LEAVES AND BUDS. 131 



163. Adaptation of the Green Leaf to its Functions. 



It is interesting to note how the structure of the ordinary 

 bifacial type of foliage -leaf is adapted for its work. 



Its flattened form and its position are the most favourable 

 for absorbing carbon dioxide and for catching light. The 

 epidermis and cuticle prevent excessive evaporation, besides 

 acting as a screen against undue heating and other ill-effects 

 of strong light. Both palisade and spongy mesophyll cells, 

 containing numerous chloroplasts, are adapted for assimila- 

 tion. The palisade tissue is adapted to protect the chloro- 

 plasts from the effects of too bright light ; when the light is 

 diffused, the plastids are arranged chiefly along the upper 

 and lower walls of the columnar palisade cells, but in strong 

 light the protoplasm moves in such a way as to arrange 

 chloroplasts along the side-walls of the cells with their edges 

 (the chloroplasts are disc- shaped) presented to the light. The 

 spongy-tissue, with numerous air-spaces communicating with 

 the atmosphere by means of the stomates, is equipped for 

 carrying on the gaseous exchanges concerned in respiration, 

 photosynthesis, and transpiration. 



The spreading veins, the larger ones projecting on the 

 lower side of the leaf, while the smaller ones run between the 

 palisade and spongy mesophyll, carry water (with dissolved 

 salts) to the mesophyll cells and elaborated food (sugar, etc.) 

 from them. 1 The mesophyll cells are arranged more or less 

 definitely in groups extending from the epidermis to the 

 veins. This is clearly seen in most leaves with well-marked 

 palisade mesophyll, the cells of the latter being arranged in a 

 fan-like manner as seen in vertical section (see Fig. 36), and 

 the cells which converge towards the veins have been called 

 " collecting " or " converging " cells. The veins also serve in 

 the best way to support the leaf ; the larger veins have bands or 

 cords of hard fibrous tissue below or above, or in both positions. 



164. Forms of Leaves. Before quitting the study of leaves we 

 must observe a number of their general characters their variety of 

 form, arrangement of veins, etc. In order to do this we shall have to 



1 The water, with dissolved salts, travels in the vessels of the wood, 

 the dissolved sugar in the long cells of the bundle -sheath, and the 

 proteids in the sieve -tubes of the bast. 



