Poroxylon is not common, and until recently had not been found in the Lower Coal Measures of England. The plants appear to have been much smaller than Cordaites, with delicate stems which bore relatively large simple leaves. The anatomy of the root was that common in Gymnosperms, but the stem had a very large pith, and the leaves were much like those of Cordaites in having parallel veins. An important character in the anatomy of the stem was the presence of what is known as centripetal wood. This must be shortly explained. In all the stems hitherto considered, the first-formed wood cells (protoxylems, see [p. 57]) developed at the central point of the wood, towards the pith (see [fig. 19], px, [p. 49]). This is characteristic of all Angiosperms and the higher Gymnosperms (except in a couple of recently investigated Pines), but among the lower plants we find that part of the later wood develops to the inner side of these protoxylem masses. The distinction is shown in [fig. 65].
Fig. 65.—A, Normal bundle of higher plant; x, protoxylem on inner side next the pith p, and the older wood w outside it, centrifugal wood. B, Bundle with wood cells c developed on inner side of protoxylem, centripetal wood; the arrow indicates the direction of the centre of the stem.
This point is one to which botanists have given much attention, and on which they have laid much weight in considering the affinities of the lower Gymnosperms and the intermediate groups between them and the ferns, which are found among the fossils. In Cordaites this point of connection with the lower types is not seen, but in Poroxylon, which has otherwise a stem anatomy very similar to Cordaites, we find groups of centripetal wood developed inside the protoxylem of primary bundles. For this reason, principally, is Poroxylon of interest at present, as in its stem anatomy it seems to connect the Cordaites type with that of the group below it in general organization.
Ginkgoales.—Reference to [p. 44] shows that Ginkgo, the Maidenhair tree, belongs to the Ginkgoales, a group taking equal rank with the large and complex series of the Coniferales. The Ginkgoales of the present day, however, have but one living representative. Ginkgo stands alone, the single living species of its genus, representing a family so different from any other living family that it forms a prime group by itself.
Had the tree not been held sacred in China and Japan, it is probable that it would long since have been extinct, for it is now known only in cultivation. It is indeed a relic from the past which has been fortunately preserved alive for our examination. It belongs to the fossil world, as a belated November rose belongs to the summer.
Because of its beauty and interest the plant is now widely distributed under cultivation, and is available for study almost as freely as the other types of living Gymnosperms already mentioned, so that but a short summary of its more important features is needed here.
Old plants, such as can be seen growing freely in Japan (in Kew Gardens there is also a fine specimen), are very tall handsome woody trees, with noble shafts and many branches. The leaves grow on little side shoots and are the most characteristic external feature of the tree; their living form is illustrated in [fig. 66], which shows the typical simple shape as well as the lobed form of the leaf which are to be found, with all intermediate stages, on the same tree. No other plant (save a few ferns, which can generally be distinguished from it without difficulty) has leaves at all like these, so that it is particularly easy to identify the fossil remains, of which there are many.
Fig. 66.—A, Tuft of Ginkgo Leaves, showing their “maidenhair”-like shape. B, Single deeply-divided Leaf to be found on the same tree, usually on young branches.