J . Linn.Soe. (Bot.),6 1 , 3 8 4 , ~137-146
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Printed i n Great Britain
July, 1968
Some aspects of cycadeoid evolution
BYT. DELEVORYAS
Department of Biology and Peabody Museum of Natural History,
Yale University, New Haven, Connecticut, U.S.A.
First definite remains of the order Cycadeoidales appear at the beginning of the Mesozoic, and
extinction occurred at the end of the Cretaceous. Reasoning on the assumption that the
characters most frequently present in a group of related plants are primitive ones, an attempt
is made to determine what the most primitive cycadeoids looked like and what the probable
ancestors were. Most likely the most primitive cycadeoids had slender, elongated, branched
stems, with numeroua leaves. Pollen was borne in compound sporangia on leaf-likestructures.
The most likely ancestral form, on the basis of known Paleozoic fossil plants, are the pteridosperms. Origin of cycadeoid cones is postulated as having occurred in one of at least three ways.
One suggestion is that cones are portions of leaf systems, and that ‘telescoping’ of the stems‘
resulted in the displacement of the cone in Cycadeoideato the axil of another leaf. Another Buggestion is that cones are fertile axillary branches. A third idea is that cones represent branches
arising from leaves.
CONTENTS
Introduction .
Primitive characteristics of the Cycadeoidales
.
Vegetative aspects
.
Fertile structures
Microsporangia
Ovulatecones
.
Origin of cycadeoid cones
Foliar theory
.
Axillary branch origin of cones
Possible cone origin from branch arising from a leaf
Concluding remarks
Acknowledgement
References
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INTRODUCTION
The term ‘cycadophyts’, as used in this paper, applies to those members of the seed
plants in the orders Cycadales and Cycadeoidales (Bennettitales) (with the Nilssoniales
being regarded as equivalent to the Cycadales).In many systems of classification, the seed
ferns, or pteridosperms,are also included,but there seemsto be sufficient differencebetween
the seed ferns and the two cycadophyte orders to warrant their separation into a separate
class (or more than one class, in view of the pronounced difference between Paleozoic and
Mesozoic seed ferns).
Generally the cycadophytes are reported as having originated in the Triassic Period,
with some evidencethat it may have been earlier, but the reports of Paleozoiccycadophytes
are not always accompanied with convincingcredentials.Sphenozamites is a group of leaves
found first in the Permian (Renault, 1896) and resembling Otozamites,although there is no
evidence that they had cycadophytic relationships.The leaves are said to resemble those of
I
138
T. DELEVORYAS
Zamia Skinneri and 2.muricuta (Seward, 1917 : 587). I n many respects the leaflet looks
considerably like those of Noeggerathia. Plagiozamites is a similar genus, again with no
proof that i t is cycadophytic, that has been reported from the Coal Measures and Permian of
Manchura (Zalessky, 1905), the Upper Carboniferous and Permian of Europe (Zeiller,
1894),as well as in the West Virginia, U.S.A., Coal Measures (Bassler, 1916).Pterophyllumlike leaves, very similar to the typical Mesozoic forms of the genus, occur as far back as the
Upper Carboniferous (Renault & Zeiller, 1886, 1890; Solms-Laubach, 1891: 85). Feistmantel (1886)reported an Anomozamites-likeleaf from the Qlossopterisflora of India.
Dr S. H. Mamay (pers. comm.) has recently demonstrated that Permian plant remains,
previously referred to the genus Tingia, could possibly be parts of fronds that were
cycadophytic. Another piece of evidence that cycadophytes may have existed in the
Permian are remains of seed-bearing structures collected by Dr Mamay from Texas, U.S.A.
These structures bear two rows of seed-like bodies and may possibly represent a leaf with
ovules on a poorly preserved lamina, or a rachis having some resemblance to a Cycas
sporophyll. There is some similarity to Spemzopteris described by Cridland & Morris (1960).
Spermopteris, itself, from the Upper Pennsylvanian of Kansas, U.S.A., could also be
interpreted as a possible cycadophyte.
All of these pieces of evidence, while not demonstrating indisputably the existence of
cycadophytes in the late Paleozoic, cannot be dismissed, and hint strongly a t the origin
of that group of plants before the Mesozoic Era.
