507
THE S E A S O N S I N .i THOJ'IUAL RAIN-NOHEST
The seasons in a tropical rain-forest (New Hebrides).
Part 2 . Botany.
R. BAKER,M.A., D.Phil., and INABAKER. (From the
By JOHN
Department of Zoology and Comparative Anatomy, Oxford.)
(Communi-
F.R.S., F.L.S.)
cated by Professor E. S. GOODRICH,
(PLATE
9)
LHead 7 November 19351
CONTENTS.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Discussion.
514
General remarks ........................................
515
Gregarious flowering
..............
The influence of the enviroiimeiit on reproductive and leaf-fall
515
seasons . . . . . . . . .
..........
516
The effect of interrial
..............................
518
Summary ..................................................
51 8
References . . . . . . . . .
....................................
INTRODUCTION.
This paper is the second of a series in which the work of the Oxford University
Expedition to the New Hebrides (Pacific Ocean) is described. The main object
of the expedition was t o make a detailed study of seasonal phenomena in
terrestrial organisms in a remarkably unvarying climate. The meteorological
results obtained by the expedition have already been reported (Baker and
Harrisson, 1936). We wish to acknowledge much help from Mr. T. H. Harrisson
and Mr. A. J. Marshall in the botanical work. Since it appears that inore is
known about the causation of seasonal phenomena. in plants than in animals,
a general discussion of the subject, forms part of this paper, although thc
expedition concerned itself more with animals than with plants.
The observations described in this paper were made in the rain-forest at
Hog Harbour (15" 15' S.) in the island of Espiritu Santo, New Hebrides, from
September 1933 until August 1934, inclusive. A typical view of the rainforest is shown in Plate 9, in which one of the photographs represents a
part of the forest meteorological station. The forest may also be studied in
the frontispiece of a book by one of UR (Baker, 1929). The forest covers
almost the whole island except thr plantation area in the south and southeast and the relatively small areas occupied by the villages, gardens, and
deserted garden-sites of the natives. It is not possible to be certain vhether
508
DR. J. R. BAKER AND MISS INA BAKER ON
any considerable area of the forest was under cultivation in the past, when the
island held a much larger native population. One cannot distinguish a primary
and a secondary forest, though the sites of gardens which were deserted within
living memory are obvious enough. The forest has the general characters
of a typical tropical rain-forest (general luxuriance of foliage, cutting off almost
all direct sunlight, together with abundance of creepers, lianas, and epiphytes,
and of trees with buttressed roots) ; but the trees are mostly rather small,
and there is not a regular succession of layers of foliage beneath the canopy
such as oicurs in rain-forests on richer soil. In thevicinityof Hog Harbour
the underlying rock is all upraised coral.
SEPT
C!oryniborchis
veratriifolia.
OCT,
NOV
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DEC. JAN.
MAR.
FEB.
......................
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..
8j
CarugaJEoribunda
..
&
Castanospermum
australe.
Eugen,ia sp.
Burringtonia
samoensis.
MAY
...................................
............ @
Piper methysticuni
APR.
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AUG.
JUNE JULY
CIZ
Diagram showing the times of flowering (rosettes) a i d fruiting (black circles) of the
selected species during 1933-4. The dotted lines indicate the absence of' flowers
and fruit.
Ten species of common wild plants were selected for study throughout the
year, in addition to certain cultivated ones. The selected wild plants range
from trees to herbs. Observations were made SO far as possible between the
10th and 20th of each month, except that in September 1933 they were made
late in the month, and in February records were unfortunately not made for
four of the species. Observations were confined t o specimens of the ten species
growing in the vicinity of Hog Harbour. Each month it was noted whether
there were expanding flower buds, flowers, unripe fruit, or ripe fruit.
@
.
THE SEASONS I N A TROPICAL RAIN-FOREST
509
The results of the investigation are spown diagrammatically on p. 508. The
five-lobed symbols indicate that flowers (not flower-buds) were observed, and
the black circles represent fruits (unripe or ripe). The dotted line indicates
that neither flower nor fruit was seen, although the plants were observed.
When for any reason observations were not made, a space is left in the diagram.
