” (Webb,
1959: 557). This example is startling, but not ambiguous: it is “sense one”.
That such a usage should be legitimate is nevertheless a danger sign.
G . SEDDON
80
Table 1. Morphological and structural modifications characteristic of Australian
sclerophylls illustrated with examples from Western Australia
Broad and leathery leaves
Stirlingia latifolia
Conospennum scaposum
Banksia grandis
Eucalyptus marginata
Acacia cyanophylla (phyllode)
Microphylly-the ericoid leaf
Micromy rtus imbricata
Astroloma macrocalyx
Bossiaea eriocarpa
Hibbertia hypericoides
Spiny stems
Cryptandra parvifolia
Psammomoya ephedroides
Sunken stomata
Hakea clavata
Acacia acuminata
Daviesia pachyphylla
Cutinization and lignification
Eucalyptus spp.
Hakea spp.
Daviesia spp.
Acicular or needle leaf
Grevillea acerosa
Hakea recurva
Hovea pungens
APhYllY
Hib bertia conspicua
Bossiaea leptacantha
Daviesia aphylla
Winged stems
Acacia alata
Sphaerolobium alatum
Trachymene compressa
Development of tannins and resinous substances
Hakea wria
Acacia acuminata
Dodonaea viscosa
Strong development of palisade mesophyll and
weak development of spongy mesophyll
Eucalyptus marginata
Presence of hairs and scales or waxy bloom on
surface
Lachnostachys verbascifolia
Eucalyptus caesia
THE ANOMALIES OF DISTRIBUTION IN AUSTRALIA
Some rain-forest species have leathery leaves (are “sclerophyllous”), but it
would be unusual to refer to such species as.“sclerophylls”. The sclerophylls
are the olive, the oaks of the Mediterranean and California the proteas, the
eucalypts, most of the Australian wattles, and all those innumerable tough
shrubs from the Mediterranean lands showing the range of xeromorphic
characters. There is a large domain of useful application here, and “sclerophyll”
would, by now, have become a neutral biogeographic term, if it were not for
two things. The first is that the Mediterranean lands have winter rain and
summer drought, and this has tended to keep alive “sense two” as part of
“sense three”, that is, the xeromorphic characters of the “Mediterranean”
plants are often assumed to be functional in withstanding seasonal drought.
The second is more dramatic: the Australian sclerophylls cannot easily be
categorized. The sclerophylls in Australia are not restricted to those parts of
south-western and southern Australia with a mild wet winter and a hot dry
summer. They are just as common in the north, with a mild dry winter and a
hot wet summer-or the eastcentral coast, which is mild and wet the year
round, or the mountains of Victoria and Tasmania, which are cool and wet the
year round, or the tablelands of New South Wales, which are different again-in
short in every climatic regime in Australia, from the most widespread one, the
arid and semi-arid lands of the interior, to the wet coastal fringe.
It is the latter that present the problem, one of no mean order: sclerophyll
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
81
forests flourish in parts of Australia where on purely climatic grounds one
would expect rain-forest. “The ecological relations of Rain forests and endemic
sclerophyllous elements are unique” (Webb, 1959: 551) in Australia, and the
attempt to understand them has tested to the full the explanatory scheme
inherent in Warming’s and Schimper ’s terminology. That terminology has
generated muddle and confusion, but it has also sharpened the perception of
anomaly, and thus has been the energising force behind much good work. The
rest of this paper is a survey of successive attempts to resolve the anomaly of
the “wet sclerophyll forests”, as the tall eucalypt forests with a dense
understorey of tree-ferns in the high rainfall areas came to be called.