With the onset of the Mesozoic, the cycadophytes began to flourish, and the appellation
‘Age of Cycads’ to that portion of geologic time is certainly an appropriate one. I n the
Triassic Period, remains of both orders, Cycadales and Cycadeoidales, are present in
abundance. Furthermore, there is evidence of reproductive structures, so that we can feel
more certain of the identification of these two orders during the early Mesozoic.
I n 1953, Professor C. A. Arnold published a concise review of the Cycadales ;this paper
will be devoted principally to the cycadeoids, a group that has long attracted interest of a
great number of botanists and that still presents many unsolved problems.
The Cycadeoidales(assumingPaleozoic cycad-like remains are still too problematical to
be called cycadeoids) are exclusively Mesozoic, recognized with certainty fist in the
Triassic, and leaving no traces beyond the end of the Cretaceous Period. Ettingshausen
(1887) reported the existence of remains of Anmommites in the Tertiary of New South
Wales, but Kriiusel(l928) was inclined to believe the leaves were those of a fern, or perhaps
even of dicotyledonous angiosperms. Two families of Cycadeoidales are generally recogllized (although most likely they are not necessarily natural ones) based primarily on
the general habit of the plants. One group, the Williamsoniaceae, are depicted as having
had elongated, often branched, stems, as opposed to the short, squat stems of the’ second
group, the Cycadeoidaceae, which are less branched. There are exceptional forms in
both of these families, however, with a number of williamsoniaceans unbranched and some
cycadeoidaceansbranched conspicuously.
PRIMITIVE CHARACTERISTICS OF THE CYCADEOIDALES
I n a paper representing a contribution to a symposium on the origin and evolution of
ferns, Wagner (1964) suggests that within a group of related plants, the characters that
most members have in common are most likely the primitive features of the group, and that
those characters that are not found commonly within the group are most likely specializations. If one is able to discern the so-called primitive characters occurring frequently
within a group, he may use them tqsuggest the features most likely present in the ancestor.
I n this way, it may be possible to gain some insight about the origin of a group of plants.
This approach is utilized in this paper with regard to the cycadeoids in an attempt to
learn about what group of plants was involved in the origin of cycadeoids, and what
directions of specialization occurred after the appearance of that group.
Some aspects of cycadeoid evolution
139
Vegetative Aspects
Obviously, the leaf type most frequently found among the cycadeoids is a pinnately
compound one. Several types of cycadeoid leaves may be entire (e.g. Nilssoniopteris), but
these are really in the minority. An ancestor with pinnately compound, frond like leaves,
then, is a good possibility.
I n general habit, cycadeoideas are typically regarded as having squat, fleshy stems, with
closely spaced leaves. This type of habit is certainly that most consistently associated with
Cycadeoidea and of certain Williamsonia-likeplants, but I prefer to think that this type of
habit was not really the typical one. I believe that more likely, most cycadeoids of the
Mesozoic, especially in the Triassic and Jurassic, had slender stems that branched
profusely. Williamsoniellaand Wielandiellaare but two examples, but there are a number
of pieces of evidence that point to the conclusion that many more plants had this habit.
There are a number of reports of williamsonian stems from India (e.g. Sahni & Rao, 1933;
Bose, 1953) that are rather slender. Williamsonian stems found in the Jurassic of Oaxaca,
Mexico, by Wieland (1914) are also quite slender and branched. My observations of
cycadophytic foliage in these Mexican deposits would suggest that the leaves came from
slender, much-branched plants. One would not expect to find leaves of the genus Cycadeoidea preserved in great abundance under a given stem, or that the leaves would be
nicely laid out, covered a few a t a time with sediment before there was extensive curling or
drying out. Cycadophytic foliage in Oaxaca, rather, occurs in densely packed layers of
mostly small, generally undistorted leaves that are found in almost as great a congestion as
are Tertiary leaves from deciduous sources. These aggregations of leaves have exactly the
appearance one would expect to see under much branched plants with deciduous leaves.
It is interesting to note that Harris (1961), in his reconstruction of Beania, chose t o
represent it with slender, elongated, and branched stems. True, Beania is a member of the
Cycadales, but Harris came to the same conclusion concerning its reconstruction as I have
with many cycadeoids, perhaps on the same kind of indirect evidence. I would envision the
primitive cycadeoid as a slender plant, branched, with many leaves borne on these branches.