The climate during the year of the investigation has been discussed in
detail in the first paper of this series (Baker and Harrisson, 1936). Speaking
generally, the climate is a remarkably unvarying tropical one. There is no
dry season. Five consecutive months (,June to October) are, however, considerably less wet, as well as a little cooler, than the rest. This is clearly shown
in the hythergraph, fig. 2 B in the paper by Baker and Harrisson quoted above.
I n the year of the investigation (September 1933 to August 1934) the climate
conformed on the whole to the usual regime, except that April and May, usually
very wet months, were not nearly so wet as usual. L4tthe forest meteorological
station April was the hottest month and July the coolest, but there was only a
difference of 2.4" C. (4.3"Pahr.) between the mean temperatures of these two
months.
We wish to thank the Keeper of the Department of Botany at the British
Museum (Natural History) for arranging for the identification of the plants.
This was carried out by Mr. J. E. Dandy, Mr. A . W. Exell, and Dr. G. Taylor,
to all of whom we are much indebted. Mr. J. S. L. Gilmour also gave helpful
advice.
This study of seasonal phenomena was made under grants from the Royal
Society and the Percy Sladen Memorial Fund.
RESULTS.
One of the ten species chosen for special study was unfortunately indeterminable, because no flower nor fruit was seen during the course of the year.
This is all the more remarkable because the herb in question (the ' Nasukonru '
of the natives) is probably the most abundant Angiosperm in the northern
New Hebrides. Nearly everywhere it forms the dominant species in the undergrowth (Plate 9, N . ) . It is a Dicotyledon having long lanceolate leaves with a
distinct vein running close to the margin. It grows about two feet high.
The natives, who are keen naturalists, state that they do not know the flower
of this species. Flower-buds were observed by one of us on one occasion in
the high mountains of western Espiritu Santo, but the specimen has been
mislaid. The remaining nine species will now be considered in turn.
1. CORYMBORCHIS
VERATRIFOLIA (Reinw.) B1. (Orchidaceae).
Native name : ' NerP vudhvekar '.
This is a herb with stiff pointed leaves and small white flowers. There
appear to be three flowering seasons in the year-in October, January, and June.
Expanding flower-buds were seen in the months preceding the October and
June seasons,
510
DR. J . R. BAKER AND MISS INA BAKER ON
2. FICUSCOPIOSA Steudel (Moraceae).,
Native name : ' Nedhelagh '.
This species is a smallish tree which bears spherical figs in dense clusters
springing directly from the trunk. I n the diagram the presence of figs is
indicated by the symbol for fruits. Figs were not observed in April, May, and
August, and some trees were bare of fruits in December. As not many of the
trees of this species were under observation, it is possible, and indeed
probable, that specimens with figs might be found a t all times of the year.
According to the natives, the figs are eaten by fruit-bats, whose breeding seasons
will be considered in the next paper in this series.
3. PIPERMETHYSTICUM Forst. f. (Piperaceae). (Wild Kava.)
Native name : ' Neviilko '.
This is a plant growing about three feet high with cordate divergentlyveined leaves (Plate 9, Pi.) and minute white flowers growing on fleshy spikes
three or four inches long. The fruits are red. As will be seen from the diagram,
fruit or flowers were seen in every month except April, and expanding flowerbuds were noted in that month. Some plants devoid of fruit or flowers were
noted in February and March. It appears that this species reproduces all
the year round. This is not surprising, for even so far south as New Zealand
the representative species ( P . excelsum Forst.) is said to flower during most
of the year (Cheeseman, 1906).
4. GARUGAFLORIBUNDA Decne. (sensu H. J. Lam) (Burseraceae).
Native name : ' Nalu '.
This appears to be the only indigenous tree of Espiritu Santo that is regularly
deciduous every year a t about the same time, though a few other species cast
all their leaves a t intervals, which may be regular (see below, p. 513). Early
in September (i.e. towards the end of the less wet season) it is almost completely devoid of leaves (Plate 9), and gives a remarkable appearance, showing
up vividly against the mass of green when the rain-forest is viewed from a boat
a t sea. Later in September the flowers are formed a t the tree-tops a t the same
time as the first leaves. The fruits, being inconspicuous, unfortunately largely
escaped our observation, and though we have one specimen in alcohol we have
no record as to the month in which it was procured. This is the most strikingly
seasonal plant in this peculiarly unseasonal climate.
