406 STOMATA. PART n. 



cellular tissue compressed into a tubular form, which 

 constitutes the cuticle. These flattened cells, which are 

 firmly united to one another by their edges, differ in 

 shape in almost every tribe of plants. In the monoco- 

 tyledons they are elongated in the direction of the 

 parallel ribs of the leaves ; in the highest class they 

 differ little from circular discs, but they have large 

 sinuosities in their edges, which make their junction 

 very irregular. The upper cells of the cuticle are lined 

 with a waxy substance, which protects the plant from 

 damp ; and in many cases it contains more or less silex. 

 This general covering, or cuticle, is perforated by 

 numerous pores, especially on the under-side of the 

 leaves, and the green tender parts of the branches; 

 they are the organs of respiration. These pores, or 

 stomata, are usually formed of two crescent-shaped 

 cells, joined together by their points or horns, so as to 

 leave an open space like a mouth, through which the 

 plant breathes. These, however, are only the guar- 

 dians : the opening between them leads into a cavity 

 full of air, which is the antechamber to an interior 

 cavity. The valves of the stomata open -and shut ac- 

 cording to the humidity or dryness of the atmosphere. 

 All plants of both classes, woody or herbaceous, have 

 stomates, except water plants, fungi, fuci, and others 

 of the lowest class. They are generally very abun- 

 dant on the under-surface of leaves. They are some- 

 times in vast numbers on both sides of the leaves, and 

 are essential to the life of the plant. According to MM. 

 Payen and Liepner, silex and azote, together with cal- 

 careous and alkaline salts, are invariably found in the 

 cells forming the skin of the roots, stems, leaves, fruits, 

 hairs, and spines. The strong cohesion of the skin, to- 

 gether with the presence of these substances, becomes 

 a defence against the wearing effect of the weather, 

 without diminishing the transparency of the tissues. 



