204 



ELEMENTARY STUDIES IN BOTANY 



are separated along an erect stem, a view from above, in 

 which all the leaves are referred to a single plane, shows 

 the mosaic arrangement. In many trees in dense forests, 

 notably in the tropics, the leaves appear chiefly and some- 

 times exclusively at the extremities of the branches, often 

 producing a magnificent dome-like mosaic. 



In the case of stems exposed 

 to direct light only on one 

 side, as the horizontal branches 

 of trees, stems prostrate on the 

 ground, and stems against a sup- 

 port (as climbers and twiners), the 

 leaf-blades are brought to the light 

 side so far as possible. Looking 

 up into a tree in full foliage, one 

 will notice that the horizontal 

 branches are comparatively bare 

 beneath, the Leaf-blades being 

 displayed on the upper side as 

 a mosaic. The most literal and 

 complete mosaic is shown by cer- 

 tain ivies, whose leaf-blades are 

 so adjusted to light that the 

 surface of a wall is covered com- 

 pletely by leaf-blades fitted to- 

 gether in a living mosaic (Fig. 

 167). The ivy mosaic is striking chiefly because the leaf- 

 blades are all displayed in approximately a single plane. 



124. Protective structures. The protection most widely 

 needed by land plants is against excessive loss of water, and 

 since the leaves are the prominent transpiring organs, the 

 chief methods of protection concern them. Drought is 

 usually accompanied by intense light, which is also danger- 

 ous to the chloroplasts. The problem of drought is pre- 

 sented to plants in three aspects : (1) the possible drought, 



FIG. 168. Section through a small 

 portion of a carnation leaf, show- 

 ing epidermis overlaid by a very 

 heavy cuticle ; a single stoma in 

 the epidermis, opening without 

 into a tubular passageway 

 through the cuticle, and within 

 into an air-chamber ; below are 

 the mesaphyll cells containing 

 chloroplasts. 



