28 



D. N. McVEAN 



Table 2 



Alder pistillate catkin measurements 







Mean 



Regional 



Mean 



No. of trees 





Population 



Length (mm.) 



mean 



Breadth (mm.) 



measured 



1. 



( Matlev Bog 

 i Holmesley Bog 



10- 8 i 2-1 



11- 2 ± 2-0 



11-0 



7-6 It 1'6 

 7-7 i 1-1 



10 

 5 



2. 



Chippenham Fen 



15-0 + 3-6 





10-4 ± 1-8 



10 



3. 



Santon Downham 



13-7 ± 2-9 



13-9 



10-0 ± 1-8 



5 



4. 



Ranworth Broad 



13-1 ± 2-5 





9-1 ± 1-0 



10 



5 . 



Conway Valley 



12-9 ± 2-3 





9-1 4i 1-5 



10 



6. 



Portmadoc 



11.4 ^ 2-0 



11-9 



7.7 1-0 



10 



7. 



Chester 



11-5 ± 2-1 





8-4 ± 1-0 



10 



8. 



Mugdock, b.W. btirimg 



lO'b ± 2-2 





8-0 ± 1-8 



10 



Q 



N. Arran i 



1 n-s -L 7.0 



J-VJ 0 ~ z, y 





0 u — 1 o 



zu 



10. 



N. Arran ii 



11-0 ± 2-4 





8-1 ± 1-5 



10 



11. 



Glen Nevis 



10-1 i 2-5 





7-7 ± 1-7 



10 



12. 



Ballater 



11-9 ± 1-8 





7-7 ± 1-4 



5 



13. 



Kinord 



9-3 ± 1-1 



10-7 



6-6 4: 0-9 



5 



14. 



E. Loch Ness 



11-1 ± 1-9 





8-2 ± 1-4 



10 



15. 



Garmouth 



11-3 i: 2-2 





8-1 ± 1-0 



10 



16. 



Sutherland i 



10-5 ± 2-5 





8-2 ± 1-3 



10 



17. 



Sutherland ii 



8-5 zr 1-5 



9-2 



7-4 ± 1-2 



10 



18. 



Sutherland iii 



8-6 ± 1-9 





7-1 ± 1-4 



10 



Moss (1914) recognised three varieties of Alnus glutinosa in Britain on the basis 

 of leaf and catkin sizes. 



Var. macrocarpa Loudon. Laminae about as long as broad (7-S cm.) and with larger and coarser serra- 

 tions. Pistillate catkins about 3 cm. long at maturity. 



Var. typica Moss. Laminae intermediate in shape and size (5-6 X 4-5 cm.). Pistillate catkins l'7-2-0 

 X 1-0-1*4 cm. "Believed to be the common south and lowland form and not yet known for Wales, Scotland 

 or Ireland." 



Var. microcarpa Rouy. Laminae 4-5 X 3-5-4-0 cm. and with smaller and finer serrations. Pistillate 

 catkins 1-5 X 1-0 cm. "Common form in hilly and northern localities though" it also occurs to some 

 extent in the south of England down to Somerset and Suffolk." 



The variation is actually continuous so that the above varieties merely cover arbitrary 

 ranges on the gradient. Furthermore, though correlation coefficients have not been 

 calculated, it is apparent in the field that the largest catkins do not necessarily grow on 

 the trees with the largest leaves, and the same applies to the small forms. Neither are 

 the large-cone trees more robust in growth and with more coarsely toothed leaves as 

 Moss suggests. 



The largest catkins obtainable during 1949-52 at Chippenham Fen, Cambs., the 

 locus classicus of var. macrocarpa, have been 27 and 28 mm. long (Plate 3). The mean 

 dimensions for twenty randomly gathered catkins from this tree were found to be 

 19 X 12 mm. 



Catkins of typica size have been collected in Arran and Glen Nevis though mean 

 dimensions for each tree were below the typica limit. 



It is apparent from the data that there is a decrease from south to north and from 

 east to west in both leaf and catkin size. The gradient is not a smooth one unless popula- 

 tions are grouped regionally, as for leaf and catkin length in Tables 1 and 2. The decrease 

 in catkin length amounts to over 5 per cent per degree and leaf length to almost 4 per cent 

 per degree of longitude. 



In Figures 2 and 3 the size distribution of 200 catkins from each region studied is 



