SECTION III. 



WATER AND DRY MATTER IN PLANTS. 



The substances which the plant stores up in its period of 

 growth are made up of the chemical elements previously de- 

 scribed. These elements are not found in the free state in plants, 

 but in various combinations, and may be classified as water and 

 dry matter. 



Kinds of Water in Plants — All plants and parts of plants 

 contain water. The water in plants is of two forms; physio- 

 logical and hygroscopic. 



1. Physiological Water is that which is contained in the plant 

 structure. It is obtained from the soil. It is used to keep the 

 leaf tissues and their cell walls moist so that carbonic acid gas 

 may be absorbed, to transfer food materials, and to regulate the 

 temperature of the plant by means of evaporation of water, just 

 as the temperature of the animal body is regulated by the 

 evaporation of perspiration. 



2. Hygroscopic Water is that which is taken up from the air 

 and may vary from day to day according to the humidity of the 

 surrounding air. On rainy days more water would be taken up 

 than on dry days. The writer has often determined the water 

 content of the same samples of corn meal, wheat bran, cotton 

 seed meal, hays, etc., on different days and found variations 

 of two per cent. Sometimes there is an increase and at other 

 times a decrease of hygroscopic water, depending upon the 

 humidity of the surrounding air. The hygroscopic moisture 

 also varies with different plant materials. 



Amounts of Water Used by Plants. — According to Whitson,^ the 

 amount of water used by plants varies greatly with the kind of 

 plant and with climatic conditions, but is always large. For 

 instance, in the growth of one pound of dry matter of corn about 

 250 to 300 pounds of water are used; for potatoes, 350 to 400 

 pounds ; for clover, 500 to 600 pounds. 



Variation of Water in Plants. — Some species of plants con- 

 tain much more water than others and the different parts of the 



1 Halligan, " Fundamentals of Agriculture." 



2 



