734 A TEXTBOOK OF THEORETICAL BOTANY 



The wood structure is therefore " abietinean," which is usually con- 

 sidered as a relatively advanced type. 



Secondary thickening follows the same plan as in Pimis, but there is 

 relatively little in the spurs. There is also an external phellogen and a heavy 

 periderm, with a considerable growth of phelloderm, or secondary cortex, 

 on the inner side of the phellogen. The medullary ravs are exceptionally 

 small, being almost invariably one cell broad and seldom more than three 

 cells high, except in the spurs. Where they cross the tracheids they connect 

 to them by large simple pits. Mucilage cavities are abundant throughout the 

 tissues. There are annual rings, rendered evident, as in Pinus, by seasonal 

 variation in the width of the tracheids, but not as a rule very prominent. 

 The wood consists almost wholly of tracheids, but there is a small amount 

 of xylem parenchyma containing Calcium oxalate crvstals. 



Outside the vascular zone is an imperfect ring of stone cells, probably 

 pericyclic, accompanied by an inner ring of thickened cells, the walls of 

 which seem to be gelatinized. Their function is unknown. In older stems 

 the phloem is exceptionally broad and appears to be banded tangentially, 

 owing to the collapse of rows of cells, between which the large sieve tubes are 

 prominent. There are some discontinuous zones of fibres in the phloem 

 and a small amount of parenchyma. 



The stem structure of Ginkgo is in fact fundamentally much more akin 

 to that of the Conifers than to that of the Cycads, though it differs in several 

 details. 



Anatomy of the Leaf 



The leaves are the most striking feature of Ginkgo (Fig. 737). They are 

 broadly fan-shaped, 4 to 7 cm. across, with a wavy margin and an apical 

 notch which sometimes almost completely divides the blade into two portions.* 

 Exceptionally there may be several notches, producing a palmatifid appearance, 

 which is an interesting approach to the leaf structure of the related fossil 

 genus Baiera and may be a relic of an originally compound leaf. The lamina 

 tapers downwards into a flexible petiole which is often longer than the lamina 

 itself. The leaves are produced in a spiral order, but on the spurs the inter- 

 nodes are suppressed, so that the leaves seem to spring from almost the same 

 level. These spur leaves are usually undivided. 



The lower margin of the leaf is thickened, the thickness increasing towards 

 the petiole, so that the blade is really shaped like a flat spoon or scoop, recalling 

 the " ascidial " or trumpet-shaped leaves w^hich are found as abnormalities 

 in many Angiosperms. 



The petiole contains a double leaf-trace of two collateral strands which 

 show secondary growth, surrounded by a sheath of thick-walled cells (Fig. 738), 

 A similar double leaf-trace is found in some Pteridosperms, in Cycads 



* The resemblance of the leaf to that of the Maidenhair Fern, Adiantu»i capiUis-veneris 

 gave rise to the name by which Ginkgo was fomierly known, Salisbiiria adiantijolia, and also 

 to the English name. 



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