8 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



parts that are modifications of leaves, especially such as are of 

 a leafy texture ; and on the stem. 



52. Stomates have not been found upon the roots, nor on 

 colourless parasitical plants, nor the submersed parts of plants, 

 nor on Fungi, Algse, and Lichens ; they are, moreover, rare, 

 or altogether absent, in succulent parts and in seeds. 



53. It frequently happens, that they are so incompletely 

 formed, as to be either altogether incapable of action, or to 

 act in a very imperfect manner ; as in succulent plants. 



54. The function of stomates is to regulate evaporation 

 and respiration. It has been thought, that the former func- 

 tion, in particular, is that for which they are destined ; and, 

 that the cause of certain parts becoming succulent, is the 

 absence of stomates in sufficient numbers to carry off the 

 watery part of the sap. But some succulent plants have 

 more stomates than ordinary plants, so that this opinion 

 requires reconsideration. 



55. HAIRS are minute expansions of transparent cellular 

 tissue proceeding from the surface of plants. They are of 

 two kinds, lymphatic and secreting. 



56. Lymphatic hairs are formed by vesicles of cellular tis- 

 sue placed end to end, and not varying much in dimensions. 



57. Glandular hairs are formed by vesicles of cellular tis- 

 sue placed end to end, and sensibly distended at the apex or 

 base into receptacles of fluid. 



