l84 FRESH-WATER BIOLOGY 



Many of the delicate submerged plants will wither rapidly when 

 taken from the water and exposed to the air. This occurs be- 

 cause the outer layer of tissue or epidermis, as it is called, is thin 

 and allows the water contained in the plant rapidly to pass into 

 the air as vapor. If a plant which bears both floating and sub- 

 merged leaves is exposed it will be noticed that the latter wilt and 

 dry out much more rapidly than the former. Examination will 

 show the cuticle of the floating leaves to be thicker and much less 

 permeable to water, if at all so, than that of the submerged leaves, 

 while special openings may be discovered through which water 

 vapor escapes instead of passing oft all over the surface as in the 

 submerged leaves. These special openings are called stomata and 

 are the same in structure as those which occur on the leaves of 

 land plants. The size of these openings may vary from time to 

 time according to the needs of the plant. Each opening is sur- 

 rounded by two ceUs, called guard cells, which also vary in size 

 and shape according to the amount of water they contain. When 

 turgid they become somewhat kidney-shaped, curving away from 

 the opening and thus making it larger. When flaccid because 

 there is little water in the plant they tend to straighten out and 

 thus make the opening smaller. Thus, by the activity of these 

 stomata whose action depends upon the amount of water in the 

 plant, the amount of water passing from the plant by transpiration 

 is regulated. 



The number of stomata occurring on the exposed surface of a 

 floating leaf may be quite large. One author counted the number 

 of stomata present in the area of i sq. mm. at five different loca- 

 tions on the upper surface of the floating leaves of one of the Pota- 

 niogetons. He found a minimum of 216 and a maximum of 276 

 with an average of 255 per sq. mm. 



It is evident that stomata are intended for leaves which must 

 endure exposure to the air, but they do occur, though rarely, on 

 the submerged leaves also (Fig. 260). Sometimes only one or 

 two submerged leaves of a given plant will have them and 

 again several specimens of the same species may be examined 

 without finding any at all. The only explanation for the occur- 

 rence of such structures on submerged leaves is, that the ancestors 



