330 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



much more robust than those of the central zone and form much denser 

 masses, while the grasses and sedges of the marginal zone form a 

 dense mass which extends high up above the water. The difference 

 in the density of the stand of Panicum hemitomon and Eleocharis 

 interstincta of the intermediate zone and the grasses and sedges of the 

 marginal zone is very noticeable, the latter forming a much denser 

 stand. It seems that the larger growth takes possession of the soil 

 occupied by a smaller one after the soil has become capable of sup- 

 porting the larger. 



That the soil is the factor which determines the distribution is evi- 

 dent after a comparison of the zones. The depth of water, however, 

 influences the distribution of some species in a zone. Eleocharis 

 robbinsii of the central zone is rarely found in water over 15 inches 

 deep; but if depth were the only factor we should expect to find the 

 plants of the marginal zone where we find Eriocaulon compressum and 

 Eleocharis robbinsii. Some of the sedges of the marginal zone are also 

 affected by the depth, but the plants of the marginal zone are just as 

 vigorous in the deepest water of the lake as elsewhere. It is evident, 

 then, that the depth of the water does not determine the disposition 

 of the zones, for where the soil is muddy the marginal zone is in the 

 deepest water, while, where the soil is sandy, the central zone extends 

 into the shallow water. 



That light and heat are not important factors in this disposition is 

 shown by the fact that the plants of all the zones are exposed to the 

 direct sunlight, while the current is diffused and not strong enough to 

 affect the heat to any appreciable extent. 



That distance from the shore and protection from wind do not play 

 any important part is shown by the projection of the tongue of the 

 marginal zone out into the lake along the canal at the western end, 

 and by the extension of the water lilies along the canal even farther, 

 in separate patches— indeed nearly to the middle of the lake. The 

 marginal zone is 2,200 feet wide at the eastern end, while at the 

 sides it may disappear altogether. Another proof of this is afforded 

 by the patches between the canals at the eastern end, where the soil 

 is removed from the current and is sandy. Here a growth resem- 

 bling that of the intermediate zone is completely surrounded by the 

 marginal. 



Summarizing, briefly, some of the more striking instances which 

 seem to show that the character of the soil determines the zonation 

 are: Between the canals at the eastern end where sandy patches are 

 surrounded by muddy soil the intermediate zone is surrounded by 

 the marginal. At the eastern end where the soil is muddy the mar- 

 ginal zone is 2,200 feet wide and extends far out into the deepest 

 water of the lake, while at the sides where the soil is sandy it may 

 disappear entirely and the central zone occur in the shallow water. 



