SOME GENERAL HINTS AND NOTES 



SECTION VII 



MOUNTAINS, HILLS, AND DRY PLACES 



The Effect of Altitude upon Plants. — For c\er)- 

 successive rise in altitude of 300 ft. the tem- 

 perature decreases by 1^ F. Hence the effect of 

 .•iUitud(^ upon plants in the first place is to drive 

 the southern species downward and the north- 

 ern species upward. A point is reached above 

 2000 ft. when .\rctic plants begin to become 

 dominant, and at the highest elevations in the 

 British Isles the vegetation is distinctly Alpine, 

 consisting of such groups of plants as Saxi- 

 frages, Pinks, and Willows. Another effect 

 of altitude is to make the conditions as a 

 whole moister, owing to the fact that moun- 

 tains act as condensers of moisture. 



As a whole eastern, and a great part of 

 southern, England has a rainfall of not much 

 more than 20 in. per annum, and these are the 

 lowland areas. The rainfall of the north and 

 west and south-west is much greater, and in 

 places amounts to 100 in., whilst in parts of 

 the Lake district it may be as much as 200 In. 

 Though the rainfall is so great in hilly dis- 

 tricts the ground Is not necessarily so satu- 

 rated, for water finds its own level very quickly, 

 and it is in the lowlands where it lies longest. 

 Only in the hollows, or on some hill-tops, does 

 the hilly tract develop boggy conditions. Else- 

 where the conditions, especially the slopes and 

 rocky summits with shallow soil, are suitable 

 for dry-soil forms, and a large bulk of the 

 plants are adapted to this state. Owing to 

 altitude, again, especially above the deciduous- 

 tree zone (1000 ft.), the ground flora Is much 

 more exposed to wind. 



A further effect of altitude (or the ex- 

 istence of hills) is to determine the prevalent 

 winds, and to regulate the distribution of 

 valleys and rivers. Another feature Is the 

 exposure of plants to mist and fog. The 

 clouding of the sky on hills has an influence 

 on the light conditions. 



Lowlands and Uplands Compared. — The 

 general conditions in the lowlands make for 

 uniformity. It is true that wide associations 

 occur also upon hills, as those formed by 

 Heather, Cotton Grass, Sedges, Rushes, and 

 some grass types, but these are, even when 

 widespread, more or less discontinuous owing 

 to physical structure, slope, &c. , whereas in tlie 

 lowlands these conditions are less variable. 

 Moreover, lowland tracts vary little in altitude, 

 so that the temperature and rainfall are more 

 or less uniform. Thus we find in the lowlands 

 wide expanses of forests, pasture, heath, arable, 

 lowland moors, and aquatic vegetation on a 

 grand scale, as formerly in the Fens. Hence 

 there Is a preponderance of southern t3'pes of 

 plants. Clay plants in particular are charac- 

 teristic of the lowlands, and those found upon 

 marls and loams, or in other words, the vege- 

 tation of the .Agrarian Zone. 



The uplands, on the other hand, exhibit the 

 opposite characteristics. The associations are 

 not so homogeneous as in the lowlands. The 

 sloping sides of hills are often unstable, the 

 talus and debris moving downwards, and the 

 soil is also of a more barren character, owing to 

 the exposure of the surface to denuding agencies 

 and the slight opportunity afforded for soil 

 formation or retention. Hence not only climatic 

 but also physical factors cause the lowlands and 

 the uplands to present entirely dissimilar types 

 of vegetation, as may be recognized at once by 

 comparing, for instance, the flora of the Welsh 

 hills wllli that of Bedfordshire. 



Prevalence of Bare Rocks in Hilly Tracts. — 

 One marked characteristic of upland areas is 

 the preponderance at high altitudes of bare 

 rock surfaces. Moreover, an outcrop where 

 soft and hard rocks are contiguous is as a rule 

 diversified, and the hard rocks are exposed as 

 hills or escarpments, whilst soft clayey or shaly 



