88 THE PHYSIOLOGY OE STOMATA. 



RELATIVE HUMIDITY AND STOMATAL MOVEMENT. 



RELATIVE HUMIDITY— ITS INFLUENCE. UPON THE CONDITION OF STOMATA. 



In some previous experiments (Sudden Change, p. 45) we have been con- 

 cerned especially with the responses of transpiration and of stomata to change 

 of illumination. In subjecting the plants to such changes other conditions 

 should remain constant, or as nearly so as circumstances will permit. During 

 the times when the experiments were made the relative humidity was very 

 low, often as low as 5 per cent, while the difference between the dark room and 

 the outside was, in this regard, not more than a very few per cent, a negligible 

 amount. The fact that the stomata are found quite open during a very high 

 temperature and a very low relative humidity — a typical desert condition — 

 leads one to suspect that the latter has very little influence upon the stomata 

 themselves, though, of course, the rate of transpiration may be very greatly 

 influenced by it, just as the rate of evaporation is. If the conditions are such 

 as to bring the plant nearly to the point of wilting, a high relative humidity 

 will have the effect of making it easier for the plant to recover than if a low 

 relative humidity prevailed; so that a plant may be so far wilted that the 

 stomata are closed, when a rise in the relative humidity may then afford the 

 condition for a returgescence and a consequent opening of the stomata. 



The supposed regulatory effect of stomata upon transpiration is said to 

 rest upon their great sensitiveness to changes of humidity (Haberlandt, 1904, 

 p. 406) but neither the opening of stomata during returgescence, because 

 of the dampening effect of humidity upon transpiration, nor the closure of 

 stomata during wilting, which may but does not necessarily follow a low 

 relative humidity, may be regarded as an expression of irritability. Indeed, 

 from a teleological point of view, the stoma should, at any rate, close before 

 the danger of water loss has become felt, or, as it has been expressed, before 

 visible wilting ensues (Leitgeb). "Visible wilting" is, however, a purely 

 artificial criterion. The danger-point, which, during wilting, we do not know 

 much about, may be reached and passed before visible wilting is manifest. 

 This phase of the subject is, however, treated in another place. The consid- 

 eration before us is the supposed sensitiveness of stomata to change in rela- 

 tive humidity. 



There is a small amount of evidence that a high relative humidity favors, 

 as a condition, the wider opening of the stomata in the ocotillo. This plant 

 has, as has been already noted, two kinds of leaves, distinguished by their 

 position as regards the chief shoot and by their thorn metamorphosis. The 

 primary leaves are of a briefer life than the secondary leaves, possessing less 

 power of resistance. It is also true that in general the stomata are larger 

 and open more widely, and it may readily be supposed that this may con- 



