49 



from one another : if they were free, each would become 

 spherical ; but since they are fixed at the ends, they can only 

 bulge out at the sides — that is, increase their curvature, for 

 they cannot increase in length. This they do, and the stoma 

 lends to open. Such a condition of things would be the case if 

 the guard cells were free and isolated, but it is not that which 

 ordinarily obtains. The guard cells, as we saw, are placed at 

 intervals among the ordinary epidermal cells of the leaf, from 

 which they may be distinguished both by their form and by 

 their containing chlorophyll, which the ordinary epidermal 

 cells do not. Their behaviour therefore is influenced by the 

 aggregate behaviour of these epidermal cells, of which they 

 only form a small part. When all the epidermal cells are tur- 

 gid the guard cells tend to swell apart and open the stoma, but 

 this is prevented by the turgid cells of the epidermis, which, 

 since they occupy more space, tend to push them together. 

 Of these two opposing forces, the influence of the epidermal 

 cells is the greater, and consequently, when the guard cells are 

 in the greatest degree of turgidity, the stoma is closed. When 

 evaporation is going on, the cells of the epidermis contract, and 

 fall apart, while the guard cells of the stoma become flaccid and 

 tend to fall together, but are pulled apart by the superior op- 

 posing force of the epidermal cells. The factors acting upon 

 the process of the opening and closing of the stomata are thus 

 not single, but the resultant of a number of forces. 



The parenchyma tissue of the leaf consists of two kinds of 

 cells : — 



First, oblong cells, which touch on all sides, and are packed 

 closely together so as to leave no intercellular spaces. The.-e 

 occur immediately below the layer of epidermis covering the 

 upper surface of the leaf, they are placed perpendicularly to 

 the surface, and they are sometimes known as the palisade 

 layer. 



Secondly, irregularly branched, more or less rounded cells, 

 which of course, in consequence of their shape, cannot touch at 

 all points, but leave spaces intervening between them, which 

 are known as intercellular spaces. These cells, like those of 



D 



