308 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 



how a whole group of the arrangements discussed above, among which the rolled 

 leaf is not the least noticeable, serve, at different seasons of the year, and often at 

 different times of the same day, two distinct purposes, as indicated. 



First, the stomata themselves. While the green tissue has need of food-salts 

 from the soil for the manufacture of organic materials, they cannot be too widely 

 opened; everything is welcome, then, which promotes transpiration, and conse- 

 quently assists in the elevation of fluid nourishment from the saturated soil. But if 

 the temperature and dryness of the air increase after the green parenchyma has 

 finished its yearly task, or if the soil from which the absorbent roots have hitherto 

 derived their supply of fluid become so dry that the water exhaled from the aerial 

 positions can no longer be replaced, it is of the greatest importance that the stomata 

 should be closed. This is brought about by the two cells bounding the stoma, 

 which have been termed the " guard " cells. 



In order to clearly understand the mechanism of the opening and closing of 

 stomata, it is necessary to examine the structure of these guard-cells more in detail. 

 Both are bean-shaped in outline, their concave surfaces being turned towards the 

 stoma; they are only connected with one another at their extremities. By their 

 convex sides they are in contact with ordinary epidermal cells; their outer walls 

 are in contact with the atmospheric air, and their inner walls with the spongy 

 parenchyma. Both the innermost and outermost walls of the guard-cell are 

 strongly thickened, but the wall by which they are connected with neighbouring 

 epidermal cells, as well as that portion which directly borders the stoma, is relatively 

 thin, elastic, and extensible. If the figure of two such guard-cells be imitated in 

 caoutchouc, and they be fitted together like an actual closed stomate — water being 

 forced into them under considerable pressure — the curvature of the thin and elastic 

 portions of the walls will be most altered. The side wall in contact with the 

 neighbouring epidermal cell bulges out, and at the same time the whole cell becomes 

 elongated in a direction perpendicular to the surface. By this means the two 

 guard-cells are forced apart. When the water is allowed to flow out of the swollen 

 caoutchouc cells, they again fall back into position, the two portions of the walls 

 which border the stoma coming into contact with each other and closing the opening. 

 The same thing occurs in the actual guard-cells of the living plant. As soon as 

 they become distended, they separate from one another; when they relax and 

 resume their original position, they come closely into contact again. This process 

 bears a strong resemblance to the changes in the cells of the pulvini at the base of 

 the sensitive leaves of Mimosa, which will be described later, and it is highly 

 probable that it may be traced back to a similar stimulation. That the guard-cells 

 actually separate from one another by swelling up, i.e. by absorbing fluid, and then 

 close together again in consequence of the loss of water, can be shown by first 

 supplying water and then withdrawing it by a solution of sugar. In the former 

 case the stomata open, in the latter they close, and it may therefore be considered an 

 established fact that a closing movement is brought about by the extra loss of water 

 in dry air. But if these pores, through which water vapour escapes when the plants 



