WATER RELATIONS OF SPECIES: COMPARISONS 295 



rate were easily detectable, as changes in the slope of curves of total leaf 

 length against time. 



All experiments were carried out in a heated glasshouse (60-70"?), with 

 supplementary Hghting with fluorescent tubes, during the winter of 1959 

 to i960. Plaster-of-Paris resistance blocks were placed in the soil at the 

 time of planting, and the experiments were commenced only after the 

 roots had grown through the whole soil volume. The blocks used were 

 individually calibrated to ensure the maximum possible accuracy in 

 conversion of block readings to atmospheres SMT. Two blocks were used 

 in each container. 



The measuring technique varied according to the species ; either the total 

 leaf length of the plant, or of certain shoots, or the length of individual 

 leaves, constituted the basic unit measured. Leaves which had ceased to 

 elongate at the beginning of the experiment were not measured since it was 

 required only to detect changes in the rate of increase of total leaf length and 

 not to obtain absolute values for total leaf length. 



Each container was subjected to several watering-to-drying-out cycles. 

 All the curves for each species were examined and the SMT corresponding 

 with the first decrease in slope of the curve (i.e., the lowest SMT causing a 

 decrease in the rate of increase in total leaf length) was determined, sub- 

 jectively, for each cycle. Similarly the SMT above which there was no 

 detectable increase in total leaf length was determined. 



Results. Figs. 2-5, inclusive, show examples of the curves obtained. 



There were marked differences in response between S. hypnoides and F. 

 vulgaris. The growth rate of the former was depressed by the smallest 

 SMTs measurable with the plaster-of-Paris blocks, i.e. by approximately 

 0-2 to 0-3 atm. There was no detectable increase in total leaf length after 

 drying-out of the soil to a SMT of, at the most, 0-5 atm. In contrast, no 

 depression of growth rate of leaves of F. vulgaris was caused by SMTs 

 below 0-7 or o-8 atm. Cessation of elongation occurred at i -o atm for about 

 two thirds of the leaves; and at 1-5 or 2-0 atm for the remainder. There 

 was apparently no differential effect of age of the leaf on the response of 

 leaf elongation rate to increase in SMT. 



For P. padus the lowest SMT which resulted in a reduction in growth rate 

 was about 0-3 atm: the corresponding value for T. sanguinea was 0-7 atm. 

 This is a difference similar to that estabhshed for S. hypnoides and F. vulgaris. 

 There was a large variation between shoots in the SMT at which there was 

 apparent cessation of growth, especially for P. padus; and this masked any 



20 



