176 General Botany 



internal air cavities is shown by the fact that the plants may 

 be exterminated by cutting off all the rootstocks at the water's 

 edge and cutting the erect stems below the water level. Even 

 water plants may be drowned. 



Summary of hydrophytes. Among hydrophytes, then, there 

 are plants with upright stems, some with floating or buoyant 

 stems, and others with underground stems. Hydrophytes 

 resemble the mesophytes in most respects but differ in having 

 aerenchyma, or air cavities, as well as in having a somewhat 

 reduced water vascular system and less mechanical tissue. 

 Hydrophytes spread rapidly by growth and branching of hori- 

 zontal stems. Many of them survive the winter, through the 

 development during the lower-temperature period of autumn of 

 short, heavier-than-water shoots and buds. 



