ig8 FRESH- WATER BIOLOGY 



Potamogetons with floating leaves and especially the Nymphaea- 

 ceae which are followed by amphibious species until finally the 

 bog vegetation appears which may later support the ordinary 

 terrestrial plants. 



It may be supposed that aquatic phanerogams have descended 

 from aquatic or from land species. The former supposition im- 

 plies that the plants, as they are now, represent the maximum of 

 complexity in structure that has thus far developed. The latter 

 supposition impHes that the simpler vascular system is not to be 

 considered as foreshadowing a more elaborate structure to be 

 evolved in the future but is a reduced form of more highly developed 

 structure present in the terrestrial ancestors. Perhaps some spe- 

 cies have had land ancestors, while others have descended from 

 aquatic forms. 



Considering the whole list of fresh-water plants, it is noteworthy 

 that the abundant groups are algae and phanerogams. Bryophytes 

 and pteridophytes do occur, but infrequently in comparison with 

 the former groups, the thallophytes and spermatophytes, which 

 include a large percentage of aquatic vegetation. 



Would the great plant groups be represented in this proportion 

 if the evolution of aquatic forms had proceeded as in the case of 

 land plants? Does not a water environment insure greater uni- 

 formity of conditions and would not the evolution proceed more 

 slowly and the intermediate forms more likely persist in such en- 

 vironment? How could the great groups of monocotyledons and 

 dicotyledons ever become differentiated from ancestors living 

 wholly submerged? How could the seed habit so essentially like 

 that of land plants ever be acquired by the descendants of sub- 

 merged forms? On the other hand if water forms have been 

 derived from land forms, why are not the bryophytes and pterido- 

 phytes which are frequent in moist locahties better represented 

 in the aquatic flora? The change from aquatic structure to ter- 

 restrial must be much more difficult than from terrestrial to aquatic. 

 When a water plant is suddenly exposed, the loss of water by drying 

 is so rapid that the plant must die, while a land plant may endure 

 submersion tor a considerable period. In one case the change of 

 environment causes a sudden demand for a complex vascular system 



