224 STRUCTURAL BOTANY 



pore between them closes (see Fig. 92, A). Now, how 

 are we to explain this ? The pure water is absorbed 

 into the guard-cells by the denser cell-sap, and so 

 their turgidity is increased ; conversely, the com- 

 paratively dense salt solution withdraws water from 

 the guard-cells, and so they tend to collapse. But 

 why do the guard-cells become more curved as their 

 turgidity increases ? This is mainly due to the ridges 

 by which the wall of each cell is strengthened on the 

 side towards the pore.- This side of the cell is more 

 rigid than the other, and so offers more resistance to 

 stretching when water is taken up. Hence the more 

 turgid the guard-cells become, the more do their outer 

 walls stretch as compared with their inner walls, and 

 hence the cells become more and more convex towards 

 the exterior, and concave towards the pore. When 

 water is withdrawn from the cells, just the reverse 

 happens ; the outer walls shorten more than the inner, 

 and so the guard-cells tend to become straight and to 

 close the pore between them. The mechanism of the 

 stomata is in reality much more complex than appears 

 from the above account, but the changes described are 

 among the most important of those which bring about 

 the opening and closing of these organs. We have 

 learnt, then, this much : the stomata open in light, 

 and they open when the guard-cells are turgid. Can 

 we, however, explain why the guard-cells become turgid 

 in light ? The following explanation has been given. 

 We know that as a rule the guard-cells of the stomata 

 are the only cells in the epidermis which contain 

 chlorophyll granules ; in other words, they are the 

 only epidermal cells which can assimilate for them- 



