32 



D. N. McVEAN 



trees. He notes that northern races of pine grown in southern regions show reduced 

 height and needle length, and first-year seedlings respond to shorter days by hastened 

 dormanc\^ as compared with the home region. After the first year of growth, duration 

 of extension- growth is unaffected by day length. 



In the Annual Report of the Swedish Association for Forestry and Tree Breeding, 1942, 

 variation in photoperiodism is reported for both two-year and one-year A. glutinosa 

 seedlings. The V^asterbotten long-day t^-pe showed marked differences in increment, in 

 time of leaf fall and in lignification of the annual shoots compared with central and southern 

 Swedish races. 



Discussion 



There seems little doubt that in Britain the alder exhibits clinal variation in some 

 features of its morphology and physiology, and that these \'ariations tend to take place 

 along the main climatic gradient S.E.-N.W. This would seem to belong to the large 

 scale, geographical character gradients designated topoclines by Gregor (1938-39). (Mor- 

 phological variation has not yet been demonstrated on the plants grown under uniform 

 conditions in the experimental garden.) 



There may be a continual selection for small leaves and catkins in the north-west by 

 some factors, possibly climatic, or the situation may ha\'e arisen in the selective elimination 

 of genes during post-glacial migration north-westward across the countr}^ The smaller 

 variability of the northern type indicates that some gene elimination has taken place. 



The failure to establish similar morphological gradients altitudinally strengthens the 

 belief that the differences are genotypic, since there are no barriers to gene flow over 

 the short distances here involved. The morphological gradients exist as an alteration 

 in the mean values of leaf and catkin size ranges rather than as a pohinorph-ratio cline 

 involving the three varieties cited above (p. 28). 



Habitat preferences in the species encourage the formation of isolated local colonies 

 which tend to intra-breed rather than inter-breed. This, along with some past planting 

 by man, may account for the occurrence of populations of small-catkin t^pes in an 

 otherwise large-catkin region and vice versa. 



It should perhaps be noted that the most serious discrepancy occurs in the New 

 Forest populations, which lie south of the maximum extent of the Pleistocene glaciation. 



Acknowledgments 



This paper comprises part of a study of the common alder submitted for the degree 

 of Ph.D. at Cambridge University. I wish to express my thanks to Dr. A. S. Watt and 

 Dr. H. Godwin for their interest and encouragement while supervising the work. My 

 thanks are also due to Dr. A. Berrie, formerly of the Glasgow Botany Department, for 

 confirming my chromosome counts. 



REFEREXCE3 



GREGOR. J. W., 193S-9, Experimental Taxonomy, II, IV, New Phyt., 37-8. 



JOHNSSON, H., 1950, On the C2 and Generations in .\lnus glutinosa. Hereditas, 36, 205-19. 

 KUJALA, v., 1924, Das Vorkommen der Schwarzerle in Finland, Comm. ex Imtit. Quaest. Forst. Fin. 

 Ed., 7. 



McVEAN, D. N., 1953, Alnus glutinosa (L.) Gaertn., in Biological Flora of the British Isles./. Ecol., 41, 2. 

 MOSS. C. E., 1914, Cambridge British Flora, 2, S7-88. Cambridge. 



ORDING. A.. 1939, Forest Tree Breeding in Denmark : Dr. Larsen's Vv''ork at Horsholm, Tidsskr. Skoghr., 

 47. P.BA. XI 1133. 



RUBNER, K., 1934, Die Pflanzengeographischen Grundlagen des Waldhaus. Xeudamm. 

 WAREING. P. F.. 194S, Photoperiodism in VVoodv Species, Forestry, 22, 211-21. 



