40 JEAN DAWSON 



that is in other respects essentially the same as that of the 

 optimum habitat of Physa, as for example, the Elodea habitat. 

 In the field Physa does not find an optimum among excessive 

 plant growth. In the laboratory this same phenomenon holds 

 true. Any one who has tried to keep snails in numbers in 

 the laboratory has found that the water will soon develop a 

 bad odor atid become turbid if sufficient jDlants are not placed 

 in it. While on the other hand, if the water is crowded with 

 such plants are Chara, Elodea, Ceratophyllum and Hypnum, 

 it remains clear, but the snail will die in a relatively short time. 

 It is therefore seen that an excessive amount of plant life in 

 the water is detrimental to snails. 



It is interesting to note in this connection that Kofoid (1903) 

 in his work upon plankton in the Illinois river observes that 

 a luxuriant vegetation is unfavorable to an abimdant pro- 

 duction of plankton. He found wherever Ceratophyllum was 

 growing in excess that the small microscopic plant and animal 

 life was correspondingly scarce. I quote the following from his 

 report, p. 392: " The dominance of the abundant vegetation is 

 inimical to large plankton production. Other things being equal, 

 plankton production is greater when the relative occupancy of 

 the water by vegetation is decreased." Again on p. 484: "Other 

 things being equal, bodies of fresh water free from vegetation 

 (submerged macro-flora) produce more plankton than those rich 

 in such vegetation." In a footnote upon this statement he says: 

 " This relation of vegetation to the plankton may be formulated 

 as follows: The amount of plankton produced by bodies of 

 fresh water is, other things being equal, in some inverse ratio 

 proportional to the amount of its gross aquatic vegetation of 

 the submerged sort." Kofoid attributes the scarcity of plankton 

 in the regions rich in vegetation to the fact that the plants use 

 the dissolved nutrition in the water and the scarcity of plankton 

 is due to lack of food. 



Pond (1905) agrees with Kofoid 's statement and interpre- 

 tation of the scarcity of plankton only where there is an abun- 

 dance of non-rooted vegetation. According to Pond the roots 

 of anchored plants are true absorbing organs taking from the 

 soil valuable mineral salts. I quote him (p. 522) in the following : 

 " These rooted aquatics are important contributors to the 

 plankton food supply, because when living they organize matter 



