WATER RELATIONS OF SPECIES: COMPARISONS 311 



factor controlling the distribution of the species. The competitive abihty 

 of plants of this species must be severely hmited in habitats where the soil 

 is not continuously moist. However, other factors, such as the effect of 

 high soil temperature on vegetative growth or of SMT and temperature 

 on flower initiation and seed production may play an equally important 

 roleQarvis, M.S., i960). 



Similarly for Pniuiis padus it is possible that spread of the species south- 

 wards, or on to south-facing slopes at the south-eastern hmit of its range, 

 may be Hmited by soil moisture factors. Under identical conditions of soil 

 moisture availabihty vegetative growth of the southern species, Thelycrania 

 sanguinea, would certainly be faster. 



AU such conclusions must be extremely tentative. The Hmitation of 

 distribution of a species, in the comparatively simple case where only 

 chmatic factors are of importance, may be caused by any one, or any 

 combination of factors acting at any one, or any combination of stages in 

 the Hfe cycle The most important effect may be a continuous reduction of 

 the competitive abihty of the species throughout the hfe cycle; or an 

 occasional catastrophic effect at the seedling stage or the flower or fruit 

 production stage, for example. 



ACKNOWLEDGMENTS 



It is a pleasure to express my thanks to Dr. C. D. Pigott for his interest 

 and supervision during the course of this work; to Professor A.R. Clapham, 

 F.R.S., for the facihties of his department; and to the Department of 

 Scientific and Industrial Research whose award of a Research Studentship 

 made the work possible. 



REFERENCES 



Catsky, J. (1959) The role played by growth in the determination of water deficit 



in plants. Biologia PLuttaruin {Praha) I, 277-286. 

 Jarvis, M.S. (i960) The influence of climatic factors on the distribution of some 



Derbyshire plants. Ph.D. Thesis, Sheffield. 

 Jarvis, P.G. (i960) The growth and regeneration of Quercus petraea in the Sheffield 



region. Ph.D. Thesis. Sheffield. 

 Levitt, J. (1958) Frost, drought and heat resistance. Protoplasinatologia, Band VIII. 

 Richards, L. A. (1941) A pressure membrane apparatus for extraction of soil solutions. 



SoilSci. 51, 377-386. 

 Russell, Sm J. E. (1950) Soil Conditions and Plant Growth. Eighth edition, London. 

 RuTTER, A.J. & Sands, K. (1958) The relation of leaf water deficit to soil moisture 



tension in Pinus sylvestris L. New Phytol. 57, 50-65. 



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