294 MARGARET S.JARVIS 



Filipendula vulgaris (85daysl 



Prunus padus (92 days) 



Saxifraga hypnoides (93 days ) 

 Thelycrama sanquinea |72daysJ 



10 20 30 ^O 50% 



Accumulated SMT atm days, percent daily total 



Fig. I. The relationship between leaf area increase and accumulated soil moisture 

 tension for four species grown in three soil moisture regimes. Increase in leaf area 

 as percentage of that for plants on soil at field capacity. For explanation accumulated 



SMT, see text. 



THE EFFECT OF SOIL MOISTURE TENSION ON 



GROWTH RATE 



These deductions as to the approximate level of SMT at which growth is 

 reduced were tested in a further series of experiments. The response of each 

 species to changes in SMT within a watering-to-drying-out cycle was 

 investigated, in order to determine, as accurately as possible, 



(a) the lowest SMT which causes a reduction in growth rate, 



(b) the lowest SMT which causes a cessation of measurable growth. 



Method. The rate of increase of SMT was sufficiently rapid to necessitate 

 daily measurements of SMT and of growth, made at the same time each 

 day. Therefore a quick and sensitive method of detecting changes in 

 growth rate of each species was essential. Daily measurement of total leaf 

 length was found to be a practicable procedure; and for plants of all the 

 species the rate of increase in total leaf length with time was constant, in 

 soil where measurable SMTs did not develop. Hence, deviations from this 



