352 



OLD AND YOUNG LEAVES. 



leaf is still comparatively short, while the numerous lateral veins, on 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 turned towards the sun, nothing can be seen of the delicate green tissue; 



Fig. 91 —Leaf-unfolding of the Tulip-tree (LirioAen&ron tulipifera). 



1 A twig at the end of which the leaves are beginning to unfold. 2 End of the same twig, the leaves being further expanded. 

 » The anterior boat-shaped stipule artificially removed from the upper bud. * One of the stipules about to fall off. 



only the thick lateral veins, devoid of chlorophyll, stand out side by side like the 

 supporting framework of a rush mat. The green portions of the leaf, which extend 

 between the veins, form projecting folds on the concave surface, i.e. on the surface 

 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 



