Factors Limiting Closeness of Planting 185 



the mouth at which the tunnel becomes dark ; th^ re- 

 peated reflections from the walls soon absorb completely 

 all of the light which enters. It is the same way with 

 close planting, especially if the individuals are tall, the 

 upper parts of the tall plants absorbing just as 

 much light as the same length of shorter plants, hence 

 leaving less light to work in the foliage and stems of 

 the lower parts. 



Possible insufficiency of carbon dioxide in close 

 planting. When a crop like maize, which grows so 

 tall and spreads its leaves so broadly, is planted closely 

 it seems not impossible that on days of exceptionally 

 bright sunshine and when very little wind is moving, 

 there may be such rapid consumption of carbon dioxide 

 from the air as to so far reduce its amount that an 

 inadequate supply may actually reach the plants. 



It has been shown on a preceding page that a clover 

 crop yielding 4,500 pounds of hay per acre demands 

 for its carbon all of the carbon dioxide contained in a 

 layer of uniform density covering the acre 3,503 feet 

 deep. But in the case of a corn crop, in which the yield 

 of water- free matter has exceeded 14,000 pounds, the 

 volume of air required to give up its carbon dioxide 

 must have exceeded that above more than threefold, 

 or a column of uniform density exceeding 10,509 feet 

 in height. Fully 80 per cent of this assimilation of 

 carbon by the corn plant must take place in the 50 

 days following July 1. Imagine, if you will, a field of 

 corn 160 rods long and 1 rod wide, enclosed by a 

 transparent structure having the same floor space and 

 rising to a height of 10,000 feet, so as to enclose the 



