128 IRRIGATION PRACTICE 



increase in transpiration, there will be no need to reduce 

 transpiration. If, on the other hand, the water-cost of 

 the crop is partly independent of the transpiration stream, 

 it may become necessary to decrease or increase trans- 

 piration to a point at which the largest yield of dry matter 

 is produced with the smallest quantity of water. Only 

 when this is done does irrigation give its greatest returns. 



93. Carbon assimilation. Practically one-half of a 

 plant consists of the element carbon. From 2 to 10 per 

 cent consists of mineral matter, taken from the soil, and 

 brought into the plant in the process of transpiration. 

 The remainder of the plant consists of the elements of 

 water combined with carbon and mineral matter to form 

 the variety of plant constituents. 



The carbon, constituting one-half or more of the dry 

 plant, is obtained by the plant from the air through leaf- 

 action. The gas carbon dioxid constitutes about three 

 parts in 10,000 parts of air. As the air washes against 

 the leaves of plants, this gas finds its way into the leaves 

 of green plants through small openings, known as stomata, 

 or breathing pores, which occur in great abundance, 

 especially on the lower side of the leaves. The stomata 

 are delicately adjusted valves which as they open and close 

 are entrances to relatively large open spaces within the 

 leaves themselves. When the carbon dioxid enters the 

 leaves through the breathing pores, it is rapidly absorbed 

 by the juices of the leaves and immediately decomposed 

 into the element carbon and the element oxygen. The 

 oxygen is returned to the atmosphere, while the carbon 

 is retained and combined with water and other substances 

 with the formation of sugars, starches and other valuable 

 plant constituents. This process of carbon assimilation 

 continues without intermission in green plants during the 



