_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. [Illustration: 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.