LEAF STRUCTURE 83 



a forest tree by diffusion alone without the aid of the tran- 

 spiration stream. We shall return to this subject at the 

 end of this chapter when we have considered the available 

 evidence. Meanwhile, we may say that, though it is cer- 

 tainly unavoidable that the plant should lose large quantities 

 of water through transpiration, it is likely that the process — 

 inevitable in an organism with the structure and function 

 of a flowering plant — has its uses. It may reduce danger 

 of overheating. It may promote the supply of salts to an 

 extensive shoot system. It also keeps up the supply of 

 growth water, and it must be doubted whether this supply 

 could be maintained in an organism of the size of a tree 

 without the special conducting system which primarily 

 meets the necessities of transpiration. 



§ 2. The Leaf 



No other organ of the plant has a wider variety of form 

 than the leaf ; yet we recognise as typical or normal the 

 possession of the thin, expanded, light-absorbing blade or 

 lamina, and the slender stalk or petiole which gives the 

 possibilities of accurate adjustment of position, and of 

 escape by bending from mechanical injury. Through the 

 stalk run the vascular bundles, entering the stem as leaf 

 traces to link up with the general conducting system, 

 entering the leaf and appearing there as the midrib, if one 

 is present, giving off, or breaking up into a network of veins. 

 The larger veins are complete vascular bundles ; in the 

 finer there may be only a few elements, or, finally, only one 

 left ; the water supply is distributed in the most thorough 

 fashion. 



The veins run through a soft tissue termed the meso- 

 phyll. Towards the upper surface one or several layers of 

 cells are arranged regularly with their long axes per- 

 pendicular to the surface of the leaf, rather close together. 

 In a cross-section of the leaf, the columnar appearance of 

 this part of the mesophyll earns it the name of palisade 

 parenchyma. Towards the lower surface the mesophyll 



