TRANSPIRATION AND ABSORPTION OF WATER 291 



within the plant usually occurs which is often of sufficient magnitude to 

 induce temporary wilting. Only if the available water supply in the soil 

 becomes so low that absorption of water virtually ceases will the plant pass 

 into a state of permanent wilting. 



The magnitude of the reduction in the water content of the leaves required 

 to induce wilting varies greatly depending upon the species of plant. Accord- 

 ing to JMaximov (1929) leaves of many "sun" species may lose from 20 to 30 

 per cent of the total water present before wilting ensues, while typical "shade" 

 species wilt upon a reduction in the amount of water present of 3 to 5 per 

 cent. Only in the leaves of the "sun" type can incipient wilting be dis- 

 tinguished as a distinct phase ; in "shade" species incipient wilting is extremely 

 transitory. The discussion of wilting and related phenomena in this chapter 

 refers primarily to wilting of the type which is characteristic of plants 

 indigenous to sunny, exposed habitats. 



Diminution in the water content of leaves with its concomitant reduction 

 in leaf turgor also influences the total volume of the leaf. The area of leaves 

 may decrease as much as 5 per cent during the midday hours of a bright warm 

 day, the exact magnitude of this shrinkage in area depending upon the species 

 and the prevailing environmental conditions (Thoday, 1909). Not only the 

 area, but even the thickness of the leaves may decrease with a reduction in 

 the turgor of the leaf cells. Bachmann (1922) has shown that a reduction of 

 as much as 5-6 per cent occurs in the thickness of the leaves of many species 

 as they pass from a turgid into a flaccid condition. 



Comparative Daily Periodicities of Transpiration and the Absorption 

 of Water. — The observed phenomenon of wilting, as well as experimental 

 results showing that a daily diminution in the water content of leaves and 

 other plant organs is of frequent daily occurrence, are both indirect evidence 

 that the transpiration rate frequently exceeds the rate of absorption of water 

 during the daylight hours. Only a few investigations have been undertaken, 

 however, in which simultaneous measurements have been made of transpiration 

 and absorption rates of the same plant over periods of 24 hours or longer. 



For the investigation of this problem Kramer (1937) grew plants in 

 metal containers which were provided with auto-irrigators of porous clay 

 buried in the soil. Each of these irrigators was connected by tubing with 

 a reservoir of water which was set at a lower level than the container. As 

 water is absorbed by the plant from the soil more moves into the soil from 

 the porous clay irrigator which is kept filled by the pressure of the atmos- 

 phere on the water in the reservoir. Prior to an experiment the containers 

 were sealed so that all loss of water occurred as transpiration from the plant. 

 By weighing the container plus the reservoir at appropriate intervals and 



