Links With the Past in the Plant World

by A. C. Seward

The Maiden Hair tree of China and Japan, which was introduced into Europe early in the eighteenth century, has now become fairly well known. Though hardy in England, it requires warmer summers for full development and regular flowering. To botanists this Eastern tree is of peculiar interest, partly because of the isolated position it occupies in the plant-kingdom and partly by reason of its great antiquity. There is probably no other existing tree which has so strong a claim to be styled a 'living fossil,' to use a term applied by Darwin to survivals from the past. In 1712 the traveller Kaempfer proposed for this plant the generic name Ginkgo, and Linnaeus adopted this designation, adding the specific name biloba to denote the bisection of the wedge-shaped lamina of the leaf into two divergent segments. In 1777 the English botanist Sir J. E. Smith expressed his disapproval of what he called the uncouth name Ginkgo by substituting for Ginkgo biloba the title Salisburia adiantifolia, but as it is customary to retain names adopted or proposed by Linnaeus, the founder of the binominal system of nomenclature, the correct botanical designation of the maiden hair tree is Ginkgo biloba. Mere personal preference such as that of Sir J. E. Smith for Salisburia is not an adequate reason for rejecting an older name.

Fig. 19. Ginkgo biloba Linn. (Slightly reduced.)

In its pyramidal habit Ginkgo agrees generally with the larch and other Conifers. Like the larch and cedar it possesses two kinds of foliage shoots, the more rapidly growing long shoots with scattered leaves and the much shorter dwarf-shoots which elongate slightly each year and bear several leaves crowded round their apex. The leaves ([Fig. 19]), which are shed each year, are similar in the cuneate form of the lamina and in the fan-like distribution of the forked veins, to the large leaflets of some species of maiden hair ferns: the thin lamina carried by a slender leaf-stalk is usually about 3 inches across, though in exceptional cases it may reach a breadth of 8 inches. The lamina is usually divided by a deep V-shaped sinus into two equal halves; it may be entire with an irregularly crenulate margin, or, on seedlings and vigorous long shoots, the lamina may be cut into several wedge-shaped segments.

The male and female flowers are borne on separate trees; the male consists of a central axis giving off slender branches, each of which ends in a small terminal knot and two elliptical capsules in which the pollen is produced. The female flowers have a stouter axis which normally produces two seeds at the apex. The seed is encased in a green fleshy substance and, as in the fruit of a cherry or plum, the kernel is protected by a hard woody shell. In the form of the leaves and in the structure of the flowers Ginkgo presents features which clearly distinguish it from the Conifers, the class in which, until recently, it was included. In 1896 the Japanese botanist Hirase made the important discovery that the male reproductive cells of Ginkgo are large motile bodies provided with a spirally coiled band of minute cilia—delicate hairs which by their rapid lashing-movement propel the cell through water. In all Flowering Plants and in Conifers the male reproductive cells have no independent means of locomotion; they are carried to the female cell by the formation of a slender tube—the pollen-tube—produced by the pollen-grain. In the Ferns, Lycopods and Horsetails—in fact in all members of the Pteridophyta—as also in the Mosses and Liverworts as well as in many of the still lower plants, the male cells swim to the egg by the lashing of cilia like those on the male cells of Ginkgo. This difference in regard to the nature of the male cells was considered to be a fundamental distinction between the higher seed-bearing plants and all other groups of the vegetable kingdom. It was, therefore, with no ordinary interest that Hirase's discovery was received, as it broke down a distinction between the two great divisions of the plant-world which had been generally accepted as fundamental; though it is only fair to say that the German botanist Hofmeister, a man of exceptional originality and power of grasping the essential, foresaw the possibility that this arbitrary barrier would eventually be removed. The Ferns and other plants in which the male cells are motile, represent earlier stages in the progress of plant development, when the presence of water was essential for the act of fertilisation, a relic of earlier days when the whole plant-body was fitted for a life in water. As higher types were produced, the plant-machinery became less dependent on an aqueous habitat, and the loss of organs of locomotion in the male cells is an instance of the kind of change accompanying the gradual adaptation to life on land. The idea of the gradual emancipation of plants from a watery environment is expressed in a somewhat extreme form by the author of a book entitled The Lessons of Evolution([52]), who states that the ocean is the mother of plant-life and that plants formed the army which conquered the land. In Ginkgo we have a type which, though similar in most respects to the Conifers, possesses in its motile reproductive cells a persistent inheritance from the past. The recognition of this special feature afforded a sound reason, especially when other peculiarities are considered, for removing Ginkgo from the Conifers and instituting a new class-name, Ginkgoales.

Ginkgo is a generalised type, linked by different characters both with living members of the two classes of naked-seeded plants and with certain existing Palaeozoic genera. It is a survivor of a race which has narrowly escaped extinction; the last of a long line that has outlived its family and offers by its persistence an impressive instance of the past in the present. Though Mrs Bishop in her Untrodden Paths in Japan speaks of forests of Maiden Hair trees apparently in a wild state, it is generally believed that they were cultivated specimens. Mr Henry who has an exceptionally wide knowledge of Chinese vegetation tells us that 'all scientific travellers in Japan and the leading Japanese botanists and foresters deny its being indigenous in any part of Japan; and botanical collectors have not observed it truly wild in China.' Moreover, Mr E. H. Wilson, after traversing the whole of the district where Ginkgo was supposed to occur in a wild state, says that he found only cultivated trees. There is no reason to doubt that China is the last stronghold of this ancient type which in an earlier period of the earth's history overspread the world.

A brief summary of the past history of Ginkgo and of the Ginkgoales supplies overwhelming testimony to the tenacity of life with which the Maiden Hair tree has persisted through the ages.

It was pointed out in the account of the past history of Araucaria that the records obtained from Palaeozoic rocks, while affording evidence of the existence of Carboniferous and Permian genera undoubtedly allied to the living species, do not enable us to speak with certainty as to the precise degree of affinity. Similarly, Palaeozoic leaves have been described as representatives of the class of which Ginkgo is the sole survivor, but the evidence on which this relationship is assumed is by no means conclusive.