A Hay Infusion.—Still another example of the close relation between plants and animals may be seen in the study of a hay infusion. If we place a wisp of hay or straw in a small glass jar nearly full of water, and leave it for a few days in a warm room, certain changes are seen to take place in the contents of the jar; after a little while the water gets cloudy and darker in color, and a scum appears on the surface. If some of this scum is examined under the compound microscope, it will be found to consist almost entirely of bacteria. These bacteria evidently aid in the decay which (as the unpleasant odor from the jar testifies) is beginning to take place. As we have learned, bacteria flourish wherever the food supply is 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 proteins, 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 and may have been cut in a field near a pool containing these creatures. When the pool dried up, the wind may have scattered some of these little organisms in the dried mud or dust. Some may have existed in a dormant state on the hay and the water awakened them to active life. In the water, too, there may have been some living cells, plants and animals.
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. Consequently a struggle for life takes place, 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 foothold within the jar. If such a thing happens, food will be manufactured within their bodies, a new food supply arises for the animals within the jar, and a balance of life may result.
[24] A small amount of nitrogen gas is returned to the atmosphere by the action of the decomposing bacteria on the ammonia compounds in the soil. (See figure of nitrogen cycle.)
Reference Books
elementary
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Sharpe, A Laboratory Manual for the Solution of Problems in Biology, pp. 133-138. American Book Company.
advanced
Eggerlin and Ehrenberg, The Fresh Water Aquarium and its Inhabitants. Henry Holt and Company.
Furneaux, Life in Ponds and Streams. Longmans, Green, and Company.