Environment and Adaptation 195 



On the basis of their relation to water, three 

 categories of plants are recognized : Hydrophytes 

 or aquatics, Mesophytes or plants in which there is 

 a normal supply of water but which are not true 

 aquatics, and Xerophytes in which the need of con- 

 servation of water is more or less acute. Of course 

 these groups are not absolutely separate from each 

 other, and may be further subdivided. 



AQUATICS 



Aside from the algae, nearly all of which are 

 true aquatics, there are r. good many flowering plants 

 and a few mosses and ferns, which are also to a 

 greater or less degree genuine water plants. How 

 far these forms are secondary, that is, are derived 

 from originally terrestrial types, is not always easy 

 to decide ; but in many cases it is perfectly clear that 

 they are modified descendants of terrestrial forms. 

 These aquatics may be completely immersed, as in 

 some of the pond-weeds and some of the sea-plants, 

 like the eel-grass (Zostera) ; or they may be float- 

 ing plants, like the little water- fern (Azolla), or 

 the duck- weed (Lemna), etc., or they may be 

 rooted below the surface of the water with floating 

 leaves and flowers, like the water-lilies, or finally 

 they may stand above the surface like the reeds, 

 cattail-rushes, etc. 



Land and Water Plants Compared. Compared 

 with the related land plants, these aquatics show 