Leaves may or may not have been seasonally deciduous on this plant.
Pertile structures
Microsporangia
Microsporangiate structures among the cycadeoids have many elements in common.
First, when the microsporangia are known, it can be demonstrated that they are compound structures-a thick walled body with a number of tubular sporangia within.
Examples of such microsporangiate structures occur in Cycadeoidea, a t least some Williamsonias, Williarnsoniella, Weltrichia, Cycadocephalus, etc. There is further similarity in the
arrangement of these capsules on sporophyll-like structures. Generally these sporophylls
are borne in a whorl, and two rows of appendages, attached to the rachis of the sporophyll,
are directed toward the center of the microsporangiate complex. On these appendages are
attached pollen-bearing capsules ; in some instances the appendages themselves are the
capsules and are considerably reduced microsporophylls, with the capsules borne on
extremely short appendages. I n Cycadeoidea, as I have recently reconstructed it (Delevoryas, 1963, 1965, 1968)) the appendages are not free structures but are more in the
nature of trabeculae attached to the outside and to the inside of a structure that might be
considered equivalent to a sporophyll.
The actual pollen-containing structures which are generally composed of more than one
chamber, might be equated to microsporangia of certain pteridosperms, namely those of
the Medullosaceae. I n that family there are a number of microsporangiate organ genera
that are composed of a system of tubular sporangia aggregated into a compound structure
140
T. DELEVORYAS
(e.g. Aulacotheca, Whittleseya, etc.). Some authors (e.g. Schopf, 1949)would consider such a
structure as a sporangium with septations. I would prefer to view it as a sporangium
system derived from a phyletic fusion of sporangia-in other words, a synangium.
If the ao-called sporophylls of a cycadeoid cone are indeed fronds (or, as indicated as a
possibility below, portions of fronds), and if the pollen-bearing structures are synangia, we
would expect to find in the precursor to this condition a frond system that bore on it many
synangia. Again, fronds of medullosan pteridosperms are examples of such a system.
Foliage generally agreed to belong to medullosans has been found bearing such synangia as
Whittleseya (Stewart & Delevoryas, 1966).Studies of presumed pteridosperm foliage in the
late Paleozoic seem to indicate that the fronds were huge, and a great number of fruiting
structures could have been borne on such a structure.
I n certain williamsonias, the pinnate organization of the frond system is retained, and
each aporophyll has a main r a c k with pinnately arranged laterals on which were borne the
synangia. These are probably the most primitive among the cycadeoid microsporangiate
structures. At the other end of the spectrum are microsporophylls that are extremely
reduced; these are found in Williamsoniellu, for example, where each sporophyll is a small,
fleshy structure, with embedded sporangia, and no evidence of any kind of pinnate structure. Cycadeoidea has been modified in another direction; here the pinnately arranged
laterals have probably fused to the distal part of the rachis that has been recurved. Such a
fusion could be considered a phyletic one if it were possible to understand how such a
structure could be formed ontogenetically. It would be impossible, however, to derive such
a ‘tangerine wedge ’ structure with fused laterals (Delevoryas, 1968) ontogenetically.
Although there is no way t o prove the supposition, I think it is likely that the lateral
structures fused during ontogeny, with no evidence of the point of fusion visible a t maturity.
This suggestion can be borne out only by early ontogenetic stages of cycadeoid cones, of
which none are known a t present.
I n some cycadeoid pollen cones, the sporophylls have become adnate to form a cup-like
structure. Williamsonia spectabilis and W . whitbiensis have a baaal cup with free distal
finger-likeprocesses. I n Cycadeoidea, there is considerable adnation a t the basal regions of
the sporophylls, but the distal parts are free. Williamsoniumexicana, described by Wieland
(1914)from Jurassic deposits in Mexico, is represented as showing an extreme condition of
fusion of ‘sporophylls’, and is generally depicted as an urn-like structure with slender
fingers bearing sporangia arising from the rim of the urn. I have examined the type specimen a t the Instituto de Geologia in Mexico City, and I am not convinced that the fossil is a
williamsonian a t all. I n fact, I doubt that it is even a sporangium bearing structure.
