CHAPTER XXII 



HYDROPHYTES 



192. Adaptations. When a plant lives entirely or par- 

 tially submerged in water, its structure differs in many 

 ways from that of an ordinary land plant, and these ad- 

 justments to water life are called adaptations. On parts 

 under water the epidermis is thin and permits absorption, 

 so that in a completely submerged plant its whole surface 

 absorbs. When this is the case, the root-system is much 

 reduced in extent as compared with a land-plant of the 

 same size, for it is not the only organ for water absorption. 

 In submerged plants the rigid tissues are less developed 

 than in land plants, for the "buoyant power of water helps 

 to support the plant. This fact may be illustrated by taking 

 from the water submerged plants that seem to be upright, 

 with all their parts spread out; upon removal they collapse, 

 not being able to support themselves. Water-plants are 

 also usually provided with air-chambers and passageways 

 that the air may be free to reach the working cells. 



A few of the most characteristic hydrophytic associa- 

 tions are given as illustrations, some of which occur in 

 almost every neighborhood. 



193. Pond weed associations. Water-lilies and pond- 

 weeds are conspicuous members of these associations, the 

 former with floating leaves (pads) (Fig. 301), the latter 

 often entirely submerged. Associated with them are nu- 

 merous other forms with floating or submerged leaves. 

 The plants are anchored by their roots and rootstocks in 



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