TRANSPIRA TION 



41 



regulation of transpiration may undoubtedly occur which are dangerous to 

 the plant, for we must not forget that the stomata are not merely the organs 

 of transpiration, but are primarily the means by which carbon-dioxide enters 

 the plant. Everything that retards transpiration must at the same time 

 hinder the absorption of carbon-dioxide, and hence clashings may occur be- 

 tween the two processes, when the plant must naturally suffer from one cause 

 or the other. Conflicting conditions of this kind are to be met with whenever 

 we investigate natural phenomena closely. 



We have now to study separately the more important factors which play 

 a part in the movements of the stomata, and foremost among these we may place 

 the vapour tension of the atmosphere. For purely physical reasons, abundance 

 of moisture in the air must, as has already been shown, retard transpiration ; 

 but at the same time it will induce the stomata to open widely, and thus the 

 physical condition will be in a sense neutrahzed. In dry air on the other 

 hand, evaporation is increased, while transpiration is reduced because many 

 plants close their stomata at once when wilting begins. It is easily seen how the 

 degree of humidity of the air will act on the guard-cells sooner than on other 

 cells ; in damp air they attain the utmost osmotic turgor which the cell-sap 

 is able to exert when water absorption is at a maximum ; when loss of water 

 takes place owing to increasing dryness of the atmosphere, if at the same time 

 an abundant supply of water is not forthcoming, a rapid reduction in the turgor 

 ensues, accompanied by closing of the stoma. Closure of the stoma is, how- 

 ever, by no means always a concomitant of wilting ; in many plants the wilted 

 leaf has its stomata wide open. Such leaves continue to give off water, and 

 shrivelling of the leaf takes place with great rapidity. Plants of this kind can 

 exist only in regions where the moisture is excessive, as, for example, among 

 our native plants, those inhabiting marshes {Alisma, Acorus, Menyanthes, &c.) 

 and shady places {Osmunda regalis). The cobalt-paper method will be found to 

 be convenient in investigations of this sort, for by this method we may convince 

 ourselves, for example, of a vigorous transpiration from the half-dried leaf in 

 the plants above mentioned, where the stomata remain open, while in Tro- 

 paeolum majus this cannot be observed owing to the rapid closure of the stomata 

 on wilting. Further, actual wetting will induce wider opening of the stomata 

 than a damp atmosphere. This explains the result obtained by Wiesner 

 (1882), who found that there was a marked increase in transpiration after the 

 leaves had been dipped in water. Under certain conditions a quite contrary 

 effect may be produced, as, for instance, when the neighbouring cells absorb 

 water and the stomata are in consequence passively compressed (Kohl, 1886), 

 or when the pores are blocked by capillary water. 



The stomata of different plants do not always react in the same way to 

 the second factor, which we may allude to here — namely, light. Frequently, e.g. 

 in Amaryllis, Aspidistra, &c., we may observe that the slits open when the leaves 

 are more brightly illuminated. Here again, since for purely physical reasons light 

 furthers evaporation, there is, the danger of excessive transpiration. In many 

 cases withering of the leaves may be prevented in spite of the brilliant illumi- 

 nation, if the stomata close at the first indication of wilting. Stahl (1894) has 

 proved that this result does not always follow, however, and it may be shown 

 that if the leaf of Tropaeolum, slightly wilted, be exposed with closed stomata 

 to direct sunlight, it does not wither any further even after some hours, 

 although a fresh leaf exposed under the same conditions dries up rapidly, just 

 because it does not close its stomata. We may conclude, therefore, that the 

 action of light on the guard-cells is dependent for its effect on the chlorophyll 

 contained in these cells. In fact, as we shall see later, the chlorophyll is able 

 under the influence of sunlight to generate osmotically active substances, and 

 hence, in a sense, to bring about opening of the stomata. Assuredly, however. 