Perhaps the specimen has become somewhat worn during the last few decades and the
surface features have rubbed off. On the other hand, the sporangia have never been
satisfactorily figured; the only photograph by Wieland that is supposed to show microsporangia is in his 1914 paper (Pl. 29, fig. 2), but in that illustration, the supposed sporangia
are outlined in ink. Williamsoniu santalensis has some similarities with the Cycadeoidea
sporophyll assemblage in that there is a fleshy distal region on each sporophyll. These
sporophylls are fused into a collar basally, but fusion is not as great as in Cycadeoidea.
Williamsoniasantalensis sporangia resemble those of Cycadocephalus sewardi, where there
are elongated, finger-likesynangia borne in two rows and directed inward.
One final feature of cycadeoid microsporangiate structures is the nature of the pollen
grain. Monosulcate grains are the characteristic type in the cycadeoids, and, presumably,
would be expected t o have been borne by precursors ofthe cycadeoids.
Thus it is obvious that the various cycadeoidalean microsporangiate fructifications can
be compared favorably with each other, and this close similarlity would seem t o suggest a
common origin for the group. Of the known groups of fossil vascular plants, only the late
Paleozoic pteridosperms have structural features which could have become modified into
the microsporangial apparatus in the Cycadeoidales. Even the pollen is of the appropriate
type.
Some aspects of cycudeoid evolution
141
Ovulate cones
Notably uniform throughout the order is the ovulate receptable. I n all known members
of the Cycadeoidales that have the ovulate structures preserved, ovules are borne on a
fleshy receptacle. It may be elongated and conical, or short and almost globose, but the
essential features are common throughout. Interspersed among the ovules are structures
called interseminal scales’ that serve to pack the ovules into a tight, compact mass on the
surface of the receptacle. I n some forms, the mature seeds may be elevated on stalks ;other
types appear to have remained sessile. I n spite of these minor variances, there is no difficulty in recognizing an ovulate cone of the Cycadeoidales. This uniform cone type occurs in
such diverse assemblages of cycadeoidalean remains as Triassic forms, Yorkshire Jurassic
ovulate receptacles of the Williamsonia type, ovulate cones from the Jurassic of Mexico,
cones from the Rajmahal Hills in India, and cones of the Jurassic and Cretaceous C y m deoidea. Certainly more than just the ovulate receptacle is necessary to establish close
relationships among these cycadophytes, but one cannot help but be impressed with this
uniformity and its pointing toward a natural grouping.
In spite of the noteworthy uniformity among ovulate receptacles, these structures are
the most difficult to derive from some ancestral condition. The seeds are much like those of
Paleozoic pteridosperms, but certainly no more so than are those of the Cycadales. Because
the cycadeoid type of ovulate receptacle was well established in Triassic times, it is natural
to look to the late Paleozoic for possible ancestors. Of the known seed plants a t that time,
none provides a completely satisfactory ancestral condition. Because the seed ferns show so
many features that most likely are ancestral to corresponding cycadeoid features, it is the
logical place to look for a condition that could have evolved into a cycadeoid ovulate
cone.
Reconstructions of certain Paleozoic pteridosperm fronds often show numerous ovules
closely arranged on the leaf system; an aggregation of such a seed-bearing system could
conceivably have led to the ovulate receptacle of the cycadeoids. If the ovulate receptacle
is to be considered the equivalent of part of a foliar system, it should be possible to equate
parts of it, both ovules and interseminal scales, to parts of a pteridosperm frond. Certainly
the ovules present no problems, because they are strictly homologous with seed-fern
ovules. Interseminal scales, however, have no obvious counterpart in seed-fern fronds, but
they may possibly represent sterile parts of the compound leaf. On the other hand, it is
quite possible that interseminal scales are equivalent to the ovules. The reasoning behind
this involves the number and arrangement of ovules on a receptacle. I n Cycadeoidea,
ovules generally are very numerous, with a micropyle evident a t just about all the angles of
continuous interseminal scales. The exception, however, is near the base, where in most
cases only interseminal scales are present. If there were some kind of precise relationship
between scales and ovules with respect to number and position, it might be reasonable to
conclude that the parts are different. I n certain Williamsonia cones that I recently collected
from the Jurassic of Oaxaca, Mexico, the number of ovules on an ovulate receptacle is small,
with micropyles evident a t the corners of only a relatively few interseminal scales, again
suggesting no precise relationship.
