THE RELATIONS OF PLANTS TO ANIMALS 189 



abundant. The water within the jar has come to contain much of 

 the food material which was once within the leaves of the grass, 

 organic nutrients, starch, sugar, and proteids, formed in the leaf 

 by the action of the sun on the chlorophyll of the leaf, and now 

 released into the water by the breaking down of the walls of the 

 cells of the leaves. The bacteria themselves release this food from 

 the hay by causing it to decay. After a few days small one- 

 celled animals appear; these multiply with wonderful rapidity, so 

 that in some cases the surface of the water seems to be almost white 

 with active one-celled forms of life. If we ask ourselves where 

 these animals come from, we are forced to the conclusion that they 

 must have been in the water, in the air, or on the hay. Hay is 

 dried grass, which may have been cut in a field near a pool con- 

 taining these creatures. When these pools dried up, the wind 

 may have scattered some of these little organisms in the dried 

 mud or dust. Some may exist in a dormant state on the hay, 

 the water serving to awaken them to active life. In the water, 

 too, there may have been some living cells, plant and animal. 



At first the multiplication of the tiny animals within the hay 

 infusion is extremely rapid ; there is food in abundance and near 

 at hand. After a few days more, however, several kinds of one- 

 celled animals may appear, some of which prey upon others. Con- 

 sequently a struggle for life begins, which becomes more and more 

 intense as the food from the hay is used up. Eventually the end 

 comes for all the animals unless some green plants obtain a foot- 

 hold within the jar. If such a thing ha opens, food will be manu- 

 factured witnin their bodies, a new food supply arises for the 

 animals within the jar, and a balance of life results. 



REFERENCE BOOKS 



ELEMENTARY 



Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American 

 Book Company. 



ADVANCED 



Eggerling and Ehrenbcrg, The Fresh Water Aquarium and its Inhabitants. Henry 



Holt and Company. 

 Furneaux, Life in Ponds and Streams. 

 Parker, Biology. The Macmillan Company. 



