THE WATER-COST OF DRY MATTER 131 



plant. Mineral food, though taken up in small quanti- 

 ties, is indispensable to plant-growth. Through the action 

 of the mineral foods, the most complicated processes of 

 the plant are initiated and completed. Finally, carbon 

 assimilation and growth cannot proceed unless there is 

 a sufficient supply of moisture in the soil. The heat and 

 light factors cannot well be controlled by the farmer; 

 the mineral food can be controlled in part, but under con- 

 ditions of irrigation the moisture environment of plant 

 roots may be rather easily controlled. It is of first impor- 

 tance, therefore, to the irrigation farmer, to know in what 

 way variation in the moisture supply will affect the total 

 production of dry matter of the crop he is growing. 



96. The transpiration ratio. Dry matter is' that 

 part of a plant which remains when all the water has 

 been driven off by heat near the temperature of boiling 

 water. It is the only part of a plant of real value to the 

 farmer in disposing of his crops for purposes of food, 

 clothing or shelter, for the water contained in vegetable 

 substances is of little more value than water drawn from a 

 stream. The quantity of dry matter produced is, more- 

 over, the best agricultural measure of the sum of the activi- 

 ties of a crop during the growing season. 



The water-cost of the dry matter of plants may be 

 expressed in various ways. The simplest and most easily 

 understood, for the purposes of this chapter, is to speak 

 of the number of pounds of water used in the production 

 of one pound of dry matter. This method of expressing 

 the water-cost of dry matter has been adopted by most 

 investigators of the subject, and it is, therefore, well 

 established. 



The pounds of water required for one pound of dry 

 matter, may, however, be determined in two ways. Water 