There is another aspect of ovule and scale that could also suggest homology, perhaps not
by itself, but considered together with all other features. This involves development of the
two structures. I n Cycadeoideu both ovules and scales in young cones are small and just
about the same length. At maturity, presumably after fertilization of ovules, the ovules
were elevated on elongated stalks ; interseminal scales, too, elongated and ultimately
ended up about the same length as ovule and stalk.
Thus, the feature that is perhaps the most consistent among cycadeoids, and the one that
would be expected to provide us with the most valuable clue concerning the ancestors of
cycadeoids, the ovulate cone, remains a puzzle. If the uniformity of that structure is
indeed an indication of its primitiveness, then the type of ovulate structure on the ancestral
142
T. DELEVORYAS
plants is still completely unknown. The ovules themselves, however, are of a typical seedfern or cycadalean type, and are not too highly specialized.
But even with difficulties encountered in searching for the cycadeoid ancestor, all of the
supposedly primitive characters found among them (ovulate receptacles excepted) point
to a pteridospermous ancestry. At least, of the known pre-Mesozoic plants, the pteridosperms have the greatest number ofthe supposedlyancestralcharacters. No one type can be
singled out as the ancestor, but collectivelythey possess the desirable characteristics for an
ancestral condition.
ORIGIN OF CYCADEOID CONES
In 1959and 1960I published two papers that included a discussion of the vascularization
of cycadeoid axes, especiallyin the corticalregion. One specimen,of Cydeoidea utopiensia
(Delevoryas, 1960),was sectioned in a series of tangential cuts through the cortex and from
these sectionsit was possible to determine the origin of the stele of a cone. In this particular
specimen,two leaf traces that were C-shaped in transverse section proximally are cylindrical a little farther out in the cortex. Still farther out these traces are open, with openings
facing each other, and the two traces become fused along the upper edges. AC-shaped trace
separates off, leaving a massive vascular strand that includes a leaf trace and the vascular
supply for a cone subtended by the leaf supplied by that trace. (Note diagrams in Figs 6 to
11in Delevoryas,1960.)I n other words, the cone vascular supply is derived ontogenetically
from parts of two leaf traces, one of which supplies the leaf subtending the cone.
In another cycadeoid stem, one of C. wyomingensis, four leaf traces were involved in a
fusion and in the formation of a cone trace. One of these leaf traces provided the vascularization for the leaf subtending the cone (Delevoryas,1960).
In Monanthesia (Delevoryas, 1969)branches of two leaf traces may fuse and give rise to
the stele of a cone; the cone is situated in the axil of still another leaf, the trace of which
contributednothing to the cone stele.
Andrews (1943) described cone stele formation in a cycadeoid in which the cone stele
appears to have been derived from parts of two traces. The cone is axillary to anot,herleaf,
whose trace was not involved in formation of the cone vascular cylinder.
These examples of the unusual origin of cone steles in ontogeny of the stem are peculiar
enough to invite speculation as to whether any phylogenetic conclusions may be drawn
from these vascular configurations. I have previously made the suggestion (Delevoryas,
1959) that one possible conclusion that could be made is that the cone is part of a leaf
system, and not actually an axillary branch. I fully realize that in many instances it is risky
to make too many phylogenetic conclusionsbased on ontogeny alone. On the other hand,
in the specimen of C.utopiensi8 mentioned above, there are vegetative branches in addition
to the fertile axes. If cones are homologouswith branches, it would not be illogical to expect
that these two structures would have the same kind of ontogeny, at least with respect to the
origin of the vascular system. The vascular supply of the vegetative branch, however, does
not originate from the leaf traces in the cortex, but from the inner part of the stele, just as
stelesof branches of most extant plants do.
Three possible ways in which cones could have originated in the cycadeoids are outlined
bel.ow, with the arguments in favor and against each suggestion.
