WATER PLANTS 255 



stemmed, and rest on the surface of the water, and can 

 be accommodated without inconvenience to changes of 

 its level. Stomata are in land plants usually developed 

 most freely on the under surfaces of the leaves, but in 

 the Water-Lilies they are on the upper surface of the leaf, 

 and consequently exposed to the air. They could not 

 be effective otherwise. Provision is also made that the 

 surface of the leaf shall not be inundated, lest the 

 stomata be rendered inoperative. There are plants 

 found in aquatic conditions that have two kinds of 

 leaves. The Water-Crowfoots are examples. They 

 have floating leaves, with the upper surface exposed to 

 the air, and submerged leaves very finely divided. The 

 latter offer little resistance to moving water, and hence 

 are not liable to be torn thereby, and they provide 

 increased surface for the absorption of carbon dioxide 

 and oxygen. The floating leaves are lobed, but not 

 dissected. They expose a broad surface to the air. If 

 the water in which the plant lives dries up, the sub- 

 merged dissected leaves die, but the floating leaves con- 

 tinue to live and carry on their useful functions. The 

 Bladderworts (Utricularia) in all European species have 

 dissected submerged leaves, and no roots. They float 

 in the water. The leaves can absorb nutrient sub- 

 stances in solution, as well as carbon dioxide and oxygen. 

 Their flowers only are aerial. Roots in their case are 

 not needed. 



Water plants, then, have no difficulty as to water- 

 supply, unless it be in its superabundance; but land 

 plants of all the higher forms have had to adopt special 

 means of water absorption and retention. They cannot 

 absorb a supply by their general surface. Were they 