This is part of the muddle. “Wet sclerophyll forest” is a self-contradiction
sensti Schimper (1898,1903).I t arose as follows: Diels (1906)first applied the
term Sklerophyllen- Wuld (sclerophyll forest) to the open-forests of Western
Australia with an understorey of xeromorphic shrubs and a poorly developed
herbaceous ground stratum. The adjective Sklerophyllen as used by Diels,
referred to the understorey, not to the sclerophyllous leaves of the ever-present
Eucalyptus trees. Later ecologists used the same term for similar vegetation o n
infertile soils in south-eastern Australia. The term was then extended to areas
of open-forest in Victoria which lack the dense assemblage of xeromorphic
shrubs, but possess an understorey of tussock grasses with a few scattered
shrubs. These too, were called “sclerophyll forest”. Finally, tall open-forests
with a dense understorey usually containing tree-ferns were included under the
term “sclerophyll forest”. To distinguish them, the prefix “wet” was added,
since they were limited to areas of rather high rainfall, hence “wet sclerophyll
forest”. The eucalypt-forests of shorter trees (10-30m tall) from drier areas
without the lush fern-stratum, become “dry sclerop%yll forest”. Specht (in
Leeper, 1970), from whom the above is paraphrased, has abandoned this
terminology, and replaced it with one based on life-form and height of the
tallest stratum-e.g. “tall tree” (over 30 m high)-and the dense or sparse
canopy that results from close spacing or otherwise. Thus rain-forest in
Australia generally becomes “closed-forest” (because of the dense canopy);
“wet sclerophyll” becomes “tall open forest’’ and “dry sclerophyll” becomes
“open-forest’’ or “woodland” or “open woodland”, depending o n how far
apart the trees are.
Schimper (1903) was well aware that “in Australia . . . sclerophyllous
woodland has a very extensive distribution”, and that it is not restricted to the
winter-rainfall areas. He had a solution, however: “Many features render it
probable that West Australia, where the winters are moist and the summers dry,
and where the sclerophyllous flora exhibits by far its greatest wealth, is the
source from which the other Australian districts have become colonized”
(Schimper, 1903: 508). The view that the south-west province of Western
Australia is the “cradle of the autochthonous elements in the Australian flora”
originated with Hooker (1856),who was especially impressed with the richness
and diversity of the “Australian” element in the south-west, where it
undoubtedly reaches its highest development. Schimper’s argument is not
Hooker’s, and it is not convincing: it would seem to be that (a) sclerophylls are
adapted to summer drought (b) therefore the “wet sclerophylls” must have
evolved in a region of summer drought, and later colonized the humid east
coast, presumably by competing successfully with rain-forests and other
6
82
C. SEDDON
pre-existing vegetation. It is now known that rain-forest genera (the
Cinnamomum flora) were widely distributed across southern and south-western
Australia in the Eocene, some 50 million years ago, and the subsequent
diversification of sclerophyll genera and the contraction of rain-forest still calls
for explanation, but the notion of colonization from the South-west Province
does not provide it, because increasing aridity might explain contraction of
rain-forest, but not the successful invasion of the humid zone by sclerophylls.
A second line of argument has been to defend the thesis that the sclerophylls
are essentially xerophytes by showing that even in areas of high rainfall, they
are subject to waterstress. For instance, the country around Sydney which has
a 48-inch rainfall fairly evenly distributed, shows a rich variety of sclerophylls,
but the thin, sandy soils over the Hawkesbury and Narrabeen Sandstones dry
out very quickly, and the plants may be subjected to severe water-stress o n
several occasions every year, despite the high rainfall. Thus these plants are
xerophytes. Sandy, excessively drained soils are fairly widespread in Australia,
so that this argument has force, but not all soils are excessively drained.
Schimper’s concept of “physiological drought’’ is also applicable in some cases:
cold, or a high content of humic acids, or an abundance of soluble salts,
especially sodium chloride, or scarcity of free water in the soil, where most of
the water is held by fine soil particles, and thus not available to the plant roots;
any of these may restrict water-uptake, so that the plant experiences
“physiological” drought. Some of these features are fairly common, especially
salty soils, but they cannot explain all cases, even in conjunction. There are
deep soils of good texture with abundant available water supporting magnificent “wet sclerophyll” forests in eastern Australia. It is not the case that all
sclerophylls are xerophytes. (The understorey of the “wet sclerophyll” forests
is lush, and only the dominants are sclerophylls, but they are enough to make
the point.)