Foliar theory
The origin of cones steles in the cortex from leaf traces rather than from the stem stele
could be interpreted to reflect the origin of cones from leaves, and each cone may then be
considered to be a part of a leaf. To determine the kind of system that might have been the
source of such a cortical vascular pattern, it is necessary to begin with a plant with an
elongated stem system and compound fronds ; certain Paleozoic pteridosperms provide
such structural prototypes. This seed fern had fronds, part of which may have been
Some aspects of cycadeoid evolution
143
fertile and part sterile, that produced some of the lateral members close to the level of
separation of the frond from the stem (Fig. 1).As the stem became ‘telescoped’ and more
fleshy, phylogenetically, the bases of these fronds could have become ‘enveloped ),so that
the branching of the traces to basal pinnae of the fronds actually occurred within the cortex
of the stem (Fig. 2). Further addition of fleshy cortex could even have resulted in ultimate
fusion (phyletically) of adjacent basal pinnae so that a t the surface of the stem, what had
been more than one fertile axis later was in the form ofa more massive, compound structure.
If leaves became more crowded, as a result of phyletic ‘telescoping ’, the fertile appendages,
now the cones, would not necessarily be in the position directly above the sterile part of the
frond from which it originally arose. Furthermore, if two or more leaves were the source of
2
Fig. 1. Portion of an idealized Paleozoic pteridosperm with the base of a leaf bearing fertile
structures.
Fig. 2. Possiblehypothetical intermediatestage betweenpteridospermsand cycadeoids with the
bmal pinna of a frond partially ‘embedded’ within the fleshy stem.
the cone, the final position of the cone may be in the axil of one of these leaves, or of none
(Fig. 3).
There are two points that might be regarded to favor this foliar interpretation ofthe cone
of Cycadeoidea. One of these is the vascularization. As mentioned above, cone steles have a n
origin different from those of vegetative branches, and if cones and branches were homologous, one would expect the source ofvascularization to be the same. The other point worth
considering is that if the pteridosperms were the ancestral forms of cycadeoids, there is not
among them an aggregation of fertile structures into any kind of a cone. Fructifications are
borne scattered over relatively unmodified foliage, or there may be instances where all of
the frond, or a t least a major part of it, is altered into a fertile region. Furthermore, axillary
branching is rather rare among the pteridosperms, and a t present we know of no case where
among that group there are forms with axillary fertile structures. In the Cycadales there is
no difficulty in regarding the cones as assemblages of fertile fronds, but such is not
apparent among the cycadeoids, especially in the ovulate region.
Axillary branch origin of cones
A second manner in which the axillary cone of Cycadeoidea may have originated that
must be considered is to assume that the cone is the morphological equivalent of a fertile
T. DELEVOBYAS
144
branch in a leaf axil (Fig. 4). In spite of the objections listed above to this interpretation,
there are other aspects that might favor the suggestion. If the cone is indeed an axillary
fertile shoot, derived from a congestion of pteridosperm sporophylls, it has somehow
become misplaced because, as was mentioned above, the cone is often not in the axil of the
leaf whose trace was the source of the vascuhrization of the cone. There are many examples
in the plant kingdom where the axillary branch is no longer, strictly speaking, in the leaf
axil. Many times it is some distance above. Nevertheless there is an obvious relationship
between branch and leaf as reflected by the vascular system. If a plant stem were to have
become 'telescoped ' phylogenetically, leaves would be considerably more crowded, and a
branch could conceivably end up in the axil of a leaf that has no relationship with the
branch. This could explain why cones need not be in axil8 of leaves with whose vascular
traces there is some connection.
I'
f
/?
y'
4
3
4
Fig. 3. Diagrammatic longitudinal section of a portion of a cycadeoid stem (e.g. Monanthsia)
showing source of vaacularization of a cone.
Fig. 4. Diagram showing a possible axillary branch origin of the cone of Cycadeoidales.
It is still necessary to explain the origin of cone traces from leaf traces, however. This is
an instance where it must be decided whether or not vascularization is always a reflection
of phylogenetic development ;in other words, does a vascular strand develop where it does
because of historical inhence, or does it develop because of immediate physiological
stimuli? Phylogeneticistsare able in some instances to follow changes through time and to
observe the resulting effects on vascularization. But there are other times that the position
of a vascular strand may have little or no reflection of phylogenetic in0uences. In a luffa
squash, for example, there are hundreds of anastomosing vascular strands that in no way
could be interpreted as reflecting branching in an ancestral condition. Vascular bundles in
that case are where they are because of the immediate environmental conditions within the
plant. Similarly, the traces that are attached to leaf traces and that become involved in the
cone stele of Cycadeoidea may simply reflect the pattern of development of procambial
strands in the growingtip ofa cycadeoidas influencedby spatial or inhibitory requirements
and internal chemical stimuli. Thus, on the basis of vascularization alone, it may be risky
to attempt to read too much in the phyletic history of cycadeoid cones. The fact remains,
however, that vegetative branches have a different origin from cones; this may be
significant in reflecting their non-homology, or, it may simply be a manifestation of an
Some aspects of cycadeoid evolution
146
often observed phenomenon where vascularization of fertile appendages often differs from
that of vegetative axes.
