3o8 MARGARET S. JARVIS 



this marked difference was not investigated; it might be attributable either 

 to genetical differences or to differences in the preceding environmental 

 conditions. Results for Filipendula vulgaris and Pimpinella saxifraga were in 

 striking contrast (Table 6). 



No leaves from F. vulgaris recovered from any treatment except that of 

 100% RH: for P. saxifraga there was only 50% recovery from 86% and 

 78%RHs, and no recovery from lower RHs. In comparison with the other 

 species tested, the LWDs developed, and the critical LWDs, in these 

 species were remarkably large. Hence, in this respect, they may be regarded 

 as being very much more drought sensitive than S. hypnoides. 



Table 6 



The mean leaf water deficit (LWD) developed and the percentage recovery of leaves 

 o{ Saxifraga liypnoides, Filipendula I'ulgaris and Piiiipiiiella saxifraga subjected to drought- 

 ing conditions. Droughting conditions as for Table 3. LWDs of leaves which did not 

 recover were calculated as for the tree species (Table 4). Date of experiment 3.8.60 



S. hypnoides F. inilgaris P. saxifraga 



r ' . , ^ . < ^ 



RH Mean LWD Recovery Mean LWD Recovery Mean LWD Recovery 

 developed (%) developed (%) developed (%) 



DISCUSSION 



The results presented above demonstrate physiological differences between 

 the species in response to conditions of increasing water stress. Growth of 

 Saxifraga hypnoides was depressed, in comparison with growth in soil at 

 field capacity, more than that of Filipendula vulgaris, by soil moisture 

 regimes where the soil was allowed to dry to i or 2 atm SMT before re- 

 watering. Confirmation of this difference was obtained from the experi- 

 ments investigating the effect of increasing SMT within a watering-to- 

 drying-out cycle. Similar differences, though not so large, were apparent 