A quite different approach is to argue that the wet sclerophylls are out of
phase with their environment, at least as regards their sclerophylly. There are
several ways in which this could be so. If the sclerophylly is not a disadvantage
to the trees in their humid environment, then it is a neutral trait, and selective
pressures will not be felt. Or the sclerophylly may be mildly disadvantageous,
but linked genetically with other advantageous traits, which thus maintain it in
a population. Or there may not have been enough time for selective pressures
to make themselves felt. Although organisms are in general adapted to their
environment, this fit is imperfect, because the environment, especially climate,
fluctuates rapidly, whereas evolutionary change is relatively slow, and is not
neccessarily uniform at all levels of plant organization. These possibilities
enormously complicate the analysis of all evolutionary phenomena. It has been
argued (Specht & Rayson, 1957) that the heath vegetation in South Australia
contains plants that have a growth phase out of rhythm with the present
Mediterranean type climate. These plants make their major growth during the
driest months of summer. In Western Australia, Eucalyptus calophylla and
many species of Banksia produce their maximum growth in rainless January
and February after flowering. “The most likely explanation seems t o be that
they retain some of the growth rhythms belonging to the climatic regimes of
earlier” [more humid] “periods” (Burbidge, 1960: 176). But these examples
do nothing for the argument that the wet sclerophylls are maladapted, almost
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
83
the reverse. In any case, it is not really possible to demonstrate non-adaptive
behaviour in the short-term. We might think it odd for plants to make growth
when we might expect them to be almost at wilting point, but the plants
themselves seem nevertheless to carry off the feat. That they do it shows that
they can, and if they can, how is it non-adaptive? Similarly, the mountain ash
and other “wet sclerophylls” seem to flourish in their inappropriate environment.
THE ROLE O F SOIL NUTRIENTS, LIGHT AND FIRE IN THE DISTRIBUTION O F
SCLEROPHYLL FOREST AND RAIN FOREST O F AUSTRALIA
There is an alternative to supposing that because we cannot explain the
xeromorphy of the Australian sclerophylls on the basis of water-stress in a
particular locality, there is possibly a xerophytic past history for the group: the
hypothesis that xeromorphy is primarily a response to water-stress could be
abandoned. In the last 20 years, other factors have been proposed as
contributory or controlling; factors such as light, fire, and soil nutrients. I t has
recently been argued (Beadle, 1966) that soil phosphate levels in the distant
past directed evolutionary development in the ancestors of the sclerophylls,
leading to the gradual development of smaller, more closely spaced and more
xeromorphic leaves. Rain-forest and wet sclerophyll eucalypt forests are found
side by side in parts of eastern Australia, but the rain-forest is generally
confined to the good soils. Such contrasts are especially striking in the Sydney
area, where there is a diverse sclerophyll assemblage on the Hawkesbury
Sandstone (about 123 sclerophyll genera, 12 rain-forest genera) and a luxuriant
rain-forest on the basaltic soils of the Illawarra coast, and Mt. Tomah and
Mt. Wilson (about 3 sclerophyll genera and 5 5 rain-forest genera at Bulli
(Beadle, 1966)). Soil control of distribution is obvious in these areas, (although
it can be argued for the primacy of water-relations, that one of the major
differences between the fine textured basaltic soils and the porous sandy soils
over the Hawkesbury Sandstone is in their capacity to retain moisture).
Beadle (1968: 3 5 5 ) has argued from an analysis of present distribution that
rain-forest genera in Australia are more effectively eliminated or excluded from
an area by low nutrients than by a dry climate, and cites the example of
Flindersia maculosa, which is found in parts of western New South Wales with
a mean annual rainfall of 10 inches, although Flindersia as a genus is generally
restricted to rain-forest. His conclusions are supported by experimental
physiology. “The probability that the physiology of the leaf is different in
xeromorphs and mesomorphs and lies in the resistance to mineral starvation is
supported by the reactions of the mesomorphs and xeromorphs to reduced
nutrient supplies”. His work on woody fruits, which are a conspicuous feature
of most sclerophylls in Australia, is especially interesting. This was investigated
by growing selected sclerophylls in their own soil, and in their own soil with
the addition of nutrients. Hakea sericea has produced fruits under both
treatments. High nutrient plants produce fruits which are much smaller than
those of low nutrient plants; the explanation seems to be that seed formation,
which requires a large amount of phosphorus and nitrogen, can go ahead
rapidly in the high nutrient plants. In the low nutrient plants, excess
carbohydrate is accumulating during the relatively long interval during which
84
C. SEDDON
the plant accumulates the phosphorus necessary for seed formation. The excess
carbohydrate is converted into wall material, and so there is a large woody fruit
by the time the seeds are mature.
