352 



OLD AND YOUNG LEAVES. 



leaf is .still comparatively short, while the numerous lateral veins, ou the contrary, 

 are already strongly developed, the latter must lie so close to one another that they 

 actually come into contact. Consequently on the under surface of the erect leaf, 

 which is tunied towards tlie sun, nothing can be seen of the delicate gi'een tissue; 



Fig. 91 — Leat-unfoldiiig of the Tulip-tree (Liriodendron tuUpifera). 



I A twig jit the end of which tlie leaves are beginning to unfoUl. - End of the same twig, the leaves being further expanded. 

 3 The anterior boat-shaped stipule artifleially removed fi-om the upper bud. * One of the stipules about to fall off. 



only the thick lateral veins, devoid of chlorophj'll, stand out side bj- side like the 

 supporting framcwoi-k of a rush mat. The green portions of the leaf, which extend 

 between the veins, form j^rojecting folds on the concave surface, i.e. on the surf.ice 

 which is turned from the sun. They are thus hidden behind the close-pressed layer 

 of ribs as if by a roof, and are consequently protected as efficiently as possible from 



