30 JEAN DAWSON 



These conclusions are based upon many observations made 

 in widely different areas and they all point to the fact that 

 an increase of plant life in quiet waters beyond a certain point, 

 is very unfavorable to snail life. 



This relation which the plant bears to Physa in point of 

 numbers, brings us to an interesting phase in its local distri- 

 bution. In newly isolated ponds Physa passes rapidly through 

 its short lived optimum to its minimum as the' pond weed cap- 

 tures the waters. It is clear that if Physa is to inhabit ponds 

 in any numbers, that it can do so only when, by some means 

 or other, the growth of the pond weed is checked or killed. 



b. Relation of Drovight to the Pond Weeds and Physa. In the 

 present study of ponds two points arise for consideration: First 

 Physa and its relation to the water plants, and second, the 

 relation of the pond plants and Physa to drought. Since a 

 definite relationship has been established for Physa and the 

 water weeds, the next question to be considered is. What is 

 the relation of the plant and snail to drought? The bearing 

 that drought has upon the plant will be considered first, followed 

 by a discussion of the desiccation of snails. 



In the study of the river and lake systems the observation is 

 made that wherever there is slack or sheltered waters there 

 is generally a more or less abundance of pond weeds. These 

 plants, however, are not found in all the ponds studied. Pond 

 weeds are abundant in the ponds shortly after they are cut off 

 from the parent body and continue to be so up to the time they 

 are subject to drought. Many of the ponds contain the water 

 plants Char a, Ceratophyllum, Hypnum and Elodea, and inves- 

 tigation shows further, that if these bodies of water have not 

 as yel reached the stage where they dry up, they almost in- 

 variably contain at least one of these common plants. On 

 the other hand, those ponds that are observed to dry up, or 

 upon inquiry are reported to do so, contain none, except Hypnum, 

 of the above cited weeds. 



In the series of swamps studied below for snail desiccation, 

 this plant seems to survive even though the swamps become 

 perfectly dry year after year. 



It is evident from these observations that drought is fatal 

 to most of the pond weeds. These plants reach their max- 

 imum growth in the quiet water, and ultimately capture the 



