104 BOTANY PAHT i 



walls. The elevation serves to increase the evaporation, and is on this account 

 met with in Ferns which grow in damp situations. The depression diminishes the 

 transpiration by creating a still atmosphere above the guard-cells. On this account 

 it is met with in plants of dry regions (xerophytes). Frequently the epidermal 

 cells adjoining the guard-cells are less thickened or lower than the other cells 

 of the epidermis (Fig. 111). Such special epidermal cells are called SUBSIDIARY 

 CELLS, and have the same use as the hinge joint mentioned above. 



The stomata are formed by the division of a young epidermal 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 loss 

 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 definitive 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 developed 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 

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

 processes of assimilation. In dorsiventral leaves the stomata occur, 

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

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

 their number may reach 700. Leaves which 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. In the tissue directly under each 

 stoma there is always a large intercellular space, termed the 

 RESPIRATORY CAVITY (Fig. Ill B), which is in direct communication 

 with the other intercellular spaces extending throughout the leaf 

 tissue. In plants grown in abundance of moisture, these intercellular 

 spaces are usually larger than in the case of plants growing in 

 drier situations. 



In contrast to the stomata, which as air-pores serve for the 

 interchange of gases, a few plants also possess WATER-STOMATA or 

 WATER-PORES, situated at the ends of the veins or nerves of the 

 leaves. These pores serve as organs for the discharge of water or 

 watery solutions. Calcium carbonate, in solution, is frequently 

 excreted in this way, and in many species of Sdxifraga it forms white 

 scales on the margins of the leaves. Although water-pores may 

 often be found at the apices and tips of the marginal teeth of young 

 leaves, they seem to dry up as the leaves become more mature. The 

 guard-cells of water-stomata usually lose their living contents early, 

 and thus the passage between them remains continually open. The 

 water-stomata (Fig. 113) are always larger than the air-stomata. 



