WATER RELATIONS OF SPECIES: COMPARISONS 305 



in an atmosphere of 100% RH, with the petioles in water) to dry weight. 

 The value of this ratio was significantly lower for leaves permanently 

 injured by the droughting treatment than for normal leaves and leaves 

 which recovered. 



Results, (i) Prunus padus and Thelycrania sanguinea. Five experiments 

 were conducted; two with each of the species separately, and the fifth with 

 shoots of the two species arranged alternately in the same drought 

 chambers. It was thought likely that the apparent differences in response 

 of leaves of a single species on successive occasions might be attributable 

 to differences in environmental conditions, such as would arise if there 

 were incomplete control of RH. Experiments using more than one species 

 per chamber ensured that the results would be comparable. 



In no experiment was there drought injury to leaves on shoots whose cut 

 ends were in water. After the drought period, at allRHs, mean LWD for 

 such leaves was less than 5%. Results for shoots to which water was 

 unavailable, are presented in Tables 3 and 4. 



Table 3 shows the difference in the abihty of the two species to recover 

 from the droughting treatments. The RH which could be tolerated was 



Table 3 



The recovery (as a percentage of the total number of leaves) of leaves on detached 

 shoots of Primus padus and Thelycrania sanguhiea after subjection to atmospheres of 

 various relative humidity, at 25°C, for a 12-hour period in the dark followed by an 

 8-hour period under a standard light source of 400 ft candles ; water was not supplied 



