X. CLIMATE OF PLANTS. 475 



there maintain itself century after century, — that is, end- 

 lessly to our present mental vision, — under those different 

 conditions of temperature. 



The like comparisons or contrasts might be made in 

 respect of humidity also. Polygonum amphibium wUl 

 grow well in ponds and canals, where its roots are under 

 water the year through ; also growing well under a 

 changed form, in meadows and other places which are 

 inundated only occasionally, and then chiefly during the 

 season of its apparent torpidity ; and being stiU able to 

 exist also on cultivated land, the surface of which is never 

 under water. Clirysosplenium oppositifolium grows vigor- 

 ously in damp and shaded places on the coast-level in 

 southern England ; and it thrives also in cold springlets 

 on the unsheltered acclivities of the Highland mountains; 

 the temperature and humidity of the two situations being 

 always widely dissimilar, unless perhaps occasionally in 

 the winter, while the water is frozen and the plants buried 

 under snow. Montia fontana occurs with the Chrysosple- 

 nium in these mountain s^Dnnglets ; being found also in 

 the lowest water-courses of the southern provinces, and 

 likewise as a weed in gardens there. 



All species, it is true, are not adapted to endure such 

 wide diversities of temperature or humidity ; and many 

 seem to have only a short range of climate. The Men- 

 zies'ia polifolia of Ireland, and the Erica vagans of Corn- 

 wall, are occasionally killed by the severity of the winter 

 frosts near London, and would probably not endure sum- 

 mers much drier, than experienced there occasionally. The 

 western Pinguicula lusitanica and Sibthorpia etiropcea are 

 apparently unfitted to endure the wider variations of mois- 

 ture and dryness to which they would be subjected in the 

 eastern counties of England. The Saxifrages of Ireland will 

 bear well enough the more severe climate of Cumberland 



