302 MARGARET S.JARVIS 



was just significantly larger than for P. padus ('P' = o-05). In higher or 

 lower SMTs the differences between species, in LWD at 7 p.m., were not 

 significant. 



There was no significant difference between the transpiration rates of the 

 two species, in any of the treatments. 



Table 2{c) shows results for S. hyptwides. 

 The 7 a.m. value was significantly larger for the plants in soil at 20 atm 

 SMT than for those in soil from 00 to 53 atm SMT, between which there 

 were no significant differences. At 7 a.m. on the same day, mean LWD 

 for leaves of F. vulgaris, SMT 20 atm, was 26-1%; for SMT 2-5 atm, 

 11-2% ; and for SMT less than 0-2 atm, 5-9%. The differences between these 

 values were significant (for 'P' = o-05, least significant difference = 2-1%). 

 It was not possible to compare transpiration rates of these two species, 

 since rehable estimates of transpiration rate for S. hyprwides were not avail- 

 able. Results obtained by measuring the loss of water from detached shoots 

 were too variable. 



Discussion. For aU four species, increased SMT was associated with 

 increased LWDs; and for P. padus and T. sanguinea, also with decreased 

 transpiration rate. Such phenomena have been described for many species 

 (e.g.,Rutter and Sands, 1958, for Pinus syhestris; Slatyer, 1955, for cotton 

 peanuts and sorghum). Despite the scantiness of the data, sufficient evidence 

 was obtained to show that the major differences between the two tree 

 species, in response of growth rate to increases in SMT, cannot be explained 

 in terms of differences in the relation between LWD or transpiration rate 

 and increase in SMT. For plants of these two species, there were no major 

 differences observed between LWDs or transpiration rates prevailing, at 

 SMTs from o-o to 20 atm, and in moderate evaporating conditions. 



Such data as were obtained for F. vulgaris and 5. hypnoides indicate that 

 the same conclusion may also be valid for these herbaceous species. Mean 

 LWD values for S. hypnoides at SMTs of less than 0-2 atm and of 2-8 atm, 

 though not significantly different, were comparable in magnitude to those 

 for F. vulgaris. 



DROUGHT RESISTANCE, AS DETERMINED USING 

 CONTROLLED RELATIVE HUMIDITIES 



Resistance to reductions in growth rate resulting from low SMTs, con- 

 sidered previously, may be completely independent of resistance to injury 



