DEPENDENCE UPON LEAVES AND CHLOROPHYL 75 



up relatively faster, on account of the crowding of population 

 and on account of the many fires kept going. 



The food of our domestic animals is in large part the 

 leaves of plants, grass, beet tops, and the greater part of 

 hay, alfalfa, clover, and corn fodder; these furnish the prin- 

 cipal green food of cattle and horses. 



The dead leaves of plants, whether those that have dropped 

 in the autumn or those that reach the ground through the 

 death of herbs etc., form the basis of the humus of the soil. 

 Humus is a mass of decaying vegetable matter, with some 

 animal matter and soil. This forms a soil covering that is 

 very helpful from the point of view of retaining moisture in 

 the soil, and to a certain extent in returning nitrogen and 

 other elements to the soil. 



108. Our dependence upon chlorophyl. The parts of a plant 

 that have no chlorophyl (for example, the root or stem of 

 a tree) are unable to make food substances out of inorganic 

 materials ; they are nourished by materials obtained from the 

 leaves. But animals and such plants as mushrooms, having no 

 chlorophyl, must get their food from the bodies of other living 

 things, and in the end all food comes from green plants. 



The value of the food materials taken from our farms in the form 

 of various crops amounts to over $3,000,000,000 a year. This does 

 not take into account the grass eaten by horses, sheep, and cattle, 

 nor the vast quantities that are destroyed by insects and fungi. All 

 of this food results from the work of leaves. It has been estimated 

 that to make a pound of starch it takes a leaf area of about fifty 

 square yards, the leaves working through ten hours of daylight. 



109. Simple food-makers. There were living things upon 

 the earth long before there were any leaf -bearing plants. 

 There must, therefore, have been some way of making food 

 out of simpler substances. Some of the species of plants that 

 are still living and that are capable of manufacturing food are 

 so simple that the whole body of one of them consists of but 

 a single cell. Some of the commoner representatives of these 



