310 MARGARET S.JARVIS 



of a difference in ability to control water loss by stomatal closure, or in 

 ability to take up water from soil of increasing SMT, but as a difference in 

 sensitivity of the metabolism of the cells of the leaf to increasing LWD. 



The results of the experiments in which detached shoots of the species 

 were subjected to atmospheres of controlled relative humidity (RH) for a 

 standard time vmder standard conditions, are particularly interesting, in 

 that they emphasise the possibihty of independence of different aspects of 

 drought resistance. For the species used in the present investigation, the 

 resistance of the leaves to desiccation at low RHs and with soil water 

 unavailable was independent of the resistance of the plant to reduction of 

 growth rate as a result of increasing SMT and concomitantly increasing 

 LWD. The growth rate of F. vulgaris and Pimpinclla saxifraga was much less 

 sensitive to the effect of increasing SMT than was that of 5. hypnoides; yet 

 the resistance of the leaves to desiccation was very much lower. Similarly 

 for the tree species; the growth rate of T. sanguinea was less affected by 

 increases of SMT than was that of P. padus; but there was no clear differ- 

 ence between the species in resistance of the leaves to desiccation. 



This type of experiment demonstrates the differential ability of plants 

 of the species tested to prevent water loss to a lethal level by stomatal 

 closure and by low rates of cuticular transpiration. The threshold RH that 

 causes injury can be determined, but, unless sufficient time is allowed for 

 equihbrium to be reached, and this was not so in the present experiment, 

 such values may correspond not only with drought tolerance but also with 

 drought avoidance of the leaf tissues. The results of such experiments can 

 probably justifiably be applied to field conditions where soil water avail- 

 abihty is negligible. Thus, where soil water is unavailable both to 5. 

 hypnoides and to F. vulgaris in the field, and similar atmospheric conditions 

 prevail, plants of the former species will survive for a longer period. 

 However such a comparison does not take into account the size and extent 

 of the root system or the capacity of the roots to grow into region of soil 

 of lower SMT; i.e., the capacity of the plant to avoid the condition where 

 soil water is unavailable to it. Such factors may be of overriding importance 

 where the drought resistance of a species in the field is concerned. The root 

 system of F, vulgaris is more extensive than that of S. hypnoides, for plants 

 growing on the slopes of the Derbyshire dales. 



It is therefore difficult to apply to field conditions the results of experi- 

 ments reveaHng physiological differences between species in their response 

 to soil or atmospheric drought. 



It is possible that the reduction in growth rate of S. hypnoides which 

 ensues in response to shght drying-out of the soil may be an important 



