The wood is compact and fine grained, the rings of secondary tissue being developed from a normal cambium as in the case of the higher Gymnosperms, and the individual tracheæ have round bordered pits. There are small male cones, but the seeds are not borne in cones. They develop on special stalks on which are no scales, but a small mass of tissue at the base of the seed called the “collar”. Usually there are two young ovules, of which often only one ripens to a fleshy seed, though both may mature.
Fig. 67.—Ripe Stage of Ginkgo Seeds attached to their Stalk. c, “Collar” of seed.
The ripe seed reaches the size shown in the diagram, and is orange coloured and very fleshy; within it is a stone encasing the endosperm, which is large, green, and starchy, and contains the embryo with two cotyledons. This embryo is small compared with the endosperm, cf. [fig. 57], [p. 76], which is somewhat similar to that of Ginkgo in this stage.
Of the microscopic characters of the reproductive organs the most remarkable is the male cell. This is not a passive nucleus, as in the plants hitherto considered, but is an actively swimming cell of some size, provided with a spiral of cilia (hairlike structures) whose movements propel it through the water. In the cavity of the unripe seed these swim towards the female cell, and actively penetrate it. The arrangements of the seed are diagrammatically shown in [fig. 68], which should be compared with that of Cycas, [fig. 76], with which it has many points in common.
Fig. 68.—Section through Seed of Ginkgo
p.c, Pollen chamber in the nucellus n, which is fused to the coat c to the level l; sc, stony layer in coat; S, the big spore, filled with endosperm tissue (in this case green in colour); e, egg cells, one of which will produce the embryo after fertilization.
The nature of the male cell in Cordaites is not yet known, but there is reason to suspect it may have been actively swimming also. As this is uncertain, however, we may consider Ginkgo the most highly organized plant which has such a primitive feature, a feature which is a bond of union between it and the ferns, and which, when it was discovered about a dozen years ago, caused a considerable sensation in the botanical world.
To turn now to the fossil records of this family. Leaf impressions of Ginkgo are found in rocks of nearly all ages back even to the Upper Palæozoic. They show a considerable variety of form, and it is certain that they do not all belong to the same species as the living plant, but probably they are closely allied. [Fig. 69] shows a typical impression from the Lower Mesozoic rocks. In this specimen, the cells of the epidermis were fortunately sufficiently well preserved to be seen with the microscope, and there is a distinct difference in the size and shape of the cells of living and fossil species, see [fig. 70]; but this difference is slight as compared with the great similarity of form and appearance, as can be seen on comparing figs. [69] and [66], B, so that the fossil is at the most a different species of the genus Ginkgo. Among the fossil leaves there is greater variety than among the living ones, and some which are very deeply lobed so as to form a divided palm-like leaf go by different names, e.g. Baiera, but they are supposed to belong to the same family. Fossil seeds and male cones are also known as impressions, and are found far back in the Mesozoic rocks. From the fossil impressions it is certain that Ginkgo and plants closely allied to it were very widespread in the past, as they are found all over Europe as well as the other continents. Particularly in the Lower Mesozoic rocks Ginkgo seems to have been a world-wide type growing in great abundance.