702 DISTRIBUTION OF PLANTS IN BRITAIN. 



plant of Laurencia dasjrphylla supplied 20-30 good-sized specimens ; 

 Mertensia and Claudia, large, but rare. 



Species which are found abundantly in one sea may be scarcely 

 present in another. This is seen in the difference between the 

 marine vegetation of the Eed Sea as compared with that of the 

 Mediterranean, and that of the Mediterranean as contrasted with the 

 Atlantic. The genera Sargassum and Oaulerpa of the Eed Sea are 

 represented in the Mediterranean by very few, and those distinct, 

 species. The genus Fucus, which is common in the Atlantic, is 

 almost entirely wanting in the Mediterranean. Many Plorideas 

 which abound in the open seas do not adorn the rocks in the Medi- 

 terranean. 



DISTRIBUTION OF PLANTS IN BRITAIN. 



The climate of Britain is influenced by its geographical position, 

 and the form and elevation of its surface., The climate is wanner 

 than that of other places in the same parallel of latitude. Its most 

 striking feature is the absence of extremes, either as regards cold or 

 heat. It is, generally speaking, mild and damp. The eastern coasts 

 partake more of the continental climate, while the western experience 

 the insular or more equable climate. While the winters are mUd, the 

 heat of the three summer months, June, July, and August, in which 

 the growth and ripening of crops take place, is by no means great, 

 beiag very little above that due to the latitude. The heat of these 

 months is most important. It should be noticed that the day and 

 night may be both mild during these months, and thus give rise to a 

 high average temperature. But the important thing is to have high 

 temperature during the day, even although the nights are cool. The 

 mean temperature varies from 46° to 52° P. Some of the mountains- 

 rise to the height of about 4400 feet, and there is a fall of 1° of 

 the thermometer for every 250 or 260 feet of ascent. Mr. Alexander 

 Buchan gives the- rate of decrease as 1° for about every 300 feet of 

 ascent. The number of Phanerogamous species of plants amounts to 

 about 1600, while the Cryptogamous are probably about 6000. 



In considering the distribution of British plants as regard areas, 

 Watson divides Britain (excluding Ireland and the Channel Islands) 

 into 18 provinces, or groups of counties, which together constitute the 

 basin of a principal river, or have some other physical peculiarity in 

 common. In each of these provinces he notices the heights attained 

 by the loftiest mountains. The details connected with those pro- 

 vinces are given in his Cybele Britannica. Many of the British 

 species appear to have been introduced, and some appear to have 



