VIII 



THE HISTOLOGY OF THE TISSUES 



95 



FIG. 124. Diagram illustrating 

 formation of stoma. i, young 

 epidermal cell ; 2, division of 

 cell ; 3, the cell-wall split to 

 form the stoma. 



The septum then splits open, the opening constitutes the stoma, 



and the cells form the guard-cells. The size of the stoma 



depends upon the movement of the guard-cells. The stomata 



are found on all the green parts of plants, but they are most 



numerous on the under side of the 

 leaves. If both sides of a leaf are 

 alike, the stomata will be equally 

 developed on both the upper and 

 lower surfaces. In those plants 

 with floating leaves the stomata 

 are found on the upper side only. 

 Stomata can open and shut by 

 the change in the shape of the 



guard-cells. The interchange of gases between the interior of 



the plant and the external atmosphere froin which interchange 



the plant obtains energy and food material goes on through 



the stomata, which also give out watery vapour. 



Some plants have openings in the epidermis by which they 



give out water in a liquid state. Such openings are called 



w a t e r-p ores. 



The water - pores 



are larger than the 



stomata and are 



always open. 

 Hairs. From 



the epidermis hairs 



are produced (a) for 



protection (b} for 



the nutrition of the 



plant. When the 



hair consists of a 



single cell it is said 



to be unicellular ; 



if a number of cells 



enter into the com- 

 position of a single 



hair it is termed 



a multicellular 



hair. The former are found on the roots of plants, where 



they take in water containing minerals in solution. The latter 



FIG. 125. Waterpore, with a portion of epidermis 

 from a leaf. (X 240.) (S.) 



