THE MAIDEN HAIR TREE

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.

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.

The generic name Psygmophyllum has been applied to some impressions of Ginkgo-like leaves discovered in the Upper Devonian rocks of Bear Island, a small remnant of land in the Arctic circle, which has furnished valuable information as to the composition of one of the oldest floras of which satisfactory remains have been found. Other examples of these lobed, wedge-shaped leaves are recorded from Carboniferous rocks in Germany, France, and elsewhere; from Permian strata in the east of Russia and from Palaeozoic beds in Cape Colony and Kashmir. A relationship between Psygmophyllum and Ginkgo is, however, by no means established and rests solely on a resemblance in the form of the leaves. The close correspondence in form and venation between some leaves from Permian rocks in the Ural mountains and from Lower Permian beds in France, and those of the recent species, is considered by some authors sufficiently striking to justify the reference of these fossils to the genus Ginkgo. Similar leaves of Permian age, which may also be related to the existing species, have been described under the name Ginkgophyllum. Other specimens of Palaeozoic age from North America and elsewhere have been assigned to the Ginkgoales; but in none of these cases, despite the resemblance in leaf-form, is there sufficiently convincing evidence of close relationship to warrant a definite assertion that the plants in question were members of the group of which Ginkgo alone remains.

It is, however, an undoubted fact that the Maiden Hair tree is connected by a long line of ancestors with the earliest phase of the Mesozoic era. From many parts of the world large collections of fossil plants have been obtained from strata referred to the Rhaetic period, or to the upper division of the Triassic system. A comparison of floras from these geological horizons in different parts of the world points to a vegetation extending from Australia, Cape Colony, and South America, to Tonkin, the south of Sweden and North America, which was characterised by a greater uniformity than is shown by widely separated floras at the present day. One of the commonest genera in Rhaetic floras is that known as Baiera; this name is applied to wedge-shaped leaves with a slender stalk similar in shape and venation to those of Ginkgo, but differing in the greater number and smaller breadth of the segments. Between the deeply dissected leaf of a typical Baiera with its narrow linear lobes and the entire or broadly lobed leaf of a Ginkgo there are many connecting links, and to some specimens either name might be applied with equal fitness. Examples of Baiera leaves, in some cases associated with fragments of reproductive organs, are recorded from Rhaetic rocks of France, the south of Sweden, Tonkin, Chili, the Argentine, North America, South Africa, and from other regions. There is abundant evidence pointing to the almost world-wide distribution of the Ginkgoales, as represented more especially by Baiera, in the older Mesozoic floras. In the later Jurassic rocks of Yorkshire true Ginkgo leaves as well as those of the Baiera type are fairly common; with the leaves have been found pieces of male and female flowers. Ginkgo and Baiera have been described from Jurassic rocks of Germany, France, Russia, Bornholm, and elsewhere in Europe; they occur abundantly in Middle Jurassic rocks in northern Siberia, and are represented in the Jurassic floras of Franz Josef Land, the East Coast of Greenland, and Spitzbergen (Fig. 20). The abundance of Ginkgo and Baiera leaves associated with male flowers and seeds discovered in Jurassic rocks, approximately of the same geological age as those on the Yorkshire coast, in East Siberia and in the Amur district, has led to the suggestion that this region may have been a centre where the Ginkgoales reached their maximum development in the Mesozoic period.

It should be added that other genera of Jurassic and Rhaetic fossils in addition to Ginkgo and Baiera have been referred to the Ginkgoales, though evidence of such affinity is not convincing. There is, however, good reason to believe that this widespread group was represented by several genera in the older Mesozoic floras.

The occurrence of the Ginkgoales in Jurassic rocks in King Charles Land and in the New Siberian Islands (lat. 78° and 75° N.), in Central China, Japan, Turkestan, California, Oregon, South Africa, Australia, and Graham's Land demonstrates the cosmopolitan nature of the group. During the later part of the Jurassic period and in the Wealden floras both Baiera and Ginkgo were abundant; leaves are recorded from Jurassic strata in the north-east of Scotland, from Lower Cretaceous or Wealden rocks in North Germany, Portugal, Vancouver Island, Wyoming, and Greenland.

During the Tertiary period, or probably in the earlier days of that era. Ginkgo flourished in North America, in Alaska and in the Mackenzie River district, Greenland, Saghalien Island, and in several European regions. In Chapter III reference was made to the volcanic activity which characterised the north-west European area in the early Tertiary period and resulted in the formation of the thick sheets of basalt on the north-east coast of Ireland and in the Inner Hebrides. There were occasional pauses in the volcanic activity, during which vegetation established itself on the weathered surface of the lava, and left traces of its existence in the leaves and twigs preserved in the sedimentary material enclosed between successive lava-floras. At Ardtun Head in the Isle of Mull beautifully preserved leaves of Ginkgo, 2-4 inches in breadth, with the median sinus and the venation characteristic of the leaves of the existing plant, have been discovered in a bed of clay which marks the site of a lake in a depression on the lava-plateau. The resemblance of these Tertiary leaves from Mull to those of the surviving Maiden Hair tree is so close as to suggest specific identity. Mr Starkie Gardner and Baron Ettingshausen have described some seeds from the London clay (Lower Tertiary) in the Isle of Sheppey as those of Ginkgo, but this identification rests on data too insufficient to be accepted without hesitation.

The recent cultivation of Ginkgo biloba in Britain may therefore be spoken of as the re-introduction of a plant which in the earlier part or in the middle of the Tertiary period flourished in the west of Scotland, and was abundant in England in the earlier Jurassic period. It is impossible to say with any confidence where the Ginkgoales first made their appearance , whether in the far north or in the south, nor are we able to explain the gradual decline of so venerable and vigorous a race.

As we search among the fragmentary herbaria scattered through the sedimentary rocks in that comparatively small portion of the earth's crust which is accessible to investigation, we discover evidence of a shifting of the balance of power among different classes of plants in the course of our survey of successive floras. Plants now insignificant and few in number are found to be descendants of a long line of ancestors stretching back to a remote antiquity when they formed the dominant class. Others which flourished in a former period no longer survive, either themselves or in direct descendants. 'The extinction of species has been involved in the most gratuitous mystery.' We can only speculate vaguely as to the cause of success or failure. Certain types were better armed for the struggle for life, and produced descendants able to hold their own and to perpetuate the race through the ages in an unbroken line. Others had a shorter life and fell out of the ranks of the advancing and ever changing army. To quote Darwin's words:'We need not marvel at extinction; if we must marvel, let it be at our own presumption in imagining for a moment that we understand the many complex contingencies on which the existence of each species depends.'