LEAF TURGOR. 69 



after a time the stomata reopen; otherwise, they remain closed 

 until nightfall brings on a decrease in the intensity of the factors 

 producing evaporation, and the turgor again rises. 



A certain amount of the water present in a turgid leaf may be 

 regarded as the working margin. The loss of part of this margin 

 does not seem to interfere in any manner with the functioning of 

 the leaf, and it is probable, even when it is wholly lost and the 

 stomata are closing, that photosynthesis and other functions are 

 still carried on, in part at least. Its presence, therefore, permits the 

 stomata to remain open and carbon-dioxid absorption to go on 

 for a time, in spite of excessive evaporation. As the amount of 

 leaf-water changes not only during a 24-hour period but also from 

 one rain or irrigation to another, this working margin also changes. 

 The content of water in alfalfa leaves has been found to change from 

 a maximum of 410 per cent of dry weight after several days of rain 

 and high humidity to 290 per cent of the dry weight when on the 

 verge of wilting as a result of low water-content a month later. As 

 the critical minimum of leaf-water at which stomatal closure occurs 

 varies in lesser proportion, there must be some means of adjustment 

 to decrease in water-content by reduction of the amount of water 

 with which a leaf can operate. 



The failure to recognize the presence of this working margin of 

 water in the leaf led Lloyd (1912) and Knight (19176) to conclude 

 that the stomata had no active part in keeping up leaf turgor. 

 Lloyd concludes from his experiment that " decrease in water in the 

 leaf occurs during the opening of the stomata. These organs are, 

 therefore, not closely regulatory of the loss of water and are in- 

 effectual in maintaining a constant supply of leaf -water." Knight 

 states that "the experiments in the present paper have confirmed 

 Lloyd's results," and later that "the stomatal aperture is not re- 

 duced by slight water deficiency in the leaf; hence the ordinary 

 view that the stomata, by their response to incipient drying, are 

 the chief factors in maintaining the water-content of the leaf is not 

 tenable. On the other hand, the stomata are very sensitive to light 

 changes, so that with increasing light intensity the stomata may con- 

 tinue to open, whilst the water-content of the leaf is decreasing." 



The assumptions made by both writers seem to be that if it 

 were not for the ineffectiveness of the stomata, the water present 

 in the leaves would be constant, and that any decrease in the water 

 contained in the leaf inhibits its normal functioning. The first 

 assumption would then mean that the normal leaf water-content is 

 constant, and that the same per cent of water is found night after 

 night, and departure from this occurs only as a result of excessive 

 transpiration during the day. This has not been found to be the 

 case, however, as the nightly maximum fluctuates greatly from one 

 rain or irrigation to another. The second assumption would mean 



