26 JEAN DAWSON 



For the purpose of this paper, water plants must be con- 

 sidered not in their food relations alone, as they have been 

 hitherto in the most favorable lake and river habitats, but 

 also with reference to the amount of gas which they give to 

 the water. In still waters, Physa has come to bear a double 

 relationship to the plant, and depends upon it for food and 

 perhaps somewhat for aerated water. This method of aerating 

 water is one of the essential differences between stream and 

 lake habitats on the one hand, and other small still water hab- 

 itats on the other. Another difference has to do with the rela- 

 tive depth of the waters and the resulting difference in the length 

 of their life histories. 



Bodies of water, no matter what has been their previous 

 history, when once they become small ponds, have much in 

 common during their short life; they all tend to become dry 

 land. The processes which cause these small bodies of water 

 to become dry may work gradually, year by year filling may 

 go on, or down-cutting of the outlet may take place, or the 

 water level may become lower and lower during a year of un- 

 usual drought and the pond become dry. This may become 

 of yearly occurrence for longer and longer periods of time until 

 finally the pond becomes permanently dry. 



a. Relation of Pond Weeds to Physa. Not only does the 

 aeration of the pond water bring the snail into a new relation- 

 ship with the water plant, but the drying up of the waters brings 

 both plants and snail into a close relationship to each other. 

 It may be remembered that in the study of lakes and streams, 

 where quiet water prevailed, the pond weeds grew luxuriantly. 

 The ponds with their quiet waters offer very favorable con- 

 ditions for the growth of the water plants and it is only a matter 

 of time until these plants entirely capture such an area. The 

 question arises as to whether waters which have a very thrifty 

 growth of water plants, offer conditions favorable to a good 

 growth of snails, or in other words whether the optimum pond 

 weed habitat coincides with the optimum snail habitat. This 

 question can be answered only by a study of the conditions 

 under which the dift'erent pond weeds grow most favorably 

 and by observing the snail life under such circumstances. 



In the examination of different bodies of water for Physa 

 habitats a well marked correlation between the large pond weeds 