According to Lam (1932) the genus Garuga is adapted to a climate which has
a rigorous dry season. It is remarkable that it should retain its deciduous
nature in the northern New Hebrides, where no part of the year is dry except
under exceptional circumstances. Lam states that Garuga Jloribunda generally
flowers in the dry season throughout its range. He gives September and October
as its flowering season in Celebes, July t o November in Java, and March to
June in the Philippines. Broun (1912) says that in the Bombay Presidency
Garugn pinnrrta is one of the trees most affected by failure of the monsoon
THE SEASONS I N A TROPICAL RAIN-FOREST
51 1
rains, probably owing to its roots being superficial. According to Troup (1921))
the leaves of G. pinnatu start falling in India in November-January, and the
tree remains leafless till March-May.
5 . CASTANOSPERMUM
AUSTRALE A. Cunn. (Papilionltceae).
Native name : ' N i s '.
This is a large tree with bright red flowers which principally attract attention
when they have fallen t o the ground beneath the tree, i t being almost impossible in the dim light of the forest to see them a t the great height a t which
they are produced. The fruits are huge woody legumes which are also only
easily observed when they have fallen to the ground, and as they are of a very
durable nature i t is difficult to know when they are freshly dropped. As will
be seen from the diagram, the flowering of this tree is definitely seasonal and
takes place from September to January, that is, mostly during the warmer
wetter part of the year. Expanding flower-buds were, however, seen in July.
Some specimens of this species were cut down and transverse sections of the
stem were polished to show growth-rings. The latter were found to be clearly
marked, and one would not have guessed that they had been formed under such
remarkably unseasonal climatic conditions. As Wright (1905) says, ' rings
of growth are not, in all cases, . . . . a guarantee of the existence of a climatic
periodicity '.
6. EUQENIA
sp. (resembling E . javunica) (Mptaceae).
Native name : ' Ne '.
This ' rose-apple ' is a small tree with smooth, thick, shiny, lanceolate leaves
and flowers with numerous magenta stamens. The flowers are borne in clusters
of three or four blossoms. All the flowers on one tree come out a t one time,
and present a very vivid appearance for a few days. The fruits superficially
resemble apples and are edible. Mr. Exell tells us that the flowers are very
like those of E . javanim Lam., but since the leaves are cuneate a t the base
instead of cordate, and the fruits larger and edible, it is probably a new species.
There appears to be a non-fruiting season during the cooler months (June to
August). During the rest of the year it is usually possible to f h d a tree with
expanding flower-buds, flowers, or fruits. The natives report that the flowers
and fruit are eaten by fruit-bats. The common Australian species of Eugenia
flower in the Australian spring and summer (Sulman, 1913). The only species
in New Zealand flowers from March to May (Cheeseman, 1906).
7. BARRINGTONIA
SAMOENSIS A . Gray (Lecythidaceae).
Native name : ' Narodh '.
The inflorescence of this tree is very striking, as the white many-stamened
flowers, measuring about two inches across, are borne on drooping panicles
about a foot long. The kernels are edible. Except during March and April,
expanding flower-buds, flowers, or fruits were found in every month. This
512
DR. J. R. BAKER AND MISS M A BAKEB ON
behaviour contrasts strongly with that of B. acutangulu Gaertn. in India,
where the flowers appear chiefly from March to May. In 1927 one of us brought
back from Hog Harbour some wood of B. speciosa, which is remarkable for the
poor development of growth-rings. Polished specimens were also prepared
of the wood of eight other species of trees from Hog Harbour, namely, Calophyllum inophyllum, Cashnospermum australe, Cerelea odollam, Cordia subcordata, Glochidion ramijorum, Hernandia peltah, Spondias dulcis, and Tournefortia argentea. Of all the nine species, Hernandia speciosa was the one whose
wood exhibited least signs of periodical growth. Most of our wood specimens
are in the keeping of Professor A. C. Seward a t Cambridge.
8. PSYCHOTRIA
sp. (Rubiaceae).
Native name : ' Yalyalrara '.
