245 



eastern border and the encroachments of the semiaquatic bot- 

 tom-land plants but slightly vary the uniformity of the vegeta- 

 tion, which at low-water stages fills the greater part of the lake. 



In such lakes as Dogfish and Quiver the vegetation by 

 reason of its predominance exerts a profound influence upon 

 the quantity and the constitution of the plankton. Its fluctua- 

 tions in quantity with the change of the seasons and the inva- 

 sion of flood water are attended by marked readjustments of 

 the plankton. 



Phelps Lake (Station F). PI. XXI. — Phelps Lake was prac- 

 tically free from vegetation throughout the period of our plank- 

 ton collections. The drying up of the lake in 1894 and its cul- 

 tivation in 1895 destroyed whatever foothold the aquatic veg- 

 etation had obtained. In the following years the ingress of the 

 aquatic flora become increasingly evident, though the alluvium 

 in the bed of the lake, hardened by the drouth, gave but scant 

 foothold to marsh-loving plants, especially to the perennial 

 species or to those with well-developed roots. Each spring saw 

 here a remarkable development of "blanket moss," a mat of 

 algee, principally Spirogyra, Zygnema, and Cladophora. In the 

 period of midsummer stagnation a dark green film of Oscillaria 

 coated the bottom or rose to the surface in scattered masses. 

 The fringe of button-bush (Cephalanthns occidental is) and 

 willows (Salix nigra and S. longifolia) at the edge of the sur- 

 rounding forest gave shelter to a few semiaquatic Compositce and 

 rushes, and beyond these there were scattered clumps of Pota- 

 mogeton natans and Nais flexilis var. robusta, which found a 

 place even in the first year in which the water reentered the 

 lake. In 1899 the margin occupied by these plants had increased 

 in width, and arrowleaf and lotus were represented by a few 

 isolated plants, while the ubiquitous CeratophyUum had made 

 its first appearance in the lake. Aside from the algae, the aquatic 

 flora formed but a small part of the environment of the plank- 

 ton in this body of water. 



Thompson's Lake (Station G). PI. XX. — This lake combines 

 in one area almost the whole range in the development of the 



