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: (i) 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 FOOD 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- 



