88 



Plant Physiology 



water elimination of about 600 tons. Land covered by 

 grass or clover may lose during the growing season from 

 500 to 750 tons of water, almost entirely through the sur- 

 faces of the growing plant. 



52. The mechanism permitting transpiration. The 

 elimination of water from the surfaces of plants takes 

 place because of the fact that the leaves or other surfaces 

 are not wholly impermeable to water-vapor. In the case 

 of delicate, especially young, leaves or shoots there may be 

 some loss of water directly through the epidermis, which 

 is then relatively uncutinized, or otherwise unprotected 

 against water-loss. In many instances this amount is 



FIG. 26. Section of tomato leaf: epidermis (e), palisade tissue (p), paren- 

 chyma (0), vascular bundles (r), and stomata (s). 



negligible, and just as the continuous cuticle commonly 

 absorbs practically no water, so it does not permit of elim- 

 ination. The epidermis, however, of one or of both sur- 

 faces of the leaf and of other delicate parts may be pro- 

 vided with numerous pores or stomata (Figs. 25 and 26) 

 which are the most important means of communication 

 between the internal tissues and the external air. 



The stomata open and close in response to complex 

 internal conditions, and under certain circumstances 

 external factors may perhaps play at least a secondary 



