WATER RELATIONS OF SPECIES: COMPARISONS 309 



between Pniniis padus and Thelycrania sanguineci.K.esu\ts of the soil moisture 

 regime experinients were inconclusive for these two species because of the 

 slow rate of increase of leaf area and the high between-plant variation for 

 P. padns. In day-by-day correlation of growth with increase in SMT, 

 reduction of growth of P. padus occurred at a lower SMT than that of 

 T. sanguinea. It was not possible to make a distinction between the two in 

 the SMT at which there was cessation of leaf elongation, because of the 

 large variation between shoots in this respect, particularly for P. padus. 

 There was no growth of either species at SMTs above 2-3 atm at the most. 

 There was cessation of measurable growth of S. hypnoides at about 0-5 atm 

 SMT and of F. vulgaris at 2-0 atm or below. In contrast, Wadleigh and 

 Gauch (1948), studying the effect of increasmg total soil moisture stress 

 on the daily elongation of cotton leaves, during several irrigation cycles, 

 found that elongation ceased at a narrow range of stress values, close to 

 15 atm. Slatyer (1957) found for privet and cotton that growth and elonga- 

 tion continued up to the point where DPD at dawn (27 and 34 atm, respec- 

 tively) corresponded with zero turgor pressure in the leaves, i.e., at the 

 permanent wilting point for the species. In the experiments described 

 above, measurable leaf elongation of the four species ceased at SMTs 

 considerably lower than those which caused wilting of the leaves at dawn. 

 This point was not precisely determined but it certainly did not occur at 

 SMTs lower than 10 atm. 



It must be noted that these low values represent only the soil moisture 

 tension (SMT) component of the total soil moisture stress (TSMS) ; and 

 that the osmotic component cannot justifiably be neglected since it may 

 be of comparable magnitude. Slatyer (1957) illustrated the relationship of 

 both SMT and TSMS to soil water content for a 'sandy clay loam' soil, to 

 which a complete fertiliser mixture had been added. For SMT values from 

 to 2 atm corresponding TSMS values were 2 to 3 atm higher. Russell 

 (1950) quotes values of 0-2 to i atm for the osmotic pressure of the soil 

 solution from field capacity to the wilting point, for 'normal leached 

 agricultural soils'. The values for SMT presented throughout this paper, 

 may therefore be up to about 2 atm lower than the corresponding TSMS, 

 but probably not more than i atm. This difference in magnitude of the 

 absolute values does not, of course, affect the comparisons made between the 

 species, since the same soil was used throughout. 



There was apparently no difference between the relation of SMT and 

 LWD, or of SMT and TR for P. padus and T. sanguinea; or of SMT and 

 LWD for 5. hypnoides and F. vulgaris. The marked difference in sensitivity 

 of growth rate to increasing SMT can possibly be explained not in terms 