Beadle (1966) has shown conclusively that soil phosphate levels play a major
role in the distribution of the sclerophylls in Australia, and he has also shown
experimentally that the level of mineral supply affects lignification and leaf
thickness and toughness, and to that extent can induce or reduce some
xeromorphic characters in plants. The role of fire is even more dramatic and it
has only gained full recognition in the last few years, although some specific
adaptations have long been understood-for example, the ability to regenerate
branches direct from the fire-blackened trunk by sprouting from latent buds;
the woody rootstock or lignotuber from which many sclerophylls can
regenerate after the plant above ground is burned off; and the woody fruits
opened only by fire, retaining their seeds for a time after the valves of the
fruiting cones have opened, releasing them on a bed of cool ash.
The sclerophylls in Australia are not merely fire-tolerant : they are
fire-promoters. The eucalypts especially, often have a peeling bark, a high
content of inflammable oils in their leaves, and high litter production. They
catch fire easily, and recover from fire relatively easily. Rain-forest does not,
and this is the best available explanation of the sclerophyll communities in
areas of heavy rainfall, in excess of 120 inches a year in parts of Tasmania. If a
mixed forest, with both rain-forest species and eucalypts, catches fire, open
conditions follow, with abundant light. Eucalypts release vast quantities of seed
after a fire, and germination and early growth are rapid. The rain-forest species
germinate with the eucalypts but have a much slower growth rate, and with
repeated fire, they are eliminated. If there is no further fire, the rain-forest
species eventually form a canopy under the eucalypts, and no further eucalypts
can germinate in the low light intensities of the forest floor, and if such a
mixed forest is not burned for 400 years, the old euclaypts die from crown
damage and fungus attack and pure rain-forest remains. Destruction of this
forest yields only rain-forest regeneration, because eucalypt seed is no longer
available for regeneration after fire. But few areas in Australia escape fire for
very long periods, and with frequent fire, the sclerophylls invade rain-forest
from the margins. Frequent fit'e also tends to decrease soil fertility-especially
those developed over nutrient-poor rocks such as sandstone and quartzites-by
increasing the rate of leaching. As we have seen, the sclerophylls are generally
better able to grow on poor soils than rain-forest. Thus there are two positive
feed-back mechanisms at work here favouring the sclerophylls: fire favours the
sclerophylls-fire improverishes the soil-poorer soils favour the sclerophyllsthe sclerophylls favour fire (Mount, 1970).
Given a knowledge of any four of the five factors; rainfall and its
distribution, temperature, soils, fire-frequency, and vegetation type, it is now
possible to predict the fifth, and although it must be conceded that a
knowledge of fire-frequency is usually learned from a study of the vegetation,
this can be supplemented by information from historical records. Prediction
makes control possible, and to the extent that control of fire is possible in the
natural environment, it is possible to decide whether certain areas should be set
aside for rain-forest or sclerophyll forest, and thus the ecological insight
outlined above has become a tool of forest management.
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
85
CONCLUSION
The most recent fashion in ecology has been a move towards systems
ecology, an “intersection of two branches of science, biology and engineering”.
Systems ecology “accepts as an operating principle that no complex system can
be fully known in all of its interactive details: and so turns to simplified
computer models. The method aims at control rather than understanding; it is a
“move away from the explanatory or cognitive criterion of truth, a soft
criterion which heuristically lends intellectual points of leverage for seeking
understanding, and toward the predictive criterion, a hard one with the
potential of leading ultimately to optimal design and control of ecosystems”
(Patten, 1971: xi$.
This sounds like Bridgman (1936) introducing
“Operationalism” in physics in the 1930s-but a lot of water has flowed under
the philosophical bridge since then. Systems analysis is likely to be a useful tool
in ecology, but its advocates need not conclude that because ecology is
complex and difficult, understanding must be sacrificed to control. In the
example quoted, control has come through understanding. In concluding my
discussion of this example, I would also like to suggest that scientific research
which has led to understanding and control is not well described as
“immature”, and further, that success in this case at least, has come almost
wholly from research strategies that are primarily ecological-mapping of soils,
analysis of the structure of the vegetation, its behaviour under natural
fluctuations in the environment, and so on.
