164 GREEN PLANTS AS FOOD MAKERS 



Evaporation of water. During the day an enormous amount 

 of water is taken up by the 'roots and passed out through the 

 leaves in the form of vapor. So rapid is this evaporation, or 

 transpiration, in a small grass plant, that the water evaporated 

 in a day may weigh more than the plant. It is estimated that 

 nearly half a ton of water may be given off into the air during 

 twenty-four hours by a grass plot 25 by 100 feet, the size of 

 the average city lot. It is estimated that a corn plant in the 

 Central West gives off more than forty gallons of water during 

 its lifetime. Nearly 20,000 lbs. of water is given off between 

 June and November by a good-sized birch tree. Fields of 

 wheat are said to give off an amount of water equal to nearly 

 20 per cent of the total rainfall on their area. The amount 

 of water lost by plants through evaporation is many times 

 more than the amount that goes into making food and living 

 matter. 



Factors in transpiration. The amount of water lost from a 

 plant varies greatly under different conditions. The humidity 

 of the air, its temperature, and the temperature of the plant all 

 affect the rate of transpiration. The stomata also tend to close 

 under some conditions, thus helping to prevent evaporation. 

 Certain experiments indicate that the plant probably has some 

 control over the stomata. The stomata are usually closed at 

 night but remain open from shortly after sunrise until late in the 

 afternoon. They begin to close in the middle of the afternoon, and 

 thus decrease the amount of water lost in the latter part of the 

 day. Plants droop or wilt on hot, dry days because they cannot 

 obtain water rapidly enough from the soil to make up for the loss 

 through the leaves. Hairs on the leaf surface, waterproofing of 

 outer cells, a decrease in leaf area, close grouping of leaves, the 

 absence of leaves, as in the cactus, and the turning of leaves 

 edgewise to light are all modifications which help to hold water 

 in the body of the plant. 



Green plants give off oxygen in sunlight. In still another way 

 green plants are of direct use to animal life. During the process 

 of sugar-making, oxygen is given off as a by-product. This may 

 easily be proved by the following experiment. 



