2 22 General Botany 



capacity of the soil and so rendering the water supply more 

 uniform throughout the growing season. It improves the 

 physical properties of the soil by making it mellow. Humus also 

 makes it possible for bacteria and other organisms that increase 

 fertiHty to hve within the soil. 



Loam. Soils containing a large percentage of humus are called 



loams. Some of the prairies of IlHnois, Iowa, and southern 



Minnesota were originally poorly drained areas largely covered 



with water during late winter and spring. During the summer 



they dried off and were covered with tall grasses that died down 



to the ground in the late autumn. During the winter they 



became matted together, forming a thick layer of plant materials. 



In time these partially decayed, and each year a new layer was 



added, until after hundreds and thousands of years humus 



accumulated to a depth of from i to 5 feet. Later, when the 



settlers broke the prairie-grass turf and the land^was drained by 



tiles and ditches, these areas became the most productive lands in 



the United States and the center of production of corn and wheat. 



Animals as a factor in plant environment. Leaf-eating insects, 



such as the potato beetle, injure the plant by destroying the 



chlorenchyma and thus preventing food manufacture. It is 



estimated that grasshoppers and other insects often eat as much 



of the grass in a pasture as do the farm animals. Plant lice, 



leaf hoppers, and scale insects remove the sap from the cells of the 



tender growing parts and may kill the entire plants. Plant lice 



and leaf hoppers may also carry disease-producing organisms 



from one plant to another. Other animals, like the earthworm, 



favor the growth of plants by loosening the soil and promote the 



formation of humus by eating and by puUing bits of leaves into 



their burrows. Herbivorous (Latin: herha, herb, and vorare, 



to eat) wild animals, like the rabbits, squirrels, and deer, markedly 



affect natural vegetation, while the domesticated cattle, sheep, 



and hogs to a large extent determine what plants can survive in 



pastures and grazing lands. 



