CHAPTER XII 

 THE LOSS OF WATER FROM PLANTS 



It is commonplace knowledge that all plants require water for their exist- 

 ence and development and that most plants require it in considerable quan- 

 tities. It is not so generally recognized, however, that in most species of 

 plants an overwhelmingly large proportion of the water absorbed from the 

 soil is lost by the plant into the atmosphere and takes no permanent part in 

 its development or in its metabolic processes. The lack of this general realiza- 

 tion is probably due to the fact that while water is supplied to and absorbed 

 by plants in its familiar liquid form, by far the greater part of that lost 

 escapes in the invisible form of water-vapor. 



The loss of water-vapor from living plants is known as transpiration. 

 Loss of water-vapor may take place from any part of a plant which is exposed 

 to the air. This applies even to roots in contact with the soil atmosphere. 

 Generally speaking, however, the leaves are the principal organs of transpira- 

 tion. Most of the transpiration from leaves occurs through the stomates; 

 this is termed stomatal transpiration. Smaller amounts of water-vapor are 

 lost by direct evaporation from the epidermal cells through the cuticle; this is 

 usually called cuticular transpiration. All aerial parts of plants lose some 

 water by transpiration, although, due to the presence on some organs of super- 

 ficial layers almost impervious to water, the rate of loss from most such 

 organs is very low. Some of the transpiration from herbaceous stems, flower 

 parts, fruits, etc. is of the cuticular type, but is small in amount. IVIost her- 

 baceous stems, fruits, and flower parts bear stomates which permit the occur- 

 rence of stomatal transpiration from such organs. Loss of water-vapor also 

 takes place through the lenticels of woody stems; this is usually called lenticu- 

 lar transpiration. 



The Mechanics of Foliar Transpiration. — The subsequent discussion 

 will deal almost entirely with transpiration from leaves, since, in most plants, 

 the amount of water-vapor lost from other organs is comparatively small. 

 The mechanics of foliar transpiration can be adequately discussed only in 

 reference to the anatomy of the leaves from which it occurs (Fig. 34, Fig. 35, 

 and Fig. 36). It should be recalled that the vacuoles of all of the living cells 



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