ROOTS AND THEIR WORK 91 



up the stem. The pressure created by this process of osmosis is 

 sufficient to send water up the stem to a distance, in some plants, 

 of twenty-five to thirty feet. Cases are on record of water having 

 been raised in the birch a distance of eighty-five feet. 



Physiological Importance of Osmosis. It is not an exaggera- 

 tion to say that osmosis is a process not only of great importance 

 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 much of .the food we eat. 



Problem XV. A study of some of the relations between roots 



a IK I tin' soil. (Laboratory Manual, Prob. XV.} 

 (a) Origin of soil. 

 () Kinds of soil. 



(c) Water-retaining ability. 



(d) Fertility of soils. 



(e) Root hairs and soil. 



(/) Hoot tubercles and, crop rotation. 



Composition of Soil. If we examine a mass of ordinary loam care- 

 full v, we find that it is composed of numerous 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 ex- 

 amine soil under the micro- 

 scope, we find considerable 

 water clinging to the soil par- 

 ticles and forming a delicate 

 film around each particle. In 

 this manner most of the water Inorganic soil is being formed by weathering. 

 is held in the soil. 



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

 soil water, it will be necessary to find out a little more about soil. Sci- 

 entists who have made the subject of the composition of the earth a study, 

 tell us that once upon a time at least a part of the earth was molten. 



