30 THE FEEDING OF ANIMALS 



which varies with the substance and with the conditions 

 that prevail. The surfaces of the particles of matter are 

 ordinarily covered with a thin film of water, which is 

 thicker on a cold, wet day than on a warm, dry day, and 

 so the same quantity of hay or gram weighs less at one 

 time than at another, because the percentage of hygro- 

 scopic water varies. An equilibrium will always be estab- 

 lished between the attraction of a substance for moisture 

 and the tension of the vapor of water in the surrounding 

 air, which accounts for the effect of temperature and of 

 the degree to which the air is saturated with water- vapor. 

 As all substances do not have the same attraction for 

 moisture, therefore, under similar atmospheric conditions, 

 one feeding-stuff may retain more water than another. 



36. Physiological water. Water that is held physio- 

 logically is that which is a constant and essential part of 

 living organisms, in which relation it is necessary to life 

 and performs certain important functions. These func- 

 tions are of three kinds: (1) The presence of water in the 

 tissues of plants and animals gives them more or less firm- 

 ness or rigidity combined with elasticity; (2) water acts 

 as a food-solvent; (3) water is the great carrier of food 

 materials and of waste products from one part to another 

 of the vegetable or animal organism. 



37. Water in living plants. Water constitutes a large 

 proportion of the weight of all living plants, especially 

 during the period of active growth. The cured hay weighs 

 much less than did the green grass when it was cut, and 

 this loss in weight is due almost wholly to evaporation of 

 water from the tissues of the plant under the influence of 

 the sun and wind. This water, which is contained in the 

 cells, tubes, and intercellular spaces of the stalk or leaf, is 

 pure water and has no more physiological value for the 



