300 MARGARET S. JARVIS 



the LWD developed, showed, for P. padtis, T. sanguinea and F. vulgaris, 

 that there was Httle variation between values obtained for all mature leaves, 

 i.e. leaves which were completely, or almost completely, expanded and 

 which were not obviously senescent. The youngest leaves had apparently 

 higher LWDs than the mature leaves, cf. Catsky (1959). 



(b) Transpiration rate. Transpiration rate was measured by determining 

 the loss in weight of detached leaves during a standard time after cutting, 

 this being the most practicable method available. Preliminary tests showed, 

 for P. padus and T. sanguinea, that there was a constant rate of loss from 

 05 to 6-0 min after cutting, followed by a slight faU-off. The method was 

 therefore considered satisfactory for these species. The experimental pro- 

 cedure adopted was to determine the loss in weight from | to 4^ min after 

 cutting. The transpiration rate was expressed as mg loss per 100 mg (at 

 ^ min) per min. 



Measurements of LWD and of transpiration rate (TR) were made on 

 the plants used for the experiments on the effect of SMT on growth rate, 

 described above ; and on P. padus and T. sanguinea planted alternately within 

 the same container, to ensure that SMT was identical for the two species. 

 All experiments were conducted in a glasshouse, on days which were 

 cloudy throughout, so that fluctuations in environmental conditions were 

 relatively small. 



Results. The results for P. padus and T. sanguinea are presented in Tables 

 2(a) and (b) ; and in Fig. 6. On July 15, for soil with a SMT of below 0-2 atm, 

 there were no significant changes in LWD through the day for either 

 species, and the differences between the LWDs of the species were not 

 significant at any time of day (Tables 2(a) and (b)). No direct comparisons 

 between the species for soil of higher SMT can be made, since the SMTs 

 obtaining were not identical. There is, however, no evidence of a major 

 difference in the LWDs developed at SMTs from 3 to 8 atm. For plants 

 in soil at '20 atm' SMT it appears that the LWDs for leaves of T. sanguinea 

 are larger than for P. padus. However, since these values of '20 atm' corres- 

 ponded with the maximum reading of the electrical resistance blocks, such 

 values may represent any value of SMT from 20 atm upwards. Hence, the 

 soil in which the T. sanguinea plants were growing may have been at a 

 higher SMT than that for P. padus. 



In the experiment of 24 August i960 (Fig. 6), designed to eliminate these 

 difficulties, the LWDs of both species were smaller than for the previous 

 experiments. With one exception, there was no overall increase in LWD 

 through the day, from 7 a.m. to 7 p.m., so that water loss did not exceed 



