in many ways, and considerable variation exists between and 

 within species and at different times of the year. The quantity of 

 organic food reserves in the basal parts of plants often determines 

 the renewal of growth and the winter survival of many plants 

 which have been subjected to adverse conditions. A plant may 

 be rich in food substances, but because of the presence of some 

 unpalatable material such as tannin in lespedeza or coumarin in 

 sweet clover, it is not eaten. The stiffening and hardening of 

 plants, caused chiefly by the formation of lignin in cell walls, 

 proceeds rapidly as plants mature, resulting in decreased forage 

 value. A number of plants growing in the western states absorb 

 selenium from the soil and accumulate it in such concentrations 

 as to cause poisoning of livestock that eat them. On the other 

 hand, some substances are especially good indicators of the qual- 

 ity of forage. For example, routine analyses are made of the 

 carotene content of hay to determine the best methods of drying. 

 For increased accuracy, the part of the plant that is actually con- 

 sumed should be analyzed rather than the entire plant, as exem- 

 plified by the leaves of clover and grass, which are eaten by sheep 

 in preference to the stems. 



Volume Occupied by Plants 



Weight is a more important characteristic when plant growth 

 or productivity is being considered, but the space occupied by the 

 above-ground parts is of greater import in understanding the 

 structure of the vegetation. The word "volume" has occasionally 

 been used in place of weight, especially in dealing with the 

 height-weight relations of range plants. The term "weight" re- 

 fers to the heaviness property of matter, "volume" to the three- 

 dimensional space occupied by an object. Except in forestry where 

 the volume of trunks of trees is measured for the yield of lumber, 

 little work has been done on volume, so it offers a productive field 

 for research. 



The volume of smaller plants may be determined by immers- 

 ing them in water in a graduated vessel and measuring the dis- 

 placement. An analysis in the USSR revealed that the total vol- 



Analytic Characteristics of the Community • lU. 