This is a bush growing to three or four feet high, with smooth green leaves
two or three inches long (Plate 9, Ps.).The small flowers are borne in insignificant clusters. The fruits are green when first produced, turning red when
ripe. Plowers and fruits are frequently seen on the same shrub, and indeed on
the same cluster a t the same time. (Another species of Psychotria, the ' Kerkerarodh ' of the natives, is very similar, and indeed certain members of the
expedition found it difficult to distinguish between them.) There appears
to be a long flowering and fruiting period from August to January, and a very
short one in May.
9. GEOPEULA
HERBACEA (Jacq.) K. Schum. (Rubiaceae).
Native name : ' Nidhvura '.
This is a small herb about two inches high, with star-like white flowers and
red berries. The leaves are cordate and grow close to the ground with the
flowers and fruit standing up slightly above them. As will be seen from the
diagram, the one month in which no flower was seen was March.
It will be seen from the diagram that there is a very defbite reproductive
period from September to January. The start of this long reproductive period
coincides with a slight rise of temperature after the end of the cooler period
of the year. The rainiest months occur during this period. All the plants
under observation had either expanding flower-buds, flowers, or fruits in
September and October, and with the exception of Garuga jloribunda and
Corymborchis veratrifolia (on which no flower nor fruit was observed in December),
all the plants had flowers or fruits until January. The records are unfortunately
very scanty for February, but by March none of the plants studied were in
flower, and only three were bearing fruit. In May and June there appears to
be another general flowering and fruiting period, but this time it occurs when
the rainfall is low and the temperature falling. This short reproductive period
was over by July. In this month Geophilu herbacea was the only one of our
selected plants in flower. I n the rain.forest of Bolivia, at a place in almost
the same latitude as Hog Harbour, Rusby (1933) found that February and
March were months of special bloom.
THE SEASONS IN A TROPICAL RAIN-FOR,EST
513
Although Garuga Jloribunda stands out as the most obviously and regularly
deciduous tree in the northern New Hebrides, yet there are others which lose
most of their leaves a t one time or nearly a t one time. Thus a specimen of
Pteromrpus indicus Willd. (Papilionaceae) a t Hog Harbour was observed to
be shedding its leaves rapidly on 23 January 1934, and on the 26th most of
its branches had only got new pale green leaves, though some of the branches
still retained the old leaves. At Singapore Pt. indicus changes its leaves
annually about the beginning of the less wet part of the year (February and
succeeding months), and Holttum (1930) mentions that certain branches change
their leaves before others. I n India (Troup, 1921) this species is leafless or
nearly so towards the end of the cold season. It thus appears that north of the
equator (India), on the equator (Singapore), and south of the equator (Hog
Harbour), the leaf-changes occur about the first quarter of the year. According
to Lely, Pterocarpus erimceus is leafless a t about the same time (December to
February) in northern Nigeria.
S p d i a s dulcis Forst. (Anacardiaceae),a tree with large yellow edible drupes,
is said by the natives to cast its leaves periodically. Our wood-specimens show
well-marked growth-rings. The fruit of this species was ripe towards the end
of January. Spondias mangifera, according to Wright (1905), is deciduous
during the dry season in the northern forests of Ceylon. In India (Troup, 1921)
this tree is leafless longer than almost any other forest tree, and, rather remarkably, ripens its fruit about the same time (December-February or DecemberMarch) as its relative a t Hog Harbour. In the New Hebrides the banyans
cast their leaves periodically, but according to our friend Mi. T. R. ff. Salisbury,
who has lived at Hog Harbour as Government Agent for many years, the period
is not annual. The natives state that Termimlia Catuppa L. (Combretaceae)
is deciduous a t Hog Harbour. This species is annually deciduous in India,
and also when grown in southern Florida (Stevenson, 1933)) while it changes
its leaves twice a year in Ceylon and a t Singapore, and twice and sometimes
three times a year on the Gold Coast. Thus it is clear that certain trees
have such a strong inherent tendency to periodical loss of leaves that they
remain deciduous even in the remarkably non-seasonal environment of Hog
Harbour.
