736 



A TEXTBOOK OF THEORETICAL BOTANY 



seem to suffer at all either from this or from insect injuries to the foliage. 

 On the other hand, this security is attained by an expenditure of material 

 in leaf thickening, occasioned by the weakness of design, which should be 

 unnecessan,' for a deciduous tree. 



The vascular bundles of the leaf are collateral and are accompanied by 

 short transfusion tracheids which increase in number towards the leaf margin 

 and finally replace the ordinary xylem tracheids at the extremity of the vein. 



The leaf traces in the stem are not girdled like those of Cycas, but they 

 do not enter the vascular zone directly, passing instead obliquely downwards 

 through the cortex for a distance of four internodes before they become part 

 of the vascular ring. Here again, as in Conifers and Cycads, and indeed in 

 all the Angiosperms, we have to do with a vascular system which is composed 

 of leaf traces. There is no portion of the system peculiar to the stem. 



I 



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^H--*^ ■ ^' ^ r-^" ^ •*'X''^ - 





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^^m 



Fig. 739. — Ginkgo biloha. Transverse section of the lamina 

 showing irregular palisade cells and transversely elon- 

 gated mesophyll. On the left a vascular bundle, on the 

 right a mucilage canal. 



The mesophyll of the leaf blade is only differentiated into palisade and 

 spongy layers in the leaves of the long shoots (Fig. 739). In the spur leaves 

 the mesophyll is practically uniform, except that the loose cells of the middle 

 tissue are drawn out parallel to the leaf surface (cf. the transfusion tissue in 

 Cycas). Long mucilage canals may be seen like dark streaks between the 

 veins. 



Although the leaves are deciduous, that is, are dropped in winter time, 

 they are rather hard in texture and have a thick cuticle and sunken stomata 

 on the lower surfaces, characters which mark them as xeromorphic, like 

 those oi Finns. 



The stomata are surrounded by small accessory cells which project 

 inwards over the stomata in a very characteristic way, which can be recognized 

 also in fossil members of the order. 



The buds of Ginkgo are covered by corky, not resinous, bud scales, and 

 during the spring growth the inner scales develop a small petiole and lamina, 

 so that, as in many Angiosperms, one can trace a series of intermediate forms 

 connecting the smallest bud scales with the fully developed leaves. 