If the “anomalous” distribution of the Australian sclerophylls can now be
adequately explained, the adaptive function of the “xeromorphic” characters
of the Australian and other sclerophylls can not, at least in full. I t is not even
clear what a full explanation would be like. It has been shown that the
production of woody fruits so common among Australian sclerophylls is
related to the metabolism of plants growing on low-nutrient soils. But there are
many Australian sclerophylls, for example, some epacrids and legumes, that
grow on the same soils, but do not have large woody fruits. Here there may be
a sufficient, if not a necessary, condition for the production of woody fruits,
but it is not even certain that it is sufficient, because it has not been shown (or
can show) that selection for fire-tolerance has not played a part in the
evolution of woody fruits. Physiological experiment and work on transpiration
rates has greatly increased our understanding of how some sclerophylls survive
water-stress, but t o show that a plant has a given structure and that this
structure functions in a given way, is still not to show that the plant has
acquired that structure as a result of natural selection to perform that function.
“Adaptive” is a theory-laden term, and to say that a character is adaptive is to
say both that it functions in a certain way in the present, and that this
successful function has given the plant some advantage over potential
competitors in the past. There is, I suggest, an ineradicable element of
speculation in such claims, although to say this is not to endorse the leap,
common 70 years ago, from gross morphology to adaptive function. (This leap
looks facile in retrospect, but some such assumption was necessary at first to
get started, as this story shows.)
That the problem remains acute is shown by one extreme response to it.
Why is the eucalyptus leaf-type so common in Australia? Why d o many shrubs
86
G. SEDDON
in Western Australia have holly-leaves, or needle-like and semi-cylindrical
leaves? Certain characters are very common in a given region, and spread right
through the most diverse genera and families. Perhaps they are not primarily
adaptive at all: Went (1971) has suggested, “that the real problem confronting
the evolutionist is not one of selection and adaptation, but of non-sexual
character transfer” and by mechanisms not yet understood. It is not my
purpose to expound or defend that proposition: its interest here is to show that
after 70 years of research, controversy can erupt even over a fundamental
ecological assumption, based on biological theory. As for the concepts and
terms with which we began, they were bound to run into difficulties from the
outset, both in deducing adaptive function from form, but also in classifying
plants according to their water-relations. Although Ecology is fundamentally
concerned with interactions, such a classification isolates a single factor, and to
that extent is bound to be misleading in the end. Yet an ecologist can’t begin
with interactions, and I suspect that all four terms are likely to limp along for
some years to come, despite their inadequacies. The challenge laid down by
Kant (1786) can be met in the case discussed here, namely, “. . . I assent that
whenever a dispute has raged for any length of time, especially in philosophy
there was, at the bottom of it, never a problem of mere words, but always a
genuine problem about things”.
ACKNOWLEDGEMENTS
I should like to thank Dr L. A. S. Johnson and his colleagues at the Royal
Botanic Gardens and National Herbarium, Sydney, N.S.W., Australia, for their
helpful comment on this ms.
REFERENCES
BEADLE, N. C. W., 1966. Soil phosphate and its role in molding segments of the Australian flora and
vegetation, with special reference t o xeromorphy and sclerophylly. Ecology, 47(6): 992-1007.
BEADLE, N. C. W., 1968. Some aspects of the ecology and physiology of Australian xeromorphic plants.
Aust. J. SC., 30: 348-56.
BOWDEN, B. V., 1956. Proposals f o r the development of the Manchester College of Science and
Technology. Manchester College.
BRIDGMAN, P. W., 1936. The nature of physical theory. Princeton University Press.
BURBIDGE, N. T., 1960. The phytogeography of the Australian region. Aust. J. Bot., 8: 75-212.
CANDOLLE, A. P. de, 1820. Essai elementaire d e giographie botanique. Dict. Sci. Not., 18: 3 5 9 4 2 2 .
CLEMENTS, F. E., 1904. The development and structure of vegetation. Bot. Survey o f Nebraska Srudies
in the Vegetation of the State. iii.
CLEMENTS, F. E., 1905. Research methods in ecology. Lincoln: Nebraska.
DAUBENMIRE. R . F., 1959. Plants and environment: a text-book of plant autecology. 2nd ed. New
York: John Wiley.
DIELS, L., 1906. Die Pflanzenwelt von West Australien. Die Vegetation der Erde, VII. Leipzig: Engelman.
EVANS, F. C., 1956. Ecosystem as t h e basic unit in ecology. Science, N . Y . . 123: 1127-8.
GRIEVE, B. J., 1955. The physiology of sclerophyll plants. J. Proc. R . SOC. West. Aust., 39: 3 1 4 5 .
