562 A TEXTBOOK OF BOTANY [Cn. XIV 



spondingly, hydrophytes lack an air system, but_instead 

 expose most of their cells at the general surface, thus pro- 

 ducing those unicellular, filamentous, filmy, or~"finery~~cut 

 forms so distinctive of water plants, which are massive only 

 where special aeration occurs through tidal or wave action. 

 This great spread of surface in water plants facilitates also 

 the absorption of light, which, weakened by its passage 

 through water, can penetrate even less deeply into the chlor- 

 enchyma of water, than of land, plants (page 53). Finally, 

 plant substance is so slightly heavier than water as to 

 need little extra aid (some gas in the tissues, or even oil 

 in the cells), to render it automatically buoyant. Corre- 

 spondingly, water plants lack those self-supporting firm 

 tissues essential in land plants, and are therefore soft 

 bodied and collapsible when brought into the air. This 

 lack of lignification permits the ready transformation of the 

 cellulose walls into the gelatinous or mucilaginous material 

 which is so abundant in the tissues of water plants, and 

 utilized by them in different ways.. 



Thus the conditions of their habitat explain the dis- 

 tinguishing characteristics of hydrophytes, viz. why 

 they are so little differentiated as to organs and tissues, 

 spread so much surface in theirjiiin or slender bodies, and 

 have such softness of texture. 



The hydrophytes, while ecologically homogeneous, include 

 members from diverse phylogenetic groups. They are 

 chiefly ^g^ which is the typical and primitive hydrophytic 

 group, but others are land forms which in course of their 

 evolution have taken to life in the water. Thus Bryophytes, 

 Pteridophytes, and even Spermatophytes (represented in 

 the Pondweeds and Eelgrasses) have become wholly sub- 

 merged hydrophytes, with all of the hydrophytic character- 

 istics, though they also retain flowers, leaves, stems, roots, 

 and even the air system, from their former habitats. 



The principal vegetation forms among hydrophytes are 

 the following, as characterized by their salient features. 