Possible cone origin from branch arising from a leaf
A third possible way in which a cycadeoid cone may have originated is as a branch
arising from a leaf. Although not a n especially common phenomenon, attachment of stems
to leaves is not really rare, especially in ferns. In certain coenopterid ferns, Anacbropteris
(Delevoryas &Morgan, 1954)and Botryopteris (Long, 1943))for example, leaves are known
to have borne on them small branches which duplicate the form in miniature of the parent
plant. If a cycadeoid cone originated as such a branch, it would be easy to understand the
connection of cone vascular bundle with that of a leaf. ‘Telescoping’ of the stem, together
with addition of fleshy tissue, would account for a separation of leaf and its daughter
fertile branch a t the surface of the stem. Again, as in the theory that the cone may be
part of a leaf system, it may be necessary to consider a cone as having been derived from a
phyletic fusion of more than one fertile axis.
Points in favor of this interpretation are essentially those that are used to support a
partial frond origin of the cone. Objections would be that among the pteridosperms, there
are no known instances of leaves bearing fertile branches on them.
No matter what the origin of the cycadeoidalean cone, it is generally true that some
relationships must exist between cone and leaf. So far as I am aware, in all cycadeoideas and
related forms, cones are always axillary. The reconstruction of Williamsoniella as presented
by Zimmermann (1933)also shows cones in leaf axils. Unfortunately precise cone insertion
among most other cycadeoidaleans is not really known.
CONCLUDING REMARKS
After the appearance of the cycadeoids in the Mesozoic (or in the late Paleozoic) a number
of trends occurred. As stated above, earliest cycadeoids were probably slender, branching
forms, and this habit persisted well into the Mesozoic. I n many forms there was a tendency
toward ‘telescoping’, with stems becoming more fleshy and with leaves borne in a more
crowded arrangement. The relatively complex anatomy of the cortex of late Mesozoic
cycadeoidsis also possibly a reflection of the ‘telescoping ’ of the stem.
Sporophylls, or parts of leaves involved in production of microsporangia, were most
likely frond-like in earliest fOiTT.M,and became progressively more fleshy, with a tendency
toward adnation.
Whether dioecy, as I suggested existed among the cycadeoids (Delevoryas, 1968), was a
primitive feature or not, is impossible to tell. Coexisting with dioecious species of Cycudeoidea are monoecious forms with cones bearing microsporangia and ovules. Quite likely
the monocarpic habit found among some cycadeoids (e.g. Monanthesia, Delevoryas, 1959)
is also a derived condition.
All evidence seems to point toward the extinction of the Cycadeoidales a t the end of the
Cretaceous; no Tertiary cycadeoid remains have been found. It is natural to speculate
about possible descendants of the cycadeoids, and to make a few comments about earlier
attempts to derive angiosperms from them. So far as we can tell, there are no known
descendants of cycadeoids. The order Cycadales is and has been quite distinct. Furthermore, unless all modern ideas concerning angiosperm morphology are in error, it is simply
impossible to derive structural patterns in angiosperm flowers from the reproductive
systems of cycadeoids.
Thus the order Cycadeoidalesrepresents another of the various enigmas in paleobotanical
studies. The problems involved, however, are probably not insoluble. I believe the most
promising avenues of research would involve a thorough study of possible early Triassic
cycadeoid-like plants, as well as a concerted effort to find in the Permian Period some
10
146
T. DELEVORYAS
plant types that would fit the pattern suggested for a cycadeoid ancestor. I n addition,
ontogenetic studies, especially of petrified material, will provide more answers, especially
about the precise nature of the cone and its origin.
ACKNOWLEDGEMENT
This work was supported by National Science Foundation Grant NSF GB-5911X.
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