130 IRRIGATION PRACTICE 



until the maximum rate of increase occurs about the time 

 of flowering. After flowering, as seed-formation sets in, 

 the rate of growth becomes smaller, for, from that time 

 on, the main energies of the plant are no longer directed 

 to the increase in dry matter, but concern themselves 

 more largely with the elaboration of the food materials 

 already gathered into seed to be used for the perpetuation 

 of the species. 



Evidently, since water is unquestionably necessary in 

 plant-growth, the needs of the plant for water probably 

 increase about as the rate of growth increases. From ear- 

 liest spring the water-need of a plant increases, until it 

 reaches a maximum about the time of flowering, after 

 which it gradually diminishes. This supposition, as will 

 later be shown, is confirmed by actual field experiments. 



95. Conditions of growth. Many factors influence, 

 to some degree, the rate of growth of a crop. Most of 

 them are uncontrollable and, therefore, of little impor- 

 tance to the farmer. Those that concern him most, 

 especially under arid conditions are (1) heat, (2) light, (3) 

 oxygen, (4) mineral food and (5) moisture supply. With 

 given vitality and inherent qualities, these factors will 

 act vigorously upon the assimilation of carbon. If the 

 temperature is too low, the life activities of the plant 

 become slower and may finally cease. The higher the 

 temperature, within a rather large range, the more rapidly 

 does growth go on. Light, especially sunlight, is another 

 powerful factor in furthering the assimilation of carbon. 

 Oxygen is a prime factor in plant-growth, for without it 

 the processes of oxidation, corresponding to breathing in 

 the higher animals, cannot proceed; and, without this 

 function, plant life cannot long persist. There must be, 

 therefore, an abundance of fresh air playing about the 



