loo Aquatic Organisms 



The population of the water includes besides the 

 original inhabitants those tribes that have always 

 lived in the water a mixture of forms descended from 



ancestors that once lived on 

 land. The more primitive 

 groups are most persistently 

 aquatic. Comparatively few 

 members of those groups that 

 have become thoroughly fit- 

 ted for life on land have re- 

 turned to the water to live. 



WATER PLANTS 



I VERY large group of plants 

 has its aquatic members. 

 The algae alone are predomi- 

 nantly aquatic. Most of them live wholly immersed; 

 some live in moist places, and a few in dry places, 

 having special fitnesses for avoiding evaporation. In 

 striking contrast with this, all the higher plants, the 

 seed plants, ferns, and mosses, center upon the land, 

 having few species in wet places and still fewer wholly 

 immersed. Their heritage of parts specially adapted 

 to life on land is of little value in the water. Rhi- 

 zoids as foraging organs, a thick epidermis with auto- 

 matic air pores, and strong supporting tissues are little 

 needed under water. These plants have all a shore- 

 ward distribution, and do not belong to the open water. 

 Only algae, molds and bacteria are found in all waters. 



