[Chap. XXIV TRANSPIRATION 225 



in Chapters IX and X, a few may influence the rate of transpiration one 

 way or the other, but a casual inspection of leaf differences is not a 

 reliable means of discovering their influence on transpiration rates. Very 

 thick cuticles, waxy layers, and resinous layers on leaves and stems un- 

 doubtedly decrease cuticular transpiration; but their effectiveness in 

 curbing stomatal transpiration of any plant cannot be judged by appear- 

 ances. It can be determined only by carefully planned experiments. 



Stomatal transpiration. Transpiration from mesophyll cells is similar 

 to that from epidermal cells, but there are a few important differences. 

 Let us first consider conditions on a spring morning when the soil is 

 moist and the whole plant is turgid with water. The sun is up and the 

 air is clear. Under these conditions the stomates are fully open. 



The water-conducting tissues of the veins are filled with water slightly 

 diluted by inorganic salts or other substances. The vacuoles, the cyto- 

 plasm, and the walls of the mesophyll cells are nearly saturated with 

 water, and likewise the walls on the inner side of the epidermal cells. 

 This internal moist surface is 6 to 30 times that of the cuticular surface 

 and bounds the labyrinthine intercellular air passages. The energy from 

 the sun increases the molecular energy of the water molecules and their 

 rate of movement is speeded up. They leave the cell surfaces more rap- 

 idly and diffuse in all directions in the intercellular spaces, from which 

 they diffuse through the stomates into the atmosphere. 



Outside the leaf, the atmosphere has a lower humidity and a lower 

 concentration of water molecules. Consequently the diffusion of water 

 vapor will be outward through the stomates. This loss of water vapor 

 from the mesophyll cells is far greater than the evaporation of water 

 from the epidermal cells. Quantitatively it amounts to 85 to 95 per cent 

 of the water that passes into the atmosphere from plants. Evapora- 

 tion of water from cell walls inside the leaf and the subsequent diffu- 

 sion of water vapor through the stomates is called stomatal transpiration. 

 When the leaf only is considered, it might also be called mesophyll 

 transpiration. 



The number of stomates is so great, they are so evenly spaced among 

 the epidermal cells, and the individual pores are so small that diffusion 

 of gases and vapor molecules from the interior of the leaf may be almost 

 as great when the pores are open as if there were no epidermis on the 

 leaf. Later in the day as the stomates gradually close, the stomatal tran- 

 spiration is also reduced, but not much until the stomates are nearly 

 closed, because the rate of diffusion through the stomates is dependent 

 not upon their area, but upon the perimeter of the pore. 



