THE MOLECULAR FORCES IN PLANTS. 



169 



tul apparatus contain chlorophyll grains. The slit of the stoma, 

 and the respiratory cavity below the slit, are also easily made out. 

 The depression above the stoma is caused by the encroachment of 

 the adjoining chlorophyll-free epidermal cells on the guard cells, 

 which they partly cover. 



This depression of the stomatal apparatus below the surface is 

 not found in leaves of Tradescantia virginica, as is shown by 

 examination of thin transverse sections. It is characteristic of 



FIG. 69. Epidermis from the under side of the leaf of Iris florentina. A, from above; 

 B, in transverse section ; /, depression ; s, stoma ; c, cuticle ; a, respiratory cavity. Magn. 

 240. (After Strasburger.) 



the stomata of Tradescantia virginica that they are almost invari- 

 ably surrounded by four epidermal cells, which, as can easily be 

 seen, contain beautiful nuclei. To prove this we examine a shred 

 of epidermis stripped from the under side of a Tradescantia leaf. 

 The stomata are far less numerous on the upper side -of the leaf 

 than on the under side (see Fig. 60). Good material for the study 

 of the stomatal apparatus is also afforded by leaves of Hyacinth us 

 orientalis and Lilium candidum. 



In studying stomata, it will soon be observed that the number 

 of them on equal areas of the leaf is very different in different 



