136 



BOTANICAL GAZETTE 



[august 



unfavorable for some other swamp plants as they are for sphag- 



nums. 



8 



The plants might be classified as to edaphic relations in the 

 following manner : argillaceous soil, unattached microscopic 

 plants, diatoms, etc.;9 calcareous and are?iaceo7is soils, Potamogeton 



Fig. 4. — Carex and Nuphar zones, showing the massing of these plants; July I 



zosteraefoHus, Blue-green algae (rare), Scirpus lacustris, Potamo- 

 geton natans, Chara coronata, Equisetum, Panicum Crus-galli; 

 carbonaceous soils, Nuphar, Tjpha, and Carex societies, Salices, 

 Sphagnum, Populus. 



{c) Atmospheric factors. — The most important of these is lights 

 because, as I believe, it is this which, aside from food supply^ 

 chiefly governs the encroachment of one zone upon another. A 

 moment's consideration will show that the plants struggling for 

 light will have better illumination on the side of the center of 

 the lake than on the side toward the land. Take, for example^ 

 the Carex zone {^Jig. 4), The young Carex and Sphagnum 



^Ganong, W. F., Trans. Roy. Soc. Canada, 3 : 131. 1897. 



9 Under better conditions of light, Chara and Potamogeton would doubtless occur- 



