206 The Living Plant 



which case a wilting results even though water is plenty in the 

 soil; but plants thus wilted can quickly recover when the weather 

 grows cooler, for then the absorption and conduction catch up, 

 so to speak, and again fill the leaf. Light increases, and darkness 

 lessens it. This harmonizes with our transpiration constants, 

 which showed that in general the process is five times more active 

 in daylight than at night; and it explains why plants that wilt in 

 the day recover at night. Dryness increases, and humidity lessens 

 it. This is the reason why most kinds of plants will not live in our 

 houses, the air of which is so dry that the leaves lose their water 

 much faster than roots and stems can supply it, no matter how 

 plenty in the soil. It explains, too, why leaves never wilt in the 

 weather called muggy, no matter how hot, and also why leaves 

 that are wilted recover when sprayed, even though experiment 

 proves that none of the spray is absorbed. As to other external 

 climatic conditions, their influence is slight, except in the case of 

 the wind, which always promotes it. Thus it is evident that in 

 general transpiration is promoted by the very same factors which 

 favor evaporation, though later studies have shown that the 

 parallel does not hold true in detail. 



We must now consider the structural basis of transpiration, 

 with which, however, the reader already has incidentally made 

 some acquaintance. If he will recall his knowledge of the cellular 

 structure of the leaf, refreshing his memory, perhaps, by another 

 inspection of figure 2, Plate I, B, and figure 54, B, it will be clear 

 that every cell borders, for purposes of respiration and photosyn- 

 thesis, upon the inter-cellular air-system, which ramifies through- 

 out the leaf and opens to the outside world through the stomata, 

 the little slit-like openings through the otherwise continuous 

 epidermis. Now these cells are all gorged with water, which 

 saturates their walls; and where these border on the air spaces 

 the water necessarily evaporates. The vapor thus formed satu- 

 rates the air inside of the leaf, and is then moved by the force of 

 its own diffusion along the passages and through the stomata to 



