32 JEAN DAWSON 



eroded from the surrounding hills and that it serves very effec- 

 tually to keep the weeds in check so that Physa flourishes. This 

 mode of checking or killing the water plants was also observed 

 in the optimum habitat of Crooked Lake, where the soil has 

 washed into the water from a neighboring field. 



c. Relation of Drought to Physa, or Snail Desiccation. That 

 some Physa can withstand a period of severe drought is' well 

 established, as in the case of the pond near the campus, but 

 how often and under what conditions drought can be endured is 

 the problem before us. To answer the question nearly a dozen 

 swamps were examined in various degrees of extinction, from 

 the last stages of standing water to dry land. These swamps are 

 found in Dayton, Tuscola County, Mich., in a morainic region 

 extending over an area of about two hundred acres. The swamps 

 are isolated and have no natural outlet but lie side by side, 

 surrounded by gently sloping hills of clay and sand. The two 

 soils retain their own individuality, lying side by side in areas 

 of an acre or so, and are not much intermingled. About fifteen 

 years ago, white cedar is reported to have filled the swamps, 

 and the surrounding region was covered with a dense growth 

 of hemlock. Burning and clearing soon reduced the hemlock 

 land to cultivated fields, while the cedar was cut and has given 

 place to black ash. Most of the ash in turn has been succeeded 

 by willow. Some of the swamps are completely cleared, thus 

 giving the open water or pond condition. 



These swamps were studied in detail for snail desiccation alone 

 and in consequence will be classified according to the moisture 

 conditions they showed during the middle of September of the 

 unusually dry season of 1904. They may be divided into groups. 

 The swamps of Group A contain more or less moisture through- 

 out the year; Group B is subject to periodic drought, while 

 Group C undergoes annual drought. 



Group A — Swamp A K This is surrounded by gently sloping, 

 ploughed hills of clay and sand, with an open growth of young, 

 soft maple and v^illows. Most of the swamp is dry and covered 

 with a young growth of mesophytic annuals which have grown 

 up since the disappearance of the water in early summer, but 

 here and there in the lower depressions, are found pools of stand- 

 ing water in which grow a small amount of Spirogyra, but no 

 other water plants are present. The substratum varies, some 



