CHAPTER I. GENERAL PHYSIOLOGY. 687 



mesophyll into the intercellular spaces, and the watery vapour 

 then escapes from the leaf by the stomata. 



A brief consideration of the peculiarities of the leaves of aquatic plants will 

 further establish these points. The characteristic feature of submerged leaves 

 is that they are very much divided, and consequently present a relatively 

 large surface by which they can absorb carbon dioxide and oxygen, as well as 

 salts, from the water. The mesophyll of these leaves is not differentiated 

 into palisade- tissue or spongy tissue; for, in the first place, the light to which 

 these leaves are exposed is not intense, and, in the second, they do not tran- 

 spire. A peculiar feature of these leaves is that the chlorophyll- corpuscles 

 are contained principally in the epidermal cells. Floating leaves, on the other 

 hand, usually have broad oval or rounded lamina, a form which is evidently 

 favourable to the retention of their position on the surface of the water. The 

 epidermis of the upper surface is well-developed and possesses stomata ; whereas 

 the walls of the epidermal cells of the lower surface are thin and uncuti- 

 cularised, so that they can absorb water and substances in solution, and there 

 are no stomata. The mesophyll is differentiated into palisade and spongy 

 tissue. The palisade-layers are arranged in the characteristic fashion towards 

 the upper surface of the leaf ; and commonly there lies between the superior 

 epidermis and the palisade-tissue, a layer of aqueous tissue which serves to 

 protect the chlorophyll of the latter frorh exposure to too intense light. 

 Towards the lower surface, the mesophyll is channelled by large intercellular 

 spaces, the use of which is to facilitate the aeration of the plant. 



The foregoing consideration of the anatomy of these three types 

 of foliage-leaves leads to the conclusion that the spongy tissue 

 of the mesophyll is that in which both the assimilatory and the 

 transpiratory processes can be most satisfactorily carried on ; that, 

 in fact, the amount of spongy mesophyll in a leaf may be taken as 

 an indication of the degree of its physiological activity. Clearly 

 the close arrangement of the cells in the palisade-tissue is not so 

 favourable for the absorption of carbon dioxide or for the exha- 

 lation of watery vapour, as is the loose arrangement of the cells in 

 the spongy tissue. The palisade-tissue is transpiratory and assi- 

 milatory tissue so adapted for action in intense light, that trans- 

 piration is diminished, and protection is afforded not only to the 

 chlorophyll in its own cells, but also to that in the cells of the 

 tissue which it covers. 



In many xerophilous plants, more or less of the mesophyll 

 may be destitute of chlorophyll, the cells containing much 

 watery sap (see p. 164), so that the leaf serves as a reservoir of 

 water. 



Leaves are adapted not only structurally, but also by their 

 irritabilities, to the performances of their functions. They are 



V. s. B. Y Y 



