114 ") 



BOTANY 



PART I 



lower tlian the otlier cells of the epidermis (Fig. 121). Such special epidermal 

 cells are called sirBSiDiARY cells, and assist the opening of the stoma. 



The stoniata are formed by the division of a young epiilermal cell into two cells 

 of unequal size, one of which, the smaller and more abundantly supplied with 

 protoplasm, becomes the stoma mother-cell ; while the larger, containing less 

 protoplasm, usually forms an ordinary epidermal cell. The stoma mother- cell 

 becomes elliptical in outline and divides again, by a vertical wall, into the two 

 guard-cells, between which, by a splitting of the wall, the intercellular passage 

 (pore) is formed. Before the formation of the definite stoma mother-cell, succes- 

 sive divisions of the young epidermal cell often occur ; in such cases the finally 

 developed stoma is generally surrounded by subsidiary cells. 



Stomata are chiefly develoiDed on the green parts of plants, but 

 are sometimes found even on the coloured floral leaves. They are 

 naturally found in greatest numbers on the leaves, as it is there that 





(^ P ^V5>> 



« 



Fig. 122. — Transverse section of the epidermis of Aloe nigricann. i, Inner, iincutinised 



thickening layer, (x 240.) 



they are most needed to facilitate the interchange of gases in the 

 pi'ocesses of assimilation. In dor.siventral leaves the stomata occur, 

 for the most part, if not exclusively, on the under surface, and average 

 about 100 to 300 to the square millimetre, although in some plants 

 their number may reach 700. Leaves Avhich are alike on both sides 

 have their stomata equally distributed on the two surfaces. Floating 

 leaves of aquatic plants have stomata only on the side exposed to the 

 air. In some cases, as in the Oleander {Nerium Oleander), numerous 

 stomata are situated together in pit-like depressions of the under 

 surfaces of the leaves. The largest stomata have been found in 

 grasses ; for instance, the stoma of wheat is 0'079 mm. long and 0"039 

 mm. broad, and has a pore 0"038 mm. in length l)y 0"007 mm. in 

 width. , In the tissue directly under each stoma there is always a 

 large intercellular space, termed the respiratory CAVITY (Fig. 121 B), 

 which is in direct communication witlPtlie" other intercellular spaces 

 extending throughout the leaf tissue. In plants grown in abundance 



