THE WALLFLOWER 



71 



which the intercellular passages of the whole plant, con- 

 taining air and watery vapour, open into the external 

 atmosphere. 



A section across a stoma shows the two guard-cells 

 with the pore between them (Fig. 26). The transverse 

 section of each guard-cell is roughly a square; the 

 walls are very thick, especially at the two corners 



FIG. 27. Portion of epidermis from the under side of a Wall- 

 flower leaf, in surface view, h, h t hairs ; S, S, stomata. 

 Magnified about 210. (R. S. ) 



towards the pore, which are provided with projecting 

 ridges. We have already learnt that the guard-cells 

 contain chlorophyll-corpuscles and starch granules, and 

 thus differ from the ordinary epidermal cells. 



The stomata have the power of opening and closing. 

 As a general rule, they open under the influence of 

 light and warmth, and close when it is dark or cold. 



