VEGETATION AND FOOD SUBSTANCES 65 
to the present shore, sand and gravel again. It is seen that this lower 
level of the lake influenced both the topography and bottom material 
locally, both of which probably have an influence on the occurrence of 
certain animals. 
6. VEGETATION 
The amount and kind of rooted vegetation is very important to 
animals. Of all the aquatic situations with which we have to deal 
Lake Michigan has fewest attached plants, and these are all algae. 
Cladophora, Chara, and filamentous algae are the most important. 
These do not appear to have been recorded below about 25 meters; 
some of them require solid bodies for attachment, and are probably most 
abundant on the rock outcrops of shallow water. 
The vegetation of the younger streams consists largely of holdfast 
algae like those along the rock shores of the lake. These are of impor- 
tance to animals. The more sluggish streams have rooted aquatic 
vegetation. 
The vegetation is used as breeding-places. Eggs are stuck into plant 
tissues by the predaceous diving beetles (Dytiscidae) and by the water 
scorpions (Ranatra). Eggs are attached to plants by the electric-light 
bugs (Belostomidae), back-swimmers, May-flies, caddis-flies, water 
scavengers (Hydrophilidae), long-horned leaf beetles (Donacia), snails, 
and many fish (Umbra, and probably Abramis). Young animals are 
often dependent upon plants for shelter, to escape from enemies, etc. 
Many animals must use plants as a means of reaching the surface for 
oxygen. The most important of these are the Dytiscidae (adults and lar- 
vae), the Hydrophilidae (adults and larvae), the back-swimmers, Zaitha, 
Belostoma, Donacia, snails, Ranatra, and Haliplidae. Some, for example 
Zaitha and dragon-fly nymphs, lie in the vegetation and wait for their prey. 
Different kinds of vegetation have different values for animals. 
The bulrush is barren for the following reasons: (1) hardness makes it a 
bad place for eggs; (2) there are no clinging-places; (3) there is little 
shade; (4) it gives a high temperature in summer; (5) there is no great 
addition of oxygen by vegetation; (6) it does not afford a suitable place 
for securing food. Equisetum is unfavorable for similar reasons. Elodea 
is excellent; Myriophyllum, good; water-lilies and Chara, only fair. 
IV. ELEMENTARY Foop SUBSTANCES (47) 
Nitrogen, in the form of nitrates, is necessary for the growth of the 
plants of a pond, lake, or stream, and an insufficient quantity is secured 
from mineral soil. Nitrogen can be taken from the air only by nitrogen- 
