232 TEXTBOOK OF BOTANY 



Summary. The principal internal factors that influence the rates of 

 evaporation and diffusion of water vapor from plants are: ( 1 ) the open- 

 ing and closing of the stomates; (2) the concentration of water vapor in 

 the internal air spaces of the leaf in comparison with that of the atmos- 

 phere; (3) the temperature of the leaf; (4) the water content of the 

 plant tissues, as affected by the rate of movement of water from the 

 soil; (5) the occurrence in the cells of colloidal gels which have a high 

 water-holding capacity, such as pectic compounds, mucilages, and 

 gums; and (6) the cutinized epidermal walls in some plants. 



Wax and resinous coatings on the epidermis may reduce cuticular 

 transpiration, but their effect on stomatal transpiration is slight. Dead 

 hairs, even when forming felt-like coverings, decrease transpiration very 

 slightly or not at all; living hairs increase it to some extent. The value 

 of any superficial structures in conserving water cannot be estimated by 

 examining the plants. Their effects can be determined only by careful 

 experimentation . 



The most important external factors that affect the rate of transpira- 

 tion are: (1) the energy of sunlight as it affects the internal tempera- 

 tures of leaves in relation to external temperature, and the opening and 

 closing of the stomates; (2) the temperature of the atmosphere and the 

 soil; (3) the concentration of water vapor in the atmosphere; (4) the 

 water conditions in the soil; and (5) the movement of air. Wind pre- 

 vents the accumulation of moist air about the plant surfaces; but when 

 the velocity is increased beyond that necessary to remove the layer of 

 moist air, there is little further increase in transpiration. 



The evaporation and diffusion of water from the mesophyll cells are 

 followed by the osmotic movement of water from the veins into the 

 mesophyll cells. This movement in turn exerts a pull on the water col- 

 umns in the veins of the leaf, stem, and roots, and probably on the water 

 in all the other cells of the stems and roots. This is commonly called the 

 pull of transpiration. 



REFERENCES 



Meyer, B. S., and D. B. Anderson. Plant Physiology. D. Van Nostrand Com- 

 pany, Inc. 1939. 



Sayre, J. D. Relation of hairy leaf coverings to the resistance of leaves to 

 transpiration. Ohio Jour. Sci. 20:55-86. 1920. 



Thut, H. F. Relative humidity gradient of stomatal transpiration. Amer. Jour. 

 Bot. 26:315-319. 1939. 



Turrell, F. M. Area of the internal exposed surface of leaves. Amer. Jour. Bot. 

 23:255-264. 1936. 



