TRANSPIRATION 29 



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 the day. 



26. Transpiration. The third important work of the 

 leaves is the 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 takes in carbon dioxid 

 and gives off oxygen in the assimilation process and also 

 gives off carbon dioxid in the respiration process. These 

 pores are ordinarily open so that water passes from them 

 freely in the form of vapor. When the supply of moisture 

 which can be obtained from the roots decreases, or the air 

 conditions are such that water is drawn from the plant more 

 rapidly than it can be supplied by the roots, these stomata 

 gradually close, thus checking transpiration and tending to 

 maintain the proper quantity of water in the leaves and 

 tissues of the plant. Some plants possess the power of 

 retaining their water content to a marked degree and are 

 able to live with a very small water supply. Cacti and other 

 desert plants have this characteristic ; the sorghums are 

 among the most drouth-resistant of cultivated crops. 



The quantity of water transpired by plants during their 

 period of growth is enormous. It is estimated that corn 

 gives off 270 pounds of water for every pound of dry matter 

 which is produced, while nearly twice this quantity is tran- 

 spired by oats in the making of a pound of dry matter. 

 The quantity of water transpired varies with the kind of 

 plant and the climatic conditions; where evaporation is very 

 -rapid, the quantity of water required by plants is greatly 

 increased. 