Two of the natives’ chief foodstuffs, the yam and the sweet potato, are planted
annually. Seasonal changes in climate are so little obvious that the reckoning
of peoples’ ages in years is unknown (except in the Christian villages). We
therefore questioned them as to how they know when to plant these crops,
and discovered that they wait for the flowering and fruiting of certain wild
trees, Yams are planted when Erythrina indica Lam. (Papilionaceae) flowers,
which happens about October. (The same species flowers in April-May in
India.) With the co-operation of our friend the Rev. W. Anderson, the
experiment is being tried of planting yams at precisely the ‘ wrong ’ period
of the year-that is, six months after the flowering of Erythrim. Sweet potatoes
are planted by the natives as soon as the fruit of Alphitonia zizyphoides A. Gray
(Rhamnaceae) is formed, about the beginning of September.
TJ”. J0URN.-ZOOLOGY,
VOL. XXXIX
37
514
DR. J . H . BAKER AND MISS INA BAKER ON
Monthly observations were made on four cultivated plants, viz. the banana,
papaw, bread-fruit, and orange.
Ripe bananas were available in every month of the year, and flowers were
seen in September, October, January, May, June, and August. They constitute a favourite food of the fruit-bats.
Bread-fruit in an unripe condition were visible on the trees at all times of the
year, except that many trees were devoid of fruit in June. They only ripened
during September, October, November, December, January, April, and a few
in June.
Oranges in an unripe state were on the trees in large numbers in September.
One or two ripe ones were seen during the ensuing months, and these became
more frequent in February and March until a climax was reached in April,
when all the fruits on the trees ripened and fell off. In May the trees were
devoid of flowers or fruit. I n June flower-buds and flowers were seen, which
had formed fruit by the following month ; and the ripening cycle then began
once more. Oranges are the only cultivated plants with a strongly marked
annual reproductive season that we had under observation.
We should be very grateful to residents in any part of the world, especially
the New Hebrides, who would send us further information about any of the
species mentioned in the preceding part of this paper. Letters should be addressed
to Dr. John R. Baker, University Museum, Oxford. Exact dates are greatly
preferable to general statements.
DISCUSSION.
General remarks.-As was stated in the introduction, it appears that botanists
have paid far more attention than zoologists to the causation of seasonal
phenomena, and i t is for this reason that i t is appropriate to make a general
discussion of the subject in the botanical part of this series of papers.
A review of what is known on the subject makes i t probable that many
plants have an inherent tendency t o flower and sometimes to lose their leaves
a t fairly regular, often approximately annual, intervals. Environmental
influences can also affect flowering and the loss of leaves. I n some species the
inherent tendency is of great importance, in others the environment. This is
well shown by contrasting leaf-fall in two species, teak (Tectona grandis) and
Bombax malubaricum. Coster (1923) remarks that in East Java, where there
is great seasonal periodicity in rainfall, teak is much influenced in leaf-fall by
climatic irregularities, while the leaf-fa11 of Bombax is scarcely affected. I n
exact correspondence with this, Holttum (1930) finds that in the much less
seasonal climate of Singapore teak trees are never bare of leaves, while Bombax
retains its long resting period. Thus we have here one species in which the
environment is particularly important, and another in which inherent tendencies
dominate. But, before these two causes of periodic phenomena are discussed,
it will be well carefully to distinguish an allied, but, separat,e, subject-namely,
gregarious flowering.
,
THE SEASONS IN A TROPICAL RAIN-FOREST
515
Gregarious $owering.--It is well known that several species of plants exhibit
this remarkable phenomenon, in which all the individuals of one species over
a wide area tend to flower a t the same time. This applies, for instance, to
species of Strobilanthes, especially S.sexennis Nees, in Ceylon (Petch, 1924).
This flowers a t intervals of about twelve years in any one place and the flowering
areas are very large, for the species forms the predominant part of the undergrowth in the up-country jungles. One series of areas flowered in 1917-18
and another series in 1922-3, and in each of these areas flowering had taken
place a t intervals of twelve years since the middle of the nineteenth century.
When the seeds have ripened the bushes die, and twelve years later the new
bushes, derived from those seeds, are old enough to flower. Other species
of the genus exhibit the same cycle, and a t least one species a shorter one.
Similarly, according to Petch, the Talipot palm (Corypha umbraculifera L.)
has an inherited tendency, not very easily influenced by the environment, to
flower a t a certain age. It is important to stress that this phenomenon is not
the same as periodicity in reproduction within the individual, which is the
main subject of the study described in this series of papers.
