24 



YULANDE HESLOP-HARRISON 



long-range dispersal rather far to assume that all of the hybrid colonies marked in Fig. 1 

 have arisen in this manner. 



The alternative explanation, that the isolated southern N. intermedia colonies originated 

 in their present stations at the time when the two parental species occurred together in 

 the neighbourhood, would seem to be more probable. It is likely that N. pumila, like 

 other boreal and montane species, has been in the process of area-contraction for some 

 considerable time. It can hardly be doubted that N. pumila did exist in N. England at 

 one time during the early post-glacial period, but without fossil evidence it is difficult 

 to say how long it is likely to have persisted in the Cheviot area. Two factors probably 

 led to its extinction: the climatic amelioration and the gradual elimination of suitable 

 habitats through the growth of peat bogs. Sm^the (1930) has suggested that Chartners 

 and the other remaining lakelets on the Northumberland Fells may have arisen as 

 moraine- dammed lakes in the closing stages of the last glaciation; how many others in 

 the area once open to colonisation by N._ pumila have now entirely disappeared we have 

 no means of knowing. 



N. lutea, although no doubt a later immigrant into the British Isles than N. pumila, 

 is known to have been present in early boreal times; reproductive contacts with N. pumila 

 may have taken place repeatedly during the replacement of one by the other in southern 

 and lowland districts. Where in the neighbourhood of expanding populations of N. 

 lutea, the hybrids probably became eliminated by competition and genetical " swamping." 

 In a few more isolated localities, particularly in relatively upland stations where their 

 " intermediate " genot>^pe may have been of adaptative value, they appear to have been 

 able to persist. 



Acknowledgments 



I wish to acknowledge the receipt of a grant from the Central Research Fund of the 

 University of London towards travelling expenses incurred during the field work carried 

 out during the course of this study. To the authorities of the British Museum, Kew, 

 Edinburgh and Cambridge University Herbaria I wish to tender my warmest thanks for 

 their most helpful co-operation. 



REFERENCES 



BAKER, J. G. & TATE, G. R., 1S6S, A new flora of Northumberland and Durham, Tram. Nat. Hist. Soc. 



Northumberland and Durham, 11, 119. 

 CASP.\RY, J. X. R., 1S69, Die Nuphar Lapplands. Konigsberg. 



r , 1S70, Die Nuphar der V'ogesen und des Scharzwaldes, Abhandlg. d. Naturf. Gesellsch. 



zu Halle, 11, lSl-270. 



, 1879, Hvilken utbredning hafva Nymphaeaceerna i Skandinavien ?, Bot. Not., 1879, 



65-93. 



COX, T. A., 1855, New Records : Nuphar pumila in Shropshire, Phyt., N.S., 1, 127. 

 HAY WARD, W. R., 1892, The Botanist's Pocket Book, ed. 3, 6. London. 

 HOOKER, J. D., 1884, The Students' Flora of the British Isles, ed. 3, 15. London. 

 HOOKER, W. J., 1821, Flora Scotica, 169. London. 

 , 1835, The British Flora, ed. 3, 1, 263. London. 



& ARNOTT, G. A. W., 1855, The British Flora, ed. 7, 15. London. 



KOCH, 1843, ex BURNAT, E., 1866, Notes on Nuphar pumilum. Dornach. 



L.\NGLET, O. F. J. &: SODERBERG, E., 1927, Ueber die Chromosomenzahlcn einigcrNymphaeaceen, Acta. 



Horti Berg., 9, 85-104. 

 LEDEBOUR, C. F., 1832. Flora Altaica, 2, 274. Berlin. 

 LUCKLEY, J. L.. 1893, A flora of Alnwick. Alnwick. 

 MEUSEL, H., 1943, Vergleichende Arealkundc. Berlin. 



MILLER, G. S. & STANDLEY, P. C, 1912, The North .Wrican species oi Nymphaea, Contributions 

 from the U.S. Nat. Herb., 16, 181-270. 



