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19. — The ORrGrN and Present Disteibtjtion of the British 



Flora. 



By the Rev. George Henslow, M.A., F.L.S., F.G.S., 

 Lecturer ou Botany at St. Bartholomew's Hospital. 



[A Lecture delivered lOth October, 1878.] 



Although climate is the most essential element to be taken 

 into account when the distribution of the plants of any flora is to 

 be considered, yet as that of our own country at the present time 

 is so well known, it will be superfluous to describe it in detail.* 

 All that will be necessary is to compare it, or rather contrast it 

 generally, as being insular and maritime, with that of the Con- 

 tinent ; and then to see what differences may be expected to exist 

 between the flora of Great Britain and that of Europe. 



The chief difference between all maritime or insular and conti- 

 nental climates lies in the predominance of moisturef in the air of 

 the former and in the greater degree of dryness in that of the 

 latter. The immediate effect of watery vapour is to moderate the 

 heat in summer by arresting its passage from the sun, and similarly 

 to arrest its radiation at night and in winter. The consequence is 

 that maritime and insular climates are far less subject to extremes 

 of temperature, diurnal or annual, than are places situate away 

 from a sea-board and many miles in the interior of a continent. 

 Another very important agent in affecting the climate is the pre- 

 valence of aerial and ocean currents ; warm in ameliorating, cold 

 in deteriorating it, as far as the magnitude and vitality of any 

 flora may be concerned. This is particularly the case with the 

 British Isles; for, were it not for the warm currents both of air 

 and water sweeping past us in a north-easterly direction across the 

 Atlantic, our climate would be very likely to be as inhospitable as 

 is that on the same latitudes in America. 



Perhaps few places could be better chosen to illustrate the above 

 statements than Edinburgh and Moscow. Thus, while the differ- 

 ence between the hottest and coldest months of the year is under 

 30^ for Edinburgh, it amounts to 60^ for Moscow ; and, it may be 



* The -word climate must be taken to represent the aggregate environment of 

 plants included under: — 1. Latitude; 2. Elevation above the sea j 3. JMaritime 

 or insular or continental position; 4. Inclination of land; 5. Mountainous 

 country or otherwise ; 6. Character of soil ; 7. Condition of soU, wet or diy, etc. ; 

 8. Degree of cultivation; 9. Prevalent winds ; 10. Rainfall; 11. Mean summer 

 and mean winter temperatures, etc. 



t As an illustration of the effect of moisture upon the distribution of plants, 

 may be mentioned the fact that tropical forms extend into subtropical regions, if 

 damp; as in South America: e.^., tree-ferns, epiphytal orchids, Myrtacece, etc. 

 Similarly the laurel, fig, and bamboo ascend the humid extra-tropical mountains 

 of Bengal and Sikkhim to 9000 feet ; while on the other hand, a temperate flora, 

 consisting of Qucrciis, SaHx, Rosa, Prunns, Riilius, Camellia, finus, etc., descends 

 to the sea in lat. 25" in India. — /. D. Hooker. 



VOL. II.— PT. IV. 10 



