PAST HISTORIES OF PLANT FAMILIES
IV. The Cycads
The group of the _Cycadales_, which has a systematic value equivalent to
the _Ginkgoales_, contains a much larger variety of genera and species
than does the latter. There are still living nine genera, with more than
a hundred and fifty species, which form (though a small one compared with
most of the prime groups) a well-defined family. They are the most
primitive Gymnosperms, the most primitive seed-bearing plants now living,
and in their appearance and characters are very different from any other
modern type. Their external resemblance to the group of the
Bennettitales, however, is very striking, and indeed, without the
fructifications it would be impossible to distinguish them.
The best known of the genera is that of _Cycas_, of which an illustration
is given in fig. 74. The thick, stumpy stem and crown of “palm”-like
leaves give it a very different appearance from any other Gymnosperm.
Commonly the plants reach only a few feet in height, but very old
specimens may grow to the height of 30 ft. or more. The other genera are
smaller, and some have short stems and a very fern-like appearance, as,
for example, the genus _Stangeria_, which was supposed to be a fern when
it was first discovered and before fruiting specimens had been seen.
The large compound leaves are all borne directly on the main stem,
generally in a single rosette at its apex, and as they die off they leave
their fleshy leaf bases, which cover the stem and remain for an almost
indefinite number of years.
The wood of the main trunks differs from that of the other Gymnosperms in
being very loosely built, with a large pith and much soft tissue between
the radiating bands of wood. There is a cambium which adds zones of
secondary tissue, but it does not do its work regularly, and the cross
section of an old Cycad stem shows disconnected rings of wood,
accompanied by much soft tissue. The cells of the wood have bordered pits
on their walls, and in the main axis the wood is usually all developed in
a centrifugal direction, but in the axis of the cones some centripetal
wood is found (refer to _c_, fig. 65, p. 97).
[Illustration: Fig. 74.—Plant of _Cycas_, showing the main stem with
the crown of leaves and the irregular branches which come on an old
plant]
In their fructifications the Cycads stand even further apart from the
rest of the Gymnosperms. One striking point is the enormous size of their
_male_ cones. The male cones consist of a stout axis, round which are
spiral series of closely packed simple scales covered with pollen-bearing
sacs (which bear no inconsiderable likeness to fern sporangia), the whole
cone reaching 1½ ft. in length in some genera, and weighing several
pounds. All the other Gymnosperms, except the Araucareæ, where they are
an inch or two long, have male cones but a fraction of an inch in length.
[Illustration: Fig. 75.—Seed-bearing Scale of _Cycas_, showing its
lobed and leaflike character
_s_, Seeds attached on either side below the divisions of the
sporophyll.]
In all the members of the family, excepting _Cycas_ itself, the female
fructifications also consist of similarly organized cones bearing a
couple of seeds on each scale instead of the numerous pollen sacs. In
_Cycas_ the male cones are like those of the other genera, and reach an
enormous size; but there are no female cones, for the seeds are borne on
special leaflike scales. These are illustrated in fig. 75, which shows
also that there are not two seeds (as in the other genera with cones) to
each scale, but an indefinite number.
The leafy nature of the seed-bearing scale is an important and
interesting feature. Although theoretically botanists are accustomed to
accept the view that seeds are always borne on specially modified leaves
(so that to a botanist even the “shell” of a pea-pod and the box of a
poppy capsule are leaves), yet in _Cycas_ alone among living plants are
seeds really found growing on a large structure which has the appearance
of a leaf. Hence, from this point of view (see p. 45, however, for a
caution against concluding that the whole plant is similarly lowly
organized), _Cycas_ is the most primitive of all the living plants that
bear seeds, and hence presumably the likest to the fossil ancestors of
the seed-bearing types. In this character it is more primitive than the
fossil group of the _Cordaiteæ_, and comes very close to an intermediate
group of fossils to be considered in the next chapter.
[Illustration: Fig. 76.—Seed of _Cycas_ cut open
_n_, The nucellus, fused at the level _l_ to the coat _c_; _sc_, stony
layer of coat; _p.c_, pollen chamber in apex of the nucellus; S,
“spore”, filled with endosperm, in which lies the embryo _e_.]
To enter into the detailed anatomy of the seeds would lead us too far
into the realms of the specialist, but we must notice one or two points
about them. Firstly, their very large size, for ripe seeds of _Cycas_ are
as large as peaches (and peaches, it is to be noted, are fruits, not
seeds), and particularly the large size they attain _before_ they are
fertilized and have an embryo. Among the higher plants the young seeds
remain very minute until an embryo is secured by the act of
fertilization, but in the Cycads the seeds enlarge and lay in a big store
of starch in the endosperm before the embryo appears, so that in the
cases in which fertilization is prevented large, sterile “seeds” are
nevertheless produced. This must be looked on as a want of precision in
the mechanism, and as a wasteful arrangement which is undeniably
primitive. An even more wasteful arrangement appears to have been common
to the “seeds” of the Palæozoic period, for, though many fossil “seeds”
are known in detail from the old rocks, not one is known to have any
trace of an embryo. A general plan of the _Cycas_ seed is shown in fig.
76, which should be compared with that of _Ginkgo_ (fig. 68). The large
size of the endosperm and the thick and complex seed-coats are
characteristic features of both these structures. Another point that
makes the Cycad seeds of special interest is the fact that the male cells
(as in _Ginkgo_) are developed as active, free-swimming sperms, which
swim towards the female cell in the space provided for them in the seed
(see _p.c_, fig. 76).
The characters of the Cycads as they are now living prove them to be an
extremely primitive group, and therefore presumably well represented
among the fossils; and indeed among the Mesozoic rocks there is no lack
of impressions which have been described as the leaves of Cycads. There
is, however, very little reliable material, and practically none which
shows good microscopic structure. Leaf impressions alone are most
unsafe—more unsafe in this group, perhaps, than in any other—for reasons
that will be apparent later on, and the conclusions that used to be drawn
about the vast number of Cycads which inhabited the globe in the early
Mesozoic must be looked on with caution, resulting from the experience of
recent discoveries proving many of these leaves to belong to a different
family.
There remain, however, many authentic specimens which show that _Cycas_
certainly goes back very far in history, and specimens of this genus are
known from the older Mesozoic rocks. We cannot say, however, as securely
as used to be said, that the Mesozoic was the “Age of Cycads”, although
it was doubtless the age of plants which had much of the external
appearance of Cycads.
From the Palæozoic we have no reliable evidence of the existence of
Cycads, though the plants of that time included a group which has an
undoubted connection with them.
Indeed, so far as fossil evidence goes, we must suppose that the Cycads,
since their appearance, possibly at the close of the Palæozoic, have
never been a dominant or very extensive family, though they grew in the
past all over the world, and in Europe seem to have remained till the
middle of the Tertiary epoch.