316 



KNOWLEDGE. 



August. 1911. 



the causes which ha\e contributed to keeping the leaf sm.iU. 

 and that this faihire may possibly be associated with the 

 presence of transfnsion tissue, which, though valuable in itself 

 under the circumstances, may tend to prevent improvement in 

 other directions, such as that of free pinnation of the veins. 



Given this cramped hereditary type of structure, ecological 

 adaptation appears to ha\e been the result of two processes 

 going on simultaneously: (1) the development of enormous 

 numbers of the rigidly-constructed leaves with a view to 

 increased assimilation and growth, and (2) the production of 

 xerophily in the indi\idual leaves as a compensation for the 

 resulting increase of surface. It thus seems clear that the 

 hereditary factor is of great importance in the ecological 

 relationships of the conifers. It is conceivable that ecological 

 .adaptation may be achieved as efficiently by a suitable 

 modification of the small-leaved type as by another kind of 

 modification of a broad-leaved type — in fact, more than one 

 kind of mechanism may be well adapted to a given set of 

 external conditions. The conifers, however, are predisposed 

 to the adoption of " architectural xerophily " as their mode of 

 adaptation by the persistent hereditary factor of microphylly 

 (possession of small leaves). 



THE MAIDENH.^IR TREE [GIXKGO BILOBA).— 



One of the most interesting groups of Gynmosperms is that 

 which is now represented only by the Maidenhair Tree. The 

 Ginkgotype is of special importance because its reproductive 

 organs show many resemblances to those of the lower 

 Gymnosperms (Cycads and Cordaitalesl, while its vegetative 

 anatomy resembles in many ways that of the Conifers, so 

 that it may be s.aid to form a connecting link between these 

 groups. 



The Maidenhair Tree, as the sole survivor of a family which 

 was well represented in the Mesozoic Period, and perhaps 

 much earlier, has naturally attracted a great deal of attention 

 from botanists, and it is proposed here to summarise some of 

 the literature on Ginkgo published during the last few years, 

 including the most recent accounts. 



Ginkgo is almost unknown in the wild state, though reported 

 by travellers in the forests of western and south-western 

 China, but it has been extensively cultivated, first in China 

 and Japan, and later in l-'urope — there is a fine tree in Kew 

 Gardens, for instance. .Apart from the strong main trunk and 

 wide-spreading branches, it hardly resembles in habit any of 

 the other Gymnosperms. while its deciduous leaves, having 

 a long slender stalk and a broad wedge-like blade with 

 repeatedly forked veins (strongly suggesting the leaves of 

 Maidenhair Fern), are so characteristic that they form 

 unusually trustworthy evidence of the existence of the 

 Ginkgoales when found in the fossil state. The tree reaches 

 a height of about one hundred feet. The branches are of 

 two kinds ; the long shoots bear scattered leaves, while the 

 short ones are dwarfed, slow-growing, and bear few leaves in 

 a cluster. A dwarf shoot may after several years become a 

 long shoot, bearing scattered leaves, and may then resume the 

 slow growth of the dwarf shoot ; sometimes it undergoes 

 branching. 



The general morphology and anatomy of Ginkgo have been 

 described by Seward and Gowan (Ann. Bot.. 1900) and in 

 greater detail by Sprecher in his invaluable monograph of this 

 plant (Geneva, 19071. while various details have been added 

 by recent writers. The stem shows the general structure of a 

 conifer, having a narrow cortex, a thick compact zone of wood 

 produced by a persistent cambium layer (contrasting with 

 Cycads), and a relatively small pith (contrasting with Cordai- 

 tales) ; in thed warf shoots, however, the wood zone is narrow 

 and the pitch large. -As in the pines and other low conifers, 

 there are well-marked annual rings, and the tracheids have 

 opposite pits and " bars of Sanio." 



Tupper iBot. Gaz., 1911) has shown that fre<iuently the 

 short shoots branch within the wood of the limb out of w-hich 

 they grow, this has recently been seen in a new Triassic 

 Araucarian, Woodwortliia, but in no other conifers so far as 

 known, and confirms the view that the araucarian conifers 

 are an old group and have possibly come from the same stock 



as the ginkgoales. Tupper also describes crystal cells and 

 wood parenchyma as occurring in rows or series running 

 longitudinally through the root oi Ginkgo in radial planes; all 

 these rows are in contact with at least one of the medullary 

 rays, and Tupper suggests that the radial distribution of this 

 parenchyma, as compared with the tangential arrangement 

 seen in conifers, shows that Ginkgo is a primitive type. 