GRIEVE, B. J.. 1957. Studies in the water relations of plants. 1.-Transpiration of Western Australian
(Swan Plain) sclerophylls. J. Proc. R . SOC.West. Aust., 40: 15-30.
GRIEVE, B. J . 84 HELMUTH, E. O., 1970. Eco-physiology of Western Australian plants. Ecol. Plant.
Gauthier-Villars, 5: 33-68.
HAECKEL, E., 1868. Natiirliche Schopfungsgescheite. Berlin.
HENRICI, M., 1937. Transpiration and water supply of South African Plants. S. Afr. J. Sci, 34: 61-72.
HENRICI. M., 1941. Transpiration of large Karroo bushes. S . Afr. J. Sci., 37: 156-63.
HOOKER, J . D., 1856. Botany of the Anrarctic Expedition. III. Flora Tasmaniae 1 , Introductory Essay.
London.
XEROPHYTES, XEROMORPHS AND SCLEROPHYLLS
87
HUMBOLDT, A. von, 1807. Essai sur la Giographie des Plantes. Paris.
KAMERLING, 2.. 1914. Welche Pflanzen sollen wir Xerophyten nennen? Flora, 106: 433-54.
KANT, 1.. 1786. Prolegomena and the metaphysical foundations o f natural science. London: Bell.
KORMONDY, k.J., 1969. Concepts ofecology. Englewood Cliffs, N.J.: Prentice Hall.
LEEPER, G. W. (Ed.), 1970. The Australian environment. Melbourne: Melbourne Univ. Press.
MAXIMOV, N. A., 1929. The plant in relation to water. London: Allen & Unwin.
MAXIMOV, N. A., 1931. The physiological significance of the xeromorphic structure of plants. J. Ecol.,
1 9 : 273-82.
McLUCKIE, J. & McKEE, H. S., 1954. Australian and N e w Zealand botany. Sydney: Associated General
Publ.
MEDAWAR, P. B., 1967. The art of the soluble. London: Methuen.
MOUNT, A. B., 1970. Euclaypt ecology as related t o fire. Tall TimbersFire Ecology Conference, Proc., 9:
75-108.
PATTEN, B. C., 1971. Systems analysis and ecosystems. New York: Academic Press.
RAVETZ, J. 1971. Scientific knowledge and its social problems. Oxford: Clarendon Press.
SCHIMPER, A. F. W.. 1898. Pflanzengeographie aufphysiologischen Grundlage. Jena.
SCHIMPER, A. F. W., 1903. Plantgeography upon a physiological basis. Oxford: Clarendon Press.
SCHOUW, J. F., 1823. Grundtraek ti1 en almindelig Plantgeografie. German ed., Berlin.
SEYBOLD, A., 1929. Die physikalische Komponente der pflanzlichen Transpiration. Gesamr. gebiet der
Bot. herausg. v. Goebel. Berlin 11.
SOLMS-LAUBACH, H., 1905. Die leitenden Gesich tspunkte; einer allgemeinen Pflanzengeographie.
Leipzig.
SPECHT, R. L., 1969. A comparison o f the sclerophyllous vegetation characteristic of Mediterranean
type climates in France, California, and southern Australia. Aust. J . Bot., 1 7 : 277-307.
SPECHT, R. L., 1970. Vegetation. In G . W. Leeper (Ed.) The Australian environment. Melbourne Univ.
Press.
SPECHT, R. L. & RAYSON, P., 1957. “Dark Island Heath (Ninety-Mile Plain, South Australia) I.
Definition of the ecosystem”. Ausr. J. Bot.. 5: 52-85.
TANSLEY, A. G., 1935. Use and abuse of vegetational concepts and terms. Ecology, 16: 284-307.
VASILJEV, I . , 1931. The water-relations of plants in Kara-Kum sand desert. Planta, 14: 225.
WARMING, E., 1895. Planresumfund. Grundtrak a f den Okologische Plantegeografi. Copenhagen.
WARMING, E., 1909. Ecology of plants, an introduction to the study of planr communities. Prepared for
publication in English by Percy Groom and lsaac Bayley Balfour. Oxford: Clarendon Press.
WEBB, L. J., 1959. A physiognomic classification of Australian Rain forest. J. Ecol., 47: 551-70.
WENT, F. W., 1971. Parellel evolution. Taxon. 20: 197-226.
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