THE LARGER AQUATIC VEGETATION 195 



Wisconsin species in the New York lake must be attributed to 

 some other cause or causes. Experimental evidence is lacking for 

 stating to what extent the chemical composition of the water 

 becomes a determining factor in fresh water. 



The important influence of temperature on distribution is at 

 once apparent, although the influence exerted by it is much farther 

 reaching than is at first supposed. Of course, the forms of the 

 arctic waters would be "cooked" by the heat of the tropical waters. 

 It has recently been ascertained that the colder waters support a 

 more abundant plankton than the tropical waters, and one writer 

 has explained this fact upon the basis that the colder waters are 

 richer in nitrates and nitrites because the decomposition of organic 

 matter proceeds much more slowly and the organisms causing decay 

 are not so numerous and less active. Such considerations offer a 

 gHmpse of how indirectly, yet effectively, the various factors may 

 operate to determine distribution. 



The competition of species for space may be considered a factor 

 in horizontal distribution, although it operates quite locally and 

 does not work to modify the flora of large areas. Chara is infre- 

 quent on the alluvial bottoms where other species are present in 

 abundance, but this is not because it prefers poorer soils but becavise 

 it is prevented from occupying the soil of its choice by the other 

 species. 



The character of the substratum is an important factor; in fact, 

 it is possible to predict the nature of the bottom from the species 

 that are found growing in it. Among the islands of western Lake 

 Erie Potamogeton heterophyllus is common on the reefs and pebbly 

 shores but is not noticeable in the coves with a good soil substra- 

 timi and so prominent is it in the former places that its presence 

 may be considered characteristic of the flora. 



Light cannot be counted a factor in horizontal distribution be- 

 cause there is always sufficient illumination within the limits estab- 

 lished by other factors. In vertical distribution light is probably 

 the most important factor. The amount of illuminating power 

 lost in passing to a given depth is quite variable in different waters. 

 Fol and Sarasin found in Lake Geneva, in September, light at 

 170 meters, and at 120 meters a photographic plate was darkened. 