The inJluence of the environment on reproductive and leaf-fall seasons.Schimper (1903) reaches the general conclosion that, in those tropical regions
in which there is no actually dry season, the plants tend to flower especially
in the less wet part of the year (though many flower all the year round).
Schimper’s conclusion, which does not tally with our results from Hog Harbour,
appears to be true of Java as a whole, but it is to be regretted that in his analysis
he did not separate the plants of East Java, which has a dry season, from those
of West Java, which has not. In West and South-west Ceylon, where there
is no actually dry season, it is true that relatively few plants flower during t h e
wettest months (May and October-November) ; though it must be admitted
that the figures show no close correlation between scarcity of rainfall and the
number of species flowering. Leaf-fall in Ceylon, however, is directly connected
with climatic changes, for it takes place especially in the hot dry season
(January-March a t Peradeniya).
Wright (1905) concludes that it is probably lack of water to replace that
lost by transpiration that induces leaf-fall in the tropics, and this agrees with
our finding that the most obviously deciduous tree of Hog Harbour, Garuga
Jloribunda, is without leaves during part of the less wet season of the year.
Wright dismisses the small temperature changes in Ceylon as unimportant.
Holttum (1930) states that a t Singapore most trees that change their leaves
once a year do so about February-March, i.e. a t the begming of the less wet
season. Schweizer (1932) says that application of artificial fertilizers to Hevm
brasiliensis may delay leaf-change by four or five weeks, and that trees a t
500-600 metres tend to be later than those at sea-level. That the environment
can directly affect the time of reproduction is also rendered likely by the
widely-accepted view that a shower of rain hastens the flowering of coffee.
Again, every gardener knows that in some species warm spells early in the year
encourage early flowering, as does transplanting in other species.
37*
516
DR. J. R. BAKER AND MISS INA BAKER ON
I n addition to this observational evidence, there is direct experimental
evidence based on laboratory research, such as that provided by Tincker (1924)
on the effect of light. Tincker subjected various plants to daylight for 6, 9,
and 12 hours daily, the controls receiving the full period of normal daylight.
Equal soil and temperature conditions were provided for all, so far as possible.
With the majority of the species investigated, longer exposure to light encouraged earlier flowering (e.g. certain clovers and many grasses). With two
species (a Chrysanthemum and a runner bean) limitation of light produced
earlier flowering. With only one species (a grass, Poa annuu) the length of the
daily exposure to light did not affect flowering.
The effect of internal rhythm.-There is also much evidence of an inherent
tendency in plants to show periodic phenomena. We propose, throughout
this series of papers, to use the term ' internal rhythm ' to indicate a tendency
on the part of any plant or animal to behave rhythmically in relation to
reproduction (and leaf-fall, in the case of plants) independently of the environment. The evidence for the existence of an internal rhythm in plants may now
be briefly reviewed.
Wright states that only three species are evergreen in the wet districts of
Ceylon and deciduous in the dry, and that some species undergo defoliation
a t approximately the same time all over Ceylon, despite the climatic differences.
Again, individual specimens of certain species, growing close together, may
have somewhat different times of leaf-fall. Wright concludes that ' in many
tropical areas the internal factors are responsible for many phases of the
vegetation '. Holttum makes the very important statement that in a number
of species a t Singapore the leaf-change takes place a t periods which are not
annual nor aliquot parts of a year, and that different individuals of these species
may differ in their times. Similarly, Smith (1923) states that an individual of
Breynia cernuu Muell. Arg. a t Buitenzorg has changed its leaves a t intervals
of about 5& months for eleven years. The leaf-fall has thus happened at
various times of the year. Holttum makes the following very illuminating
remark, which bears upon much that we shall have to say when we write of the
breeding seasons of animals a t Hog Harbour :-' I n Singapore the climate is
such that there is never a time when the heat or drought is so intense that a tree
cannot carry leaves ; therefore trees which have a natural tendency to a leaf
period which is not annual have a free scope to display their peculiarities, which
are kept in check in countries which have strongly seasonal climates '.
