THE STOMATA OF PLANTS. 



Cl 



Pig. 107. Exhibiting a front 

 vie\v of four stomata at , im- 

 bedded in hexagonal cellular 

 tissue. 



the cell-walls over which it is placed, Hartig has divided it into three layers an 

 internal, an external, and an intermediate layer ; but such is not the experience of 

 other observers. Its use is to protect the delicate structures lying beneath it, and is 

 analogous to the scarf-skin which protects the skin of man. 



The True Cuticle is composed of one or more layers of cells, the outer one being 

 much flattened (Fig, 106 a). The cells are mostly of hexagonal figure and wavy out- 

 line. Some anatomists have denied the cellular nature of this structure, on the grounds 

 that the cells are not demonstrable, and that the skin may readily be peeled from the 

 subjacent tissue ; but this theory is not usually admitted. 

 Moreover, in the cactuses and orchids, and also in the Ne- 

 rium Oleander, there are several layers of cuticular cells, the 

 whole of which may be demonstrated (Figs. 106 d, and 1 10). 

 Whenever any shred of cutis is removed from the stem 

 of a herb, some portions of woody fibre are removed with 

 it, so that it may be questioned if woody tissue is not a 

 .component of the skin ; but it is perhaps more correct to 

 associate the wood with the structures immediately be- 

 neath the skin rather than with the cellular skin itself. 



Stomata (Fig. 107) are mouths by which respiration 

 and exhalation are carried on in vegetables. They con- 

 stitute openings into and channels through the epidermis, 

 and lead into cavities beneath (Fig, 108, A). Their 

 structure is somewhat complicated, since, for the most 



part, there are a series of rounded cells bounding the opening, with two larger kidney- 



sb.aped cells in the centre, pressing 



A < closely against each other when the 



a stomate is closed, and cemented to the 

 surrounding cells by something in the 

 nature of a hinge, which permits 

 toem to rise and fall with consider- 

 able force (Fig. 108, C a). In the 

 centre of the stomate there is a raised 

 line when it is closed, and a slit when 

 it is open (Fig. 108, C c) ; and through 

 this opening an entrance is effected to 

 the cavity beneath (Fig. 108, A c). 

 This cavity varies in figure and form ; 

 but it is always surrounded by cells, 

 which communicate freely with other cells of the epidermis (Fig. 108, A). It is thus 

 that air and moisture, having entered by the stomata, act not only in the cavity beneath 

 that organ, but in the surrounding open cellular net- work of the leaves or cuticle. 



Such is a general description of the stomata; and before entering further into 

 detail we will request our readers to verify the above account by an examination of 

 these structures. Take a very thin slice from the under surface of a leaf or flower of any 

 plant, as of the lily (Fig. 109, A), the Zea Mays (Fig. 109, B), or the common geranium ; 

 or strip a thin piece of the cuticle of a herb, as of the parsley, and place it in water 

 between two pieces of glass, and examine it with the microscope. First examine tho 

 outer surface, on which may be seen the cells and slit referred to, and then turn over 



Fig. 108. 



A, stomata of the IRIS, a a, green cells bounding: the 

 orifice, b b, cells of the parenchyma, c, air chamber. 



B, the same as seen from above, a a, cells of the stoma. 

 c, opening between them. 



C, stoma of the apple leaf, a, cells of the stomate. bb, 

 cells of the cuticle, c, opening of the stoma. 



