74 General Botany 



Importance of transpiration. Just how important transpira- 

 tion is to the plant may be easily seen by a study of the energy 

 changes that take place in a leaf. First of all, we must understand 

 that light energy is very readily changed to heat energy, and that 

 when heat energy accumulates in a body it raises its temperature. 

 When a body loses heat energy, it is cooled. When the sun shines 

 on a leaf, it is estimated that about lo per cent of the light is 

 reflected by the leaf surface and about 25 per cent goes through 

 the leaf. Sixty-five per cent is retained by the leaf. This is 

 sufficient energy to raise in a few minutes the temperature of the 

 leaf from air temperature to the danger point for protoplasm. 



As soon as the temperature of a leaf rises through exposure to 

 sunlight, the water particles become more active, and as they 

 leave the surface and fly off into the air the excess heat energy is 

 reduced. In this way the leaf is kept at, or within a few degrees 

 of, the air temperature. Transpiration cools the leaf just as 

 water evaporation from your hand cools the skin. Transpiration 

 is important to plants because it helps to regulate the temperature and 

 prevent overheating. As will he shown later, it is also an important 

 factor in raising water and mineral salts from the roots to the leaves. 



It is estimated that nearly one half the energy of sunhght that 

 falls on a cornfield in Illinois is used in transpiration. 



Transpiration and stomata. Most of the water vapor that 

 leaves the plant in transpiration (80-97 per cent) is derived from 

 the mesophyll cells and passes through the stomata from the 

 intercellular spaces. Comparatively httle (3-20 per cent) is 

 lost through the epidermis directly into the air. It is evident, 

 then, that the movements of the guard cells, as they result in 

 opening or closing the stomata, modify the rate of transpiration. 



In most plants the stomata are closed at night, and as there is 

 little or no heat energy added to the leaves, the rate of transpira- 

 tion is very low. The stomata open slowly after sunrise, but as 

 soon as the light strikes the leaf its temperature rises. Trans- 

 piration increases rapidly. Toward the middle of the afternoon 