An important contribution to the literature of seasonal phenomena has been
made by Schweizer (1932), who studied leaf-change in the rubber tree, EZevea
brasiliensis, a t Djember in Java. He came to the general conclusion that an
internal rhythm is the important factor controlling the time of leaf-change,
though certain environmental conditions affect it. In seedlings of this tree,
new leaves are rapidly produced a t certain periods only, and these flush-periods
are separated by different times in different seedlings. If, however, cuttings
are taken from a single tree and planted, they show a remarkable uniformity
THE SEASOBS IN A TROPICAL RAIN-FOREST
517
in their leaf-flush periods, wherever they may be planted. Thus there appears
to be an inherent rhythm of flushing. As the plants grow into trees, the periods
between the flushes grow longer, until they generally become annual. That the
internal rhythm is still retained is shown by observations on cuttings which
were taken from single trees and distributed over Java and Sumatra. It was
noticed that cuttings from certain trees tended to have their leaf-changes early,
those from other trees tended to have theirs late, while those from yet others
had them a t irregular periods ; and the periodicity agreed with that of the
individual trees from which the cuttings were taken (for certain individual
trees always change their leaves early (MayJune) and others late (August)).
It may be mentioned that Wright similarly found that cuttings of Erythrina
indicu sometimes retain the leaf-fall period of the tree from which they were
taken. Schweizer carried out the experiment of stripping rubber trees of their
leaves three or four months before they would normally fall, and noticed that
the new ones which are formed fall nevertheless a t the time which is usual for
the tree concerned, although they are only three or four months instead of nine
months old. This, however, cannot in itself be regarded as strong evidence
of an internal rhythm. Schweizer says that there is evidence that in other
species (Poinciana regia and Lagerstroemia sp.) there are different stocks, each
of which changes its leaves a t its special time.
Karmarkar (1934) has shown that in the apple the amount of water in the
wood and bark of the current season’s shoots rises during the swelling of the
buds and flowering, reaching a maximum in June. The amount of nitrogen
(whether total nitrogen or protein nitrogen is considered) shoots suddenly
up to a peak in June. The normal seasonal cycles in water and nitrogencontent is uninfluenced by manuring in spring with sodium nitrate. Miss
Smythe (1934) shows that in the same species there is a seasonal rhythm in the
amount of sugar, starch, cellulose, and lignin in the current season’s shoots,
and this again is unaffected by a spring dressing of sodium nitrate.
Although such strong evidence has been brought forward for the existence
of internal rhythms, yet we wish to stress that there is no possibility that
annual phenomena can be wholly controlled by them in any species of plant or
animal. It has been suggested, in so many words, that as the pituitary gland
functions as a sort of compensated clock in controlling the oestrous cycles
of Mammalia, so also it might be thought to control breeding seasons without the necessity for the environment to act like the finger of the owner
of the clock in occasionally putting it right. This idea, however, only requires
t o be thought out in detail to be rejected. If in any plant or animal there were
an internal rhythm controlling an annual cycle, and if t’his rhythm were incorrect even to the minute extent of six minutes in the year, then if it were
breeding in the the spring a t the end of the last glacial epoch in the northern
hemisphere it would be breeding a t precisely the wrong season of the year
(autumn) now ! Despite this, nearly all birds in Britain do breed between
late winter and summer, and none has a special breeding season in the autumn
518
DH. J. K. BAKER A N D MISS INA BAKER ON
or early winter. Now it is absolutely inconceivable, from what is known in
general about the time-factor in biological reactions, that any organism could
have an internal rhythm exact to a few minutes in a year, and we are therefore
forced to the conclusion that in temperate countries the environment from
time to time ' puts the clock right ', to use the simile that we have employed
above. I n a relatively unvarying climate, where it is not impossible for
flowering or breeding to occur a t m y season, there may be nothing to put the
clock right, and then nearly annual internal rhythms may be disclosed.
SUMMARY
1. In the relatively unvarying climate of the northern New Hebrides (about
15" S. in the Pacific Ocean), most of t,he plants studied present seasonal
reproductive phenomena.
2 . There is a tendency for plants to flower and fruit from September to
JanuQry or February and again in May and June.
3. The start of the longer flowering season coincides with the rise of temperature after the end of the cooler period of the year. The rainiest months of the
year occur during this long flowering season.
