THE LOSS OF WATER BY THE PLANT 163 



be multiplied several times. This accounts for the great difference 

 between day and night transpiration, which is many times higher 

 than a similar difference in evaporation from any transpiring 

 surface. 



Besides the direct heating effect, light increases transpiration 

 also indirectly. It favors the opening of stomata and increases 

 the permeability of the protoplasm of evaporating cells. Both of 

 these circumstances favor the loss of water by plants. 



The influence of environmental factors controls the daily march 

 of transpiration. It is low in the early morning hours, increases 

 rapidly with the rise of the sun, and the rise in temperature, and 

 increases the saturation deficit, attaining its maximum in the 

 early afternoon and again falling rapidly with the setting of the 

 sun. If curves are plotted for the diurnal changes in transpira- 

 tion, solar radiation, the saturation deficit, and temperature, it will 

 be noted that these curves will almost coincide. Radiation, how- 

 ever, reaches its maximum somewhat ahead of transpiration, while 

 the highest point in the saturation deficit is attained somewhat 

 later (Fig. 69). This shows that sunlight is the chief factor deter- 

 mining the intensity of transpiration. 



A typical daily trend in transpiration is naturally observed 

 only on absolutely cloudless days, when meteorological conditions 

 are normal. It will show sharp fluctuations in changeable weather, 

 largely in accordance with changes in temperature and light. 



52. Leaf Structure as a Factor in Transpiration. Stomatal 

 and Cuticular Transpiration. — The rate of transpiration is deter- 

 mined not only by external factors, but to a high degree likewise 

 by the structure of the leaf, as well as the state of its cells and tis- 

 sues during transpiration. These factors complicate considerably 

 the process of water loss. 



Evaporation proper, that is, the transition of water from a 

 liquid to a vaporized condition, takes place at the surface of the 

 parenchymatous cells lining the intercellular spaces of a leaf. 



These open spaces represent a special aeration system, the out- 

 lets of which are the stomata. The rest of the leaf is covered by a 

 solid layer of epidermal cells the outer walls of which are coated 

 with a cuticle very little permeable to water or water vapor (Figs. 

 11 and 80). 



The cuticle not being entirely impermeable to water, it does not 

 completely stop transpiration. This may be demonstrated by 



