CHAPTER XI
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]).
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.
Fig. 75.—Seed-bearing Scale of Cycas, showing its lobed and leaflike character