4. Two of the plants studied-namely, the herb Gwphila herbacea and the
shrub Piper methysticum-reproduce throughout, the year, and possibly the same
applies to the tree Ficus copiosa.
5. The natives rely upon the flowering and fruiting of certain wild trees to
determine the time for planting certain crops (yams and sweet potatoes).
6. Of the cultivated perennial plants, the bread-fruit and orange are seasonal,
while the banana and papaw produce fruit throughout the year.
7. Seasonal phenomena in reproduction and leaf-fall depend partly on an
internal rhythm and partly on the environment, and in the interplay of these
two controlling factors the one dominates in some species, the other in others.
REFERENCES.
BAKER,
J. R. 1929. Man and animals in the New Hebrides. Routledge, London.
-& T. H. HARRISSON.1936. The seasons in a tropical rain-forest.-Part 1. Meteorology. Journ. Linn. Soc., Zool., xxxix, pp. 443-463.
UROUN,A. F. 1912. Silviculture in the Tropics. Macmillan, London.
T. F. 1906. Manual of the New Zealand flora. Wellington, N.Z.
CHEESEMAN,
COSTER,CH. 1923. Lauberneuerung und andere periodische Lebensprozesse in dem
trockenen Monsun-Gebiet Ost-Javas. Ann. Jard. Bot. Buitenzorg, xxxiii. p. 117.
It. E. 1930 (szc , should be 1931). On periodic leaf-change and flowering of
HOLTTUM,
trees in Singapore. Gardens Bull. Straits Settlements, v, p. 173.
KARMARKAR,
D. V. 1934. The seasonal cycles of nitrogenous and carbohydrate materials
J . Pomol. xii, p. 177.
in fruit trees.-I.
LAW,I<. J. 1932. Bull. Jard. Bot. Biiitenzoiy, ser. 3, xii, p. 281.
S A K E R , & SAKER
JOURN.
LINN.
SOC., Z O O L . V O L . X X X I X . P L 9.
Figs I & 2 , G A R U G A FLORIBUNDA
Figs 3 8. 4 T Y P I C A L VIEWS OF T H E FOREST UNDERGROWTH
LELY,H. V. 1925. The useful trees of Northern Nigeria. Crown Agents for t,he Colonies,
London.
PETCH,
T. 1924. Gregarious flowering. .41in. Roy. Bot. Gardens, Peratleniya., ix, p. 101.
KUSBY,H. H. 1933. Jungle memories. MoGraw-Hill, London.
SCHIMPER,
A. F. W. 1903. Plant geography upon a physiological basis. Oxford.
SCHWEIZEH,
J. 1932. Ueber die Periodizitiit des Blattwechsels bei tropischen BSiumeii.
M i l t . Naturforsch. Gesell. Berri, p. xi1 (sic). (Vols. not numbered.)
SMITH,J. J. 1923. Perioclischer Laubfall bei Breynici cernua Muell. Arg. Ann. Jard.
Bot. Buitenzorg, xxxii, p. 97.
SMYTHE,
E. S. 1934. The seasonal cycles of nitrogenous and carbohydrate materials
in fruit trees.-2.
J. Pomol. xii, p. 24!).
STEVENSON,
N. I. 1933. A pocket guide t,o sixty distinctive tropical trees. E’ayette,
U.S.A.
SULMAN,
F. 1913. A popular guide to the wild flowers of New South Wales. 2 vols.
London.
Nature ’,
TINCKEH,
M. A. H. 1924. Effect of length of day on flowering and growth.
cxiv, p. 360.
TROUP,R. S. 1921. The silviculture of’ lnclian trees. ( 3 vols.) Oxford.
WRIGHT,
H. 1905. Ann. Roy. Bot. Gurdens, Peraderaiya, ii, p. 415.
EXPLANATlON O F PLATE 9.
Fig. 1. Clawgo jloribzcnda, photographed on ‘3 September 1933.
Fig. 2. The same tree, photographed on 23 September 1933.
Fig. 4 represents part of the
Figs. 3 & 4. Typical views of the forest undergrowth.
forest meteorological station.
K., katathemometer, hanging in the position in which readings were taken daily.
N . , ‘ Nasukonru ’.
Pi., Piper methysticum.
Ps.,Psychotria ap.