MOUMAIXS, HILLS, AND DRY PLACES 



241 



flourish up to 2000 ft., and above this raiiije 

 the Arctic and Alpine species. 



Rainfall and Hills. — Hills exercise a decided 

 influence incausing^ the moisture in the atmos- 

 phere to be condensed and precipitated as rain. 

 The position of the range of hills in relation to 

 the wind is important, rain being precipitated 

 to the Ice of the hills. When the axis of the 

 range is oblique, the effect upon the air current 

 is similar to that of a less steep slope, and 

 more so than in the case of a range at right 

 angles, and the air ascends more slowly, rain 

 being precipitated over a wider area. The 

 snow-line on a hill necessarily also affects 

 the atmospheric conditions and rainfall in 

 such regions. 



The Exposure of Hills. — The direction of the 

 hills is an important factor, especially in rela- 

 tion to the prevailing winds, causing different 

 effects upon different types of vegetation. 

 Plants with the grass habit are less affected 

 than those having the heath habit, for in- 

 stance, the latter forming crescentic patches 

 under certain conditions. The vegetation on 

 the lee side of a hill is much more luxuriant 

 than that upon the windward side. But the 

 existence of a series of winds blowing from 

 different directions is liable to obscure the in- 

 fluence of prevailing winds. The effect of the 

 direction of the range is again seen in the 

 greater exposure of plants to the sun on the 

 south and west sides, and their xeromorphic 

 conditions of growth, less noticeable upon 

 other aspects. 



Effect of Hills upon Types. — .As one ascends 

 a hill from the surrounding lowlands, one can 

 readily notice a difference in the characters of 

 many common types of plants. .Such a plant 

 as the Common Dandelion, in the v.alleys, has 

 a tall and thick scape with a large flowerhead. 

 The radical leaves are long and broad, with 

 few lobes, or lobes not deeply cut back. The 

 achenes are green as a rule. The root is thick 

 and deep-reaching. The whole plant is, in 

 other words, suited to conditions where there 

 is a uniform and considerable supph' of mois- 

 ture, and a thick or deep soil. .At 500-600 ft., 

 the normal type of Dandelion is replaced by a 

 form with a short narrow scape, a smaller 

 flowerhead, and narrower, shorter, and more 

 deeply-divided leaves. The fruits are reddish 

 in colour. The root is short, oflen dividing at 

 the extremity. The leaves are usually pros- 

 trate upon the surface, rarely becoming ca;spi- 

 to.se or ascending. 



The types to be met with upon hills differ 

 to a great extent from those of plants found 

 in the lowlands. There is an increase in the 

 members of the grass tvpe, the rosette habit, 

 &c. 



VOL. IV, 



Aspect. — The chief types of natural lialiilat 

 that affect aspect are sea-coasts, where cliffs 

 have ;i delinite aspect, hedgerows in fields, or 

 roadsides. 



It is estimated that a southerly aspect has 

 the effect of an increase of 2° to 3" C. near the 

 surface, and this is proportionally more at 

 higher altitudes. Slope, moreover, and its 

 direction have an effect upon the temperature 

 of the soil ; for radiation is more rapid upon 

 a sloping surface than upon a flat surface. 

 Hence the beneficial effect of a southerly 

 aspect, especially in hilly regions. 



.As a rule the western ;md southern sides of 

 a hill or mountain are more suitable for plant 

 growth, as the first is more uniformly moist, 

 while the second has a higher average tem- 

 perature, and the thermal constants of plants 

 on such aspects are greater. .Aspect thus has 

 an efi'ect upon the dispersal of plants by 

 natural selection, the sun-loving plants seek- 

 ing the sunnier aspects, the cold-lovers being 

 found upon the north and east. 



Absence of Aquatic Vegetation upon Hills. 

 — One result of the existence of hills is to 

 delimit the areas given up to aquatic vegeta- 

 tion. For such vegetation is more or less 

 entirely confined to plains, valleys, or lowlands 

 generally. 



The hygrophilous types, such as those that 

 form part of the moorlands, peat-bogs, cotton- 

 grass associations, and others formed by 

 Cariccs, Rushes, some Grasses, &c., are not 

 aquatic types in the strict sense. 



-Some exceptions must, however, be made 

 to this general rule, for though lakes as a rule 

 are lowland, yet there are a number of lakes, 

 e.g. in Wales and the Lake District, that are 

 distinctly of upland type. And there are many 

 lochs and tarns in -Scotland which Iiave a 

 characteristic vegetation. 



Ascending and Descending- Types. — Owing 

 to the differences in temperature, effect of 

 wind, &c. , between the lowland and uplatid 

 regions, there are several types of plant groups 

 which may be distinguished as montane or 

 descending and lowland or ascending. The 

 montane plants are as a rule of wider range in 

 the northern regions, and the lowland plants 

 more widespread in the south. 



The montane plants include, first of all, 

 eu-montane species, chiefly of .Arctic type, as 

 the Cloudberry. They seldom descend below 

 the Infer-.Arctic Zone, or about iSooft., where 

 the temperature is 39^ F. to 42'^ F. In Britain 

 the eu-monfane group ranges between 2000 ft. 

 and 3200 ft. The general montane group is 

 found at altitudes between 2000 ft. and 1000 ft., 

 and is most common in the north generally 

 speaking, as Juniper, Whortleberry, Cowberry, 



62 a 



