172 



BURTON EDWARD LIVINGSTON 



Let US now suppose that our cell be removed from solution and 

 placed in air. Water will evaporate from the cell wall, unless, 

 indeed, the environmental air be constantly saturated with water 

 vapor and be enclosed within a chamber having walls impermeable 

 to water vapor. (It is not sufficient merely to presuppose con- 

 stant atmospheric saturation for this exception.) Evaporation of 

 water will result in two changes of primary importance, namely, an 

 increase in the concentration of the wall solution, and a decrease 

 in the imbibed water content of the wall itself. The first of 

 these changes should act in a manner similar to that already con- 

 sidered, where the environmental solution was supposed to be 

 increased in concentration, but it is probable that the present 

 change, from solution to air, would, in most cases, soon result in a 

 much greater increase in concentration of wall solution than would 

 the former one. Nevertheless, this is probably not nearly as 

 important in conditioning physiological reaction as is the second 

 change just noted. 



This other change, decrease in the imbibed water content of the 

 wall substance, is undoubtedly of very far-reaching import. Like 

 the osmotic increase of the wall solution it also must produce an 

 exit of water from protoplasm and vacuole, with whatever results 

 may accompany this exit. Furthermore, the rise in magnitude of 

 the force of imbibition in the wall, which must be assumed here, 

 acts to decrease the vapor tension of the imbibed water (as does 

 also, to a less degree, the increased concentration of the wall solu- 

 tion) , and consequently to hinder evaporation. The more water 

 is removed from the wall, the greater will be the imbibitional force 

 attracting water thereto, the more water will pass out of the proto- 

 plasm and vacuole, and the lower will be the rate of evaporation. 



The removal of water from cell walls, with its concomitant 

 results, first causes what Renner,i has termed Sdtigungsdefizit 

 and what Brown and have called incipient drying. If the proc- 



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

 Flora 103: 171-248, 1911. 



' Livingston, B. E., and Brown, W. H., Relation of the daily march of transpira- 

 tion to variations in the water content of foliage leaves. Bot. Gaz. 63 : 309-330, 

 1911. 



