122 WILLIAM H. BROWN 



Lloyd 1 has found that certain stomata, at least, do not close be- 

 fore wilting occurs, and points out that this closure is to be re- 

 garded as a wilting of the guard cells along with the other cells 

 of the leaf. Even with all stomata closed, the rate of transpir- 

 ation must vary with the evaporating power of the air. At 

 any rate, the wilting of leaves, and the accompanying closure 

 of their stomata is to be considered as a critical or end point of 

 a drying process which has been going on for some time pre- 

 viously. Livingston and Brown 2 and Lloyd 3 have found that, 

 under arid conditions, as the evaporating power of the air in- 

 creases, the percentage moisture content of plant foliage mark- 

 edly decreases; and that, apparently as a result of this, there 

 is a corresponding fall in the rate of transpiration as compared 

 with that of evaporation. This diminution of the percentage 

 water content of leaves which has been termed incipient drying, 

 is more pronounced under a high rate of evaporation than under 

 a lower one. Renner, 4 in his thoroughly critical studies on the 

 physics of water movement in plants concludes that there is 

 always a saturation-deficit ("Sattingungsdefizit") in transpir- 

 ing leaves. The magnitude of this deficit should depend upon 

 the transpiration rate and upon the rate of water intake by the 

 roots; this deficit is identical with the incipient drying of Living- 

 ston and Brown. It appears that if the process of incipient 

 drying be continued to a certain point, wilting occurs. This is 

 quite in harmony with the view above stated, that the soil mois- 

 ture content at the time of wilting should vary with the evapor- 

 ating power of the air. 



This conclusion does not imply, however, that the percentage 

 of ' 'non-available " soil water should vary with the rate of evapo- 



1 Lloyd, F. E., The physiology of stomata. Publication 82 of the Carnegie 

 Institution of Washington. 1908. 



2 Livingston, B. E., and Brown, W. H., Relation of the daily march of trans- 

 piration to variations in the water content of foliage leaves. Bot. Gaz. 53: 311- 

 330. 1912. 



3 Lloyd, F. E., The relation of transpiration to stomatal movements and to 

 the water content of the leaves in Fouquieria splendens. Plant World 15: 1-14. 

 1912. 



4 Renner, O., Experimentelle Beitrage zur Kenntnis der Wasserbewegung. 

 Flora 103: 171-247. 1911. 



