BIOLOGY OF PHYSA 27 



and the amount of motion in the waters was observed. The 

 plants that were anchored to soil, such as Chara and Elodea, 

 found their optimum in water where there was more or less 

 motion at times, while the floating plants of the type of Cerato- 

 phylliim, Spirogyra and Hypnmn were found to live in the very 

 quiet water where there is more or less decaying debris. Two 

 reasons may be given for floating plants inhabiting this sort 

 of water, ist, any motion of the water would cause an unan- 

 anchored plant to drift, and 2nd, according to Pond (1905), 

 the plant' may absorb from the debris-laden waters the mineral 

 matters which the anchored plant gets from the soil. The 

 optimum habitat of some of the more common plants found in 

 this study and their relation to Physa are as follows: 



I. Optimum Chara Habitat. The most favorable Chara habitat 

 was found in the pond of Kavanaugh swamp. This pond is 

 filled with Chara almost to the very center and nearly to the 

 exclusion of other plants. The shallow water near the shore is 

 so filled that it comes above the surface on the south side, and 

 great banks of it may be seen as the lake shelves off suddenly 

 into deeper water. Kavanaugh Lake, lying north of these ponds 

 a few rods, has no visible Chara on its wave-washed shores but 

 the plant finds a foothold in its deeper, less active waters. It 

 is in Crooked Lake, however, with its many shore modifica- 

 tions that one finds Chara showing a marked correlation to 

 wave and current. Here, as in Kavanaugh Lake, the wind- 

 swept shores have no visible sign of it; but if a small area of 

 shore is slightly protected from the full sweep of the wind, little 

 tufts of Chara not an inch in height and about two inches across 

 may be seen growing here and there near the shore. All grada- 

 tions in size and abundance of Chara are seen, depending appar- 

 ently, upon the protection from waves and the amount of humus 

 in the sand. Chara is found growing in a fairly rapid current 

 in the ditch at Kavanaugh where the loose peaty substratum 

 is held by it from being washed away. In shallow, quiet waters 

 where Chara grows in abundance, no Physa are found, but in 

 luxuriant growths of this plant, where the water courses gently 

 over it, or where it grows in a minimum amount in quiet water, 

 the snail thrives well. 



II. Ceratophyllum Habitat. Very favorable habitats of Cera- 

 tophyllnm are found in the sheltered quiet waters of Crooked 



