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 “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 
