THE WAY OF THE WILD 59 



and the weakest are doomed. Under such conditions there is, 

 of course, a competition for food from the limited amount of 

 soil at hand, as there is also for moisture in time of drought ; 

 but the chief competition is for sunlight. 



All growth in weight of plants is attended by the fixation of 

 carbon from the carbon dioxide of the air, but the process is a 

 chemical one that takes place only in the direct rays of the sun. 

 The growth of plants is therefore absolutely dependent upon 

 their leaves being constantly exposed to direct sunlight. When, 

 consequently, individuals are closely crowded together, only the 

 tallest can push their leaves up into the light, while the others 

 are overshadowed and shut away from the only power that can 

 put carbon into their structure. Accordingly they must die, not 

 exactly from starvation but rather from inability to make use 

 of the plant food of the air. 



This is the principal way in which tall, quick-growing weeds 

 injure crops by getting the start, and, being able to keep it, they 

 kill the crop or greatly check it by shutting off completely or 

 partially the direct sunlight. This is why sweet corn and Kafir 

 corn are so much more difficult to raise than is Indian corn, 

 especially in the moist climate of the so-called corn belt. The 

 plants themselves are at first small and slow-growing, while the 

 weeds of this region are quick-growing with rank stems and 

 broad leaves, which quickly overtop and shut out the sunlight 

 from the crop. 



The same effects will follow the attempts to get a M stand " 

 of alfalfa unless these weeds are kept cut off. The young alfalfa 

 sends up at first but a slender stem with few leaves, and until 

 the root is well established it is no match for rank weeds that 

 reverse the process, namely, expend their first energies in pro- 

 ducing stem and leaf. Indian corn, on the other hand, will, 

 with a fair chance, grow almost as fast as any weed, and in any 

 event always " keeps its head up." 



We take advantage of this principle in killing especially 

 troublesome weeds like Canada thistle and quack grass, which 



