WATER RELATIONS OF SPECIES: COMPARISONS 293 



It was not feasible to measure SMT iii the containers more than once 

 daily. The watering-to-drying cycles of treatments I and II were therefore 

 not necessarily to exactly i or 2 atni, unless the SMT had reached these 

 levels precisely at the time of measurement. Since the rate of drying-out 

 of the soil depended both on environmental conditions and on the trans- 

 piring surface of the plants in the container, it is apparent that the length of 

 the watering-to-drying cycles could neither be regular through the 

 experimental period for one species, nor precisely comparable between 

 species. It was therefore necessary to use some method of summing SMT 

 in order that comparisons between species might be made. Graphs of SMT 

 against time were constructed for each container and accumulated SMT 

 up to a level of i atm was measured, using a planimeter. This arbitrary level 

 was chosen on the assumption that i atm would be approximately the 

 SMT at which a considerable reduction in growth rate would occur. (This 

 assumption, and the choice of the levels of i and 2 atm at wliich the soil 

 was watered in treatments I and II, were based on the results obtained for 

 Pinus sylvestris by Sands andRutter (1959) and other results in the hterature, 

 quoted by these authors.) The accumulated SMT, in 'atmosphere-days' was 

 finally expressed as a percentage of the total number of the days in the period, 

 so that, for example, a resulting figure of 25 % would represent the situation 

 where the SMT was i atm or more on a quarter of the days of the period 

 or 0-5 atm for half the days of the period. The obvious disadvantage of this 

 method of summing SMT lies in the premise that these two situations will 

 have the same effect on growth. The relation between accumulated SMT 

 and increase in total leaf area per plant throughout the experimental period 

 (increase in mean plant area, for S. hypnoides) as a percentage of that for 

 plants growing in soil at field capacity, is shown in Fig. i. Tentative 

 deductions can be made from these curves. For T. sanguinea and 5. hypnoides 

 growth is reduced to 50% of that in saturated soil by a soil moisture regime 

 in which the SMT is at less than 0-5 atm for 50% of the period; and this 

 indicates, therefore, that leaf area increase is reduced to a neghgible amount 

 by a soil moisture tension of 0-5 atm or less. Extrapolation of the curve for 

 F. vulgaris indicates that leaf area increase would be reduced to 50% of that 

 in saturated soil by a soil moisture regime in which the SMT is at a level of 

 I atm or more for 50% of the time; hence the growth of this species is 

 considerably less sensitive to increases of SMT than that of 5. hypnoides or 

 T. sanguinea. P.^a^//^ appears intermediate in response, but the high between- 

 plant variation and the low rate of leaf area increase rendered the differences 

 between treatments not significant, so that deductions based on the curve 

 for this species are of doubtful vahdity. 