The primary wood in the stem is endarch (with the earliest 

 wood vessels innermost, and the development of the wood 

 therefore centrifugal), but distinct mesarch structure ii.e.. part 

 of the wood centripetal, the rest centrifugal, therefore the 

 protoxylem not innermost) occurs in the bundles of the cotyle- 

 dons. The leaf receives a double bundle (leaf trace), as in all 

 the more primitive gynmosperms, each bundle forking at the 

 base of the blade and breaking up into the forking system of 

 veins, some of which show traces of centripetal wood and 

 therefore indistinct mesarch structure. The veins are not 

 joined up into a network, so that if a few veins are cut across 

 near the base of the leaf, long streaks of the leaf become 

 withered and dry. The leaves vary much in size and lobing, 

 showing e\ery transition from deeply lobed to almost entire, 

 though typically there is a notch at the middle of the blade. 

 The leaf structure resembles that in cycads ; the stomata are 

 chiefly on the underside, and between the veins the loose 

 mesophyll cells are elongated parallel with the leaf surface. 



The plant is dioecious ; the flowers have no bracts, and they 

 are developed on the dwarf shoots. The male flowers or cones 

 consist of an axis bearing loosely arranged stamens ; each 

 stamen has a stalk ending in a knob which bears usually two 

 pendent stamens, sometimes three or four or as many as seven. 

 Miss Starr [Bot. Gaz.. 1910/ has shown that in the young 

 knob there develop, in addition to the stamens, patches of 

 apparently sporogenous tissue which degenerate into mucilage 

 cavities, suggesting that these cavities have replaced abortive 

 sporogenous tissue. 



The stamen of Ginkgo suggests comparison with the 

 epaulet-like form seen in the " crossotheca " type of stamen 

 in the pteridosperms. It may also be compared with the 

 stamens of the cordaitales, the whole male dwarf shoot of 

 Ginkgo corresponding to the male inflorescence of cordaites, 

 which consists of a thick axis bearing bracts among which are 

 inserted the stamens, each stamen consisting of a long stalk 

 bearing at its tip a cluster of three to six erect pollen-sacs. If 

 the sterile sporophylls in the Cordaites cone were suppressed, 

 the general structure of the male flower of Ginkgo would 

 be attained, but in Ginkgo the pollen-sacs are borne in 

 a different way, being dependent on the lower side of the knob 

 or reduced blade of the stamen. In Antholttlius zeilleri, 

 regarded as the male cone of Baicra (a Mesozoic member of 

 the Ginkgoales), the stamens are repeatedly forked and bear 

 eight pollen-sacs, one on each of the ultimate divisions. 

 Whatever interpretation is placed on these diff'erent 

 structures, the facts point to close relationship between 

 ginkgoales, cordaitales and pteridosperms. 



The female cones are much reduced, and are carried in 

 groups on dwarf shoots. Each cone consists of a long stalk 

 bearing at its tip two ovules, sometimes more, and below each 

 ovule there is a narrow cup or collar. The latter has given 

 rise to much discussion. Fujii and, later, Seward and Gowan, 

 were led by the study of numerous abnormal forms to conclude 

 that the stalk is a shoot bearing normally two rudimentary 

 carpels represented by the two collars. However, Shaw [^ew 

 Pliytologist. 190SI, examining the vascular structure of the 

 flower, found that the bundles of the collar show inverse 

 orientation, and suggested that the collar is no carpel but a 

 vestige of the cupule that surrounded the seeds of the 

 pteridosperm genus Lagcnostonia. If correct, this would 

 even more strongly point to an affinity between ginkgoales 

 and pteridosperms. Shaw regards the general anatomy of the 

 female axis and the occasional occurrence of an apical bud 

 between the ovules as proof that it is a shoot bearing two 

 lateral-stalked ovules, each ovule being attached to the axis 

 by a short pedicel, with the collar at the junction between 

 pedicel and ovule. On the other hand, the female axis has 

 four bundles, which would be expected in a stem bearing two 



