PHYSIOLOGICAL IMPORTANCE OF STOMATA 457 



As the stomata are capable of opening and closing, they 

 are not mere perforations in the epidermis, placing the inter- 

 cellular spaces in communication with the outer air ; on the contrary, 

 they are the organs which regulate the activity of gaseous interchange 

 in general, and of transpiration in particular. As a matter of fact, 

 there is no need for the stomatic orifices to be adjustable, so far as 

 respiration and photosynthesis are concerned ; both these functions 

 could be carried out quite satisfactorily, if the stomata were always open, 

 so as to admit of rapid diffusion. For it is unlikely that plants would 

 ever suffer injury, under natural conditions, through excessive photo- 

 synthesis or respiration ; hence gaseous interchange need never be 

 restricted in the interests of either of these functions. Most land plants, 

 on the other hand, are frequently threatened by the dangers attendant 

 upon excessive transpiration; and it is for this reason that such plants 

 must be furnished with the means of temporarily suppressing stomatic 

 transpiration altogether, and thus of preventing, for the time being, 

 all further escape of water-vapour, except for the small amount that 

 is unavoidably lost by cuticular transpiration. 



In the case of non-chlorophyllous organs [which are incapable 

 of photosynthesis], such as petals, stamens, the pneumatophores of 

 Nephrodium stipellatum and the shoots of certain Phanerogamic 

 saprophytes and parasites and for which transpiration is a function 

 of secondary importance, the stomata must be utilised mainly or 

 exclusively for the purpose of respiration. The stomata of foliage- 

 leaves, and of chlorophyll-containing organs in general, on the con- 

 trary, play a very important part in connection with photosynthesis, 

 since, as has already been explained (p. 437), the gaseous interchange 

 incidental to that function is carried on almost entirely through the 

 mediation of the ventilating system. Sachs long ago remarked that 

 wilted leaves are unable to manufacture starch, because their stomata 

 are shut; for a similar reason, no starch is formed, according to Stahl, 

 in leaves the stomata of which have been blocked by artificial 

 means. 



It is in connection with the transpiratory process, that the control- 

 ling action of stomata has been studied in the greatest detail. Garreau, 

 Unger, Deherain, Boussingault and others have demonstrated, by a great 

 variety of experiments, that, as a rule, the abaxial surface of a leaf 

 evolves a far larger amount of water-vapour than the aclaxial side. This 

 difference is undoubtedly due to the unequal distribution of stomata 

 on the two surfaces, the majority of leaves having all or most of 

 their stomata on the abaxial surface. In these circumstances, the 

 transpiration from the upper surface is purely cuticular, and hence 

 comparatively insignificant, whereas in the case of the lower surface 



