AERATING TISSUE 157 



tained. If the guard cells come in focus first, they are above 

 the surface; if the guard cells and the surrounding cells come 

 in focus at the same time, the stomata are even with the sur- 

 face; if the stomata come in focus after the surrounding cells, 

 they are below the surface of the leaf. The relationship of 

 the stoma to the surrounding cells should always be ascertained, 

 not only in cross-sections of the leaf, but also in powders. 



There is the greatest possible variation in the size of guard 

 cells. This fact must always be kept in mind when studying 

 leaves. This variation in the size of the guard cells is clearly 

 illustrated by coca, senna, and by deer's- tongue. In coca the 

 stomata are very small; in senna they are larger; while in 

 deer's-tongue the stomata are very large. 



The width and length of the stoma or opening between the 

 guard cells are of a character which must not be overlooked. 

 Generally speaking, those leaves which have large guard cells 

 will have correspondingly large stomata. 



The guard cells usually contain chloroplasts showing various 

 stages of decomposition. 



In bay-rum leaf the guard cells are of a bright reddish- 

 brown color, but in most leaves the guard cells are colorless. 



LENTICELS 



Lenticels are small openings occurring in the bark of plants. 

 The lenticels bear the same relationship to the stem that the 

 stomata do to the leaves. Lenticels, like stomata, have a three- 

 fold function — namely, exchange of gases in photosynthesis, 

 in respiration, and the giving off of water. 



Lenticels are macroscopically as well as microscopically 

 important. When unmagnified the lenticels are circular, lens- 

 shaped, or irregular in outline. They are arranged in parallel 

 longitudinal lines or parallel transverse lines, or they are ir- 

 regularly scattered. The latter is the usual arrangement. In 

 most cases they are elevated slightly above the surface of the 

 bark. In root barks particularly the lenticels stand out promi- 

 nently from the surface of the bark and in many cases appear 

 stalked. 



The color of the lenticels differs greatly in the different 



