HOOTS AND THEIR WORK 



91 



to a plant, but to an animal as well. Foods are digested in the 

 food tube of an animal; that is, they are changed into a soluble 

 form so that they may pass through the walls of the food tube 

 and become part of the blood. Without the process of osmosis 

 we should be unable to use the food we eat. 



Composition of Soil. — If we examine a mass of ordinary loam 

 carefully, we find that it is composed of a number of particles of 

 varying size and weight. Between these particles, if the soil is 

 not caked and hard packed, we can find tiny spaces. In well- 

 tilled soil these spaces are constantly being formed and enlarged. 

 They allow air and water to penetrate the soil. If we examine 

 soil under the microscope, we find considerable water clinging to 

 the soil particles and forming a delicate film around each particle. 

 In this manner most of the water is held by the soil. 



Experiment to illustrate the kind of soil which best retains water: A, gravel; B, sand; 

 C, barren soil; D, rich soil; E, leaf mold; F» dry leaves. 



Kind of Soil Favorable to Evaporation. — The picture shows an 

 easily constructed apparatus to show which kind of soil can retain most 

 water. Fill each of the vessels with a given weight (say 100 grams 

 each) of gravel, sand, barren soil, rich loam, leaf mold, and 25 grams 

 of dry pulverized leaves, then pour equal amounts of water (100 c.c.) on 

 each. Measure all that runs through. The water that has been retained 

 constitutes the water supply that plants could draw on from such soil. 



How Water is held in Soil. — To understand what comes in with the 

 soil water, it will be necessary to find out a httle more about soil. Scientists 

 who have made the subject of the composition of the earth a study, tell us 



