WATER RELATIONS OF SPECIES: COMPARISONS 307 



Both, mean LWDs developed as a response to the treatment, and critical 

 LWDs, were generally larger for T. sanguinea than for P. padus (Table 4). 

 There was some variation between the successive experiments, notably for 

 P. padus from the same tree. In May and July LWDs developed were 

 similar on the two occasions but the critical LWD was 29% for the older 

 leaves and between 43 % and 51% for the younger leaves collected in May. 

 Hence there was a higher percentage recovery in May, as mentioned above, 

 indicating that drought resistance may change through the season. 



Thus, although the critical LWD and the LWD developed at any RH 

 was larger for T. sangtiinea than for P. padus, there was insufficient evidence 

 to demonstrate any appreciable difference in drought resistance, as 

 measured by the abihty of leaves to recover turgidity. 



(2) The herbaceous species. For shoots whose stems were in water the 

 LWDs developed were not related to the RH prevailing during the 

 droughting treatment. 



In four experiments with Saxifraga hypnoides from different locahties 

 there was 100% recovery by the shoots whose stems were not in water, 

 after allRH treatments, down to 25%RH. Comparing results for samples 

 from different localities, it is apparent that at the lower RHs larger LWDs 

 were developed by plants from Cressbrook Dale, Derbyshire (24.9.59) 

 than by plants collected from the Wimiats, Derbyshire, both three weeks 

 later (14.10.59) and in the following year (3.8.60) (Table 5). The reason for 



Table 5 



The mean leaf water deficit of Saxifraga hypnoides after droughting. Conditions as for 



Table 3 



N.B. In all four experiments, after all RH treatments, there was 100% recovery of 

 turgidity by the shoots. 



