28 FIELD CROPS 



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, j ust as ani- 

 mals must breathe continually to live. By this process a 

 portion of the carbon of the plant is oxidized, or changed 

 back to carbon dioxide; 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 dioxide constantly, but the vol- 

 ume given oft' during the day is much less than that taken 

 up, so that the air is purified. At night, no carbon dioxide 

 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 summer day than it is the following morning. 

 For this reason, also, growing plants are not desirable in a' 

 sleeping room at night, though they help to purify the air 

 in the house during tlie day. 



26. Transpiration. The third important work of the 

 leaves is tlie giving off of water, or transpiration. On the 

 green, growing portions of the plant, but more particularly 

 on the under side of the leaves, are minute pores, or stomata. 

 It is through these pores that the plant takesin carbon dioxide 

 and gives off oxygen in the assimilation process and also 

 gives off carbon dioxide in the respiration process. These 

 pores are ordinarily open so that water passes from them 

 freely in the form of vnpoi-. When the supph^ of moisture 



