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 ; 



i <-. ^H j/a^ 



Fig. 91. Leaf -unfolding of the Tulip-tree (Liriodendron tulipifera) 



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

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

 etween 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 

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



