Readaptation to Life in the Water 269 



Readaptations to life in the water — The more primitive 

 groups of aquatic organisms have, doubtless, always 

 been aquatic; but the aquatic members of several of 

 the higher groups give evidence of terrestrial ancestry. 

 Among the reasons for believing them to have devel- 

 oped from forms that once lived on land is the possession 

 of characters that could have developed only under 

 terrestrial conditions, such as the stomates for intake of 

 air in the aquatic vascular plants, the lungs of aquatic 

 mammals, and the tracheae and spiracles of aquatic 

 insects. Furthermore, they are but a few members 

 (relatively speaking) of large groups that remain 

 predominantly terrestrial in habits, and there are among 

 them many diverse forms, fitted for aquatic life in very 

 different ways, and showing many signs of independent 

 adaptation. 



I 



The vascular plants are restricted in their distribution 

 to shores and to shoal waters. They are fitted for 

 growth in fixed position and they possess a high degree 

 of internal organization with a development of vessels 

 and supporting structures that cannot withstand the 

 beating of heavy waves. As compared with the land 

 plants of the same groups, these are their chief structural 

 characteristics : 



1. In root: — reduced development. With submer- 

 gence there is less need of roots for food-gathering, since 

 absorption may take place over the entire surface. 

 Roots of aquatic plants serve mainly as anchors; in a 

 few floating plants as balancers; sometime they are 

 entirely absent. 



2. In stems: — many characteristics, chief of which 

 are the following: 



a. Reduction of water-carrying tubes, for the ob- 

 vious reason that water is everywhere available. 



