66 



BOTANY: PRINCIPLES AND PROBLEMS 



place (Fig. 35). These are much more numerous in the lower 

 epidermis than in the upper, and, indeed, are often absent from 

 the latter altogether. Each stoma is a slit-like pore formed by 



Fig. 34. — Cross section through the blade of a typical leaf. A, upper epi- 

 dermis, covered with cuticle. B, lower epidermis, also covered with cuticle. 

 C, palisade layer of the mesophyll. D, spongy layer of the mesophyll. E, 

 stoma. F, vein. (After Kny. FromGanong,'" Textbook of Botany" , copyrighted 

 by the Macmillan Company. Reprinted by permission). 



Fig. 35. — A stoma. A, face view, showing the two guard cells (containing 

 chloroplasts) ; the pore between them, and several adjacent cells of the epidermis. 

 B, transverse section, with the two guard cells, several adjacent cells of the 

 epidermis, and a portion of two palisade cells below. {After Kny. From 

 Ganong, " Textbook of Botany", copyrighted by the Macmillan Company. Reprinted 

 by permission) . 



the pulling apart of two modified epidermal cells, the guard- 

 cells, which are unlike other cells of the epidermis in containing 

 chlorophyll. These guard-cells are so constructed that when 



