28 FIELD CROPS 



Many plants which live from year to year store large 

 quantities of food in their roots over winter and are thus 

 able to start into strong growth very early the following 

 spring. If no leaves are produced, no starch can be made 

 and hence none can be stored in the roots. This supplies 

 us with an excellent method of fighting weeds like quack 

 grass and Canada thistles, which are serious weed pests 

 largely because of the food they store and the resulting vigor 

 of their growth. If these plants are prevented from reach- 

 ing the light by continuous cultivation, they will be unable 

 to store additional food, while each attempt at growth 

 reduces the supply in the roots; eventually this will become 

 exhausted and the plant will die. We can readily see that 

 a plant must be well supplied with leaves in order to produce 

 a good crop of seed or fruit, for the leaves furnish the starch 

 from which a large part of the matter in these seeds and 

 fruits is made. If the leaves are destroyed by insects or 

 disease or in any other manner, the production of seed is 

 naturally lessened. 



25. Respiration. The process of respiration is in large 

 measure exactly the opposite of assimilation. It is con- 

 stantly taking place in all parts of the plant, just as animals 

 must breathe continually to live. By this process, a portion 

 of the carbon of the plant is oxidized or changed back to 

 carbon dioxid, but this change is much less rapid than the 

 formation of starch during the day by the leaves, else there 

 could be no growth or increase in weight by the plant. 

 Plants are giving off carbon dioxid constantly, but the volume 

 given off during the day is much less than that taken up, so 

 that the air is purified. At night, no carbon dioxid is taken 

 up, while the process of respiration continues to give it off. 

 For this reason, the air is purer at the close of a sunshiny 



