loo MECHANISM OF STOMATA 



size of the stomatal apertures directly measured under the 

 microscope (cf. Appendix VII). 



Either of these methods will serve to demonstrate the ten- 

 dency of the stomata (except in many shade and marsh plants, 

 e.g. the Water Plantain, Alisma plantago) to " close " when the 

 shoot becomes flaccid owing to deficiency of water, although, 

 as shown especially by the porometer, they often open more 

 widely at the first commencement of wilting. " Closure " is 

 also brought about by a change from light to darkness, which 

 emphasises the fact that alteration in turgidity of the plant 

 as a whole is not necessarily involved. It may be noted that 

 Fungi, which have no stomata, also exhibit diminished trans- 

 piration in darkness. The sensitiveness of the stomatal mecha- 

 nism is so great that even the effect of temporary shading, 

 as by a big cloud, can be observed with the help of the porometer. 

 Moreover, shaking of a leaf may cause a more or less marked 

 temporary " closure," and for this reason it is best to allow a 

 short interval to elapse, after fixing the porometer, before read- 

 ings are taken. 



We thus realise that the stomatal mechanism furnishes an 

 automatic control on the escape of water-vapour from the leaf. 

 The rate of transpiration has been shown to decrease as the 

 humidity increases, though it becomes zero only when the air 

 is slightly supersaturated. 



The stomata are therefore highly irritable, and the ultimate 

 cause for their response to different conditions must, at all events 

 in part, reside in the living protoplasm of the guard-cells. It 

 would seem that conditions causing " closure " of the stoma 

 must lead to a reduction in the permeability of the protoplasm, 

 and vice versa. In the case of the stomata present on the cap- 

 sule of Mosses (cf . p. 284), it has been found that the guard-cells, 

 when open, have nearly five times the osmotic pressure of the 

 surrounding epidermal cells, but when " closed " have the same 

 osmotic pressure. 



A much modified type of stoma is found in the Gramineae 

 and Cyperaceae (Sedge-family), though the mechanism is essen- 

 tially similar. The much elongated guard-cells surround a large 

 pore having the form of a flattened hexagon (Fig. 46, A). In 

 the middle portion of each guard-cell the outer and inner walls 



