170 



PHYSIOLOGY OF NUTRITION. 



plants, or on the upper and lower surfaces of the same leaf. So, 

 e.g., according to Weiss, 1 the number of stomata on 1 sq. mm. of 

 leaf surface is in : 



The counting of the stomata is certainly troublesome, but the 

 method of procedure is simple. We remove strips of epidermis 

 from the upper and under sides of mature leaves, lay them in a 



FIG. 60. Epidermis of the under side of the leaf of Tradescantia virginica. A, from 

 above; B, in transverse section; I, leucoplasts on the cell nticlei. Magn. 240. (After 

 Strasburger.) 



drop of water on the slide, and put on the cover-glass. We then 

 count the stomata observable in the field of view, and take the 

 mean of a series of such observations. The actual area of the 

 field of view is easily calculated from its diameter as determined 

 by means of a stage micrometer. It only remains to estimate 

 from the number of stomata on this area, the number on unit area, 

 e.g. 1 sq. mm. 



In speaking of the stomata of plants, it is important to make 

 mention of a peculiarity which is of great significance in connec- 

 tion with the gas exchange, and also in connection with the process 

 of transpiration to be discussed later. It is that the slit between 

 the guard cells of the stoma is by no means always of the same 



