THE WATER-COST OF DRY MATTER 129 



time of bright daylight or of sunshine. Chlorophyll, the 

 green coloring matter of higher plants, and sunshine are 

 indispensable for this wonderful decomposition and new 

 composition. 



A simple calculation will show how actively the leaves 

 of the plant must be at work decomposing carbon dioxid 

 and building up the new compounds derived from the 

 assimilated carbon. It is not uncommon in the irrigated 

 section that an acre of well-developed lucern yields, in 

 one season, 10,000 pounds of dry matter. One-half, or 

 5,000 pounds, of this crop consists of the element carbon, 

 obtained from the gas carbon dioxid of the air by the 

 countless leaves that have swayed back and forth in the 

 air throughout the growing season. Each tiny leaf has 

 done but a small part of the work, but the total gives a 

 lively appreciation of the tremendous activity of plant 

 leaves. 



94. Plant age and carbon assimilation. The rate at 

 which this assimilation of carbon, or "growth," takes place 

 varies wuth the maturity of the plant. To illustrate, at 

 the Utah Station careful measurements were made of the 

 total acre weight of dry matter in a crop of lucern from 

 May 4 to August 24, covering practically the whole of 

 the growing season. In early May, when the plant was 

 well established, the weekly gain of dry matter was 

 something over 300 pounds to the acre; this increased 

 steadily until just before the time of flowering, when it 

 was nearly 800 pounds, after which it gradually decreased 

 until late in July, when there was a loss instead of gain. 

 This represents a general law of plant-growth. At the 

 beginning of the growing season the daily or weekly gains 

 of the crop are small, but they increase steadily and rather 

 rapidly, providing the conditions of growth are favorable, 



