222 TEXTBOOK OF BOTANY 



vapor from them, so that they may be safely stored for future use. The 

 harvesting of many seeds, grains, and hay involves the drying, or the 

 acceleration of transpiration, under natural or artificial conditions. High 

 transpiration may result in the death of young flowers and in the cessa- 

 tion of sexual reproduction. 



Of the various plant organs the thin blades of the leaves of most plants 

 have the largest evaporation surface exposed to the atmosphere, and it 

 is in them that the greatest amount of transpiration occurs in the grow- 

 ing plant. 



The bulk of the growing parts of plants is water. As we learned earlier, 

 all the living cells of a plant contain protoplasm and a vacuole filled 

 with water in which various substances are dissolved. Young tissues of 

 plants may contain as much as 95 per cent water by weight, and older 

 parts 60 to 75 per cent. When the water content of the active cells of 

 plants is gradually decreased they become less and less active until a 

 point is reached where injury or death results. 



The leaves of most plants, therefore, with their high water content 

 (60-85 per cent), their relatively large areas, and their exposure to an 

 atmosphere that is usually only 10 to 75 per cent saturated, constitute 

 the most important surface of water loss from plants. Discussion in this 

 chapter, therefore, will be confined largely to the processes and factors 

 that influence transpiration as it occurs in leaves. 



Circumstances of water loss from leaves. In Chapter VIII the organi- 

 zation of water-conducting tissues and their distribution among the 

 mesophyll cells are described. Attention was called to the air spaces 

 among these myriad cells and also to the epidermis which encloses the 

 mesophyll cells, air spaces, and veins. 



The upper epidermis of the leaves in many species of plants is an 

 unbroken layer of cells. Its outer cellulose walls may be thick and have 

 a cutinized outer layer which is commonly called the cuticle. The lower 

 epidermis, and frequently the upper, have stomates distributed among 

 the epidermal cells. Hence when the stomates are open the epideiTnis of 

 the leaf contains many thousands of minute passages through which 

 gases may diffuse into the labyrinth of air passages among the mesophyll 

 cells, or diffuse out of them. Any gas may do this, and its diffusion is 

 quite independent of the diffusion of other gases or vapors. 



Molecular motion and consequently evaporation are accelerated by a 

 rise in temperature. When the surface molecules of the water acquire 

 sufficient momentum to overcome the cohesion bv which thev are held 

 in the liquid state, they diffuse into the surrounding air. The same is 



