BIOLOGY OF PHYSA 31 



water to the exclusion of other plants. At the first appearance 

 of drought this luxuriant grovvth of pond weeds is killed and 

 the pond henceforth takes on a new appearance. The per- 

 ennial plants usually give place to annuals which are not so 

 aggressive, since they take most of the summer to mature. 



Since, in excess, the pond weed is very unfavorable to Physa, 

 and since drought is so disastrous to the pond weed, it is nec- 

 essary to ascertain the relation of drought to the snail. 



A study of the most favorable pond habitat for Physa may 

 throw some light upon the question and may show an equilib- 

 rium established between the plant and the snail in quiet waters 

 so as to enable the latter to live there in large numbers. A 

 small pond situated a few rods north of the University campus, 

 Ann Arbor, is the remnant of a small, deep pond of glacial origin, 

 which some twenty years ago had fully twenty feet of water in 

 it, but which now does not exceed two feet in the deepest parts. 

 Great numbers of P. gyrina hildrethiana are found on all sides of 

 the pond reaching nearly to the center, instead of in a narrow 

 belt of a few inches in width as is generally the case in such 

 bodies of water. The conditions in this habitat are as follows : 

 Shallow water throughout; full sunlight; no enemies observed; 

 very little decaying debris; no pond weeds, but a very small 

 amount of Spirogyra, and Alisma plantago L. in moderate numbers. 



It was noticed that this habitat has none of the usual pond 

 weeds, but a form of vegetation which is less aggressive. Upon 

 a closer study of this pond a relationship was found between 

 such pond weeds as Chara, Ceratophylhim, Elodea and Physa. 

 As this pond dries up during seasons of drought and as these 

 pond weeds have no means of withstanding such a period, and 

 cannot as a consequence gain a permanent foothold in water of 

 this nature, but must give way to plants less sensitive to drought ; 

 the following observations show that other means than drought 

 retard the growth of pond weeds. This is the case at Dayton, 

 Tuscola county, Mich., in a cedar swamp which has been 

 nearly cleared, and in which water stands the year around. 

 There were a moderate number of water weeds and a great 

 many Physa were crawling about upon a soft clay substratum. 

 This swamp is of long standing and the presence of a few water 

 weeds together with a clean clay substratum is worthy of note ; 

 also the fact that the clay forming the substratum has been 



