152 



PLANT PHYSIOLOGY AND ECOLOGY 



These may consist of the epidermal cells proper, of layers of water 

 cells just below the epidermis, or of swollen cells found upon its 

 surface. Cells of the chlorenchym are often transformed into 

 large clear water cells, which may be scattered singly or arranged 

 in groups. The groups of water cells are sometimes scattered, 

 but they usually occur in transverse bands, or in horizontal layers. 

 Such layers lie between the palisade and sponge tissues, and 

 connect the bvmdles. A few plants possess tracheid-like cells 

 which serve to store water. In the case of succulent leaves, 

 practically the whole chlorenchym is used for storing water. 

 Such leaves retain their water tenaciously by virtue of mucilage 

 or other substances. 



172. Adaptation to excessive water supply. Plants which 

 grow in water but have their leaves exposed undergo changes 



Fig. 48. Cross-sections of floating (5a), submerged (56), and deeply sub- 

 merged leaves (5c) of Sparganium angustifolium. The palisade tissue 

 of the floating leaf is replaced by a single row of sponge cells in the 

 submerged one and the air-passages correspondingly increased. The 

 deeply submerged leaf lacks palisade tissue and the air-passages are 

 much reduced. 



that increase the water loss and decrease the water supply. The 

 absorbing surface is much reduced by the uniform lack of root 

 hairs, and the relatively small development of roots. In a few 

 extreme cases the roots become mere vestiges or are entirely 

 wanting. The leaves of water plants show a marked tendency 

 to increase the exposed surface. This is clearly shown by the 



