240 



HINTS AND NOTES 



beds are weathered down rapidly to lowland 

 conditions. It is usually where quarries have 

 been made in the lowlands that rock surfaces 

 are to be seen, and even these are less frequent 

 than along a hillside itself. Rock may be 

 exposed in a valley or level region, but except 

 in the case of coal-pits and granite quarries 

 this is unusual. 



Influence of Hills upon Drainage. It has 

 already been mentioned that a hilly tract may, 

 in spite of a higher rainfall, have really less 

 moisture relatively than a region with a smaller 

 rainfall. The wind is a very important factor 

 in this connection, its drying effect at certain 

 seasons, when not itself moisture-laden, tend- 

 ing to counteract the condensing effect of the 

 mountain itself. 



It is only where hollows are formed in a 

 hillside by the oozing out of underground 

 water through porous strata or other causes 

 that water remains upon a hill permanently. 

 It is aided in this by the effect of bog-mosses 

 such as Sphagna, and the rapid growth of 

 moorland plants that help to retain the water 

 in such pools or bogs. 



In the case of hills such as those formed by 

 chalk or limestone, where streams do not 

 carry the water away, the whole surface acts 

 as a sort of sponge and water collects in under- 

 ground reservoirs. In both cases the effect of 

 carbonic acid gas in the water in dissolving 

 the lime is to form pipes, fissures, and cavities 

 in the rock, and to produce caverns, as in 

 Derbyshire and elsewhere. 



The Limits of Agriculture. As one ascends 

 from the lowlands a noticeable feature is the 

 absence of, or decrease in, the areas given over 

 to cultivation. This is due largely to the same 

 causes that control the existence of wild plants 

 at high altitudes, such as increased cold, 

 moisture, wind, a high degree of insolation, ex- 

 posure, shallower and in general more barren 

 soil. Also fog and mist may be prevalent. 



Watson indeed established a zone, called the 

 Agrarian Zone, within which cultivated plants 

 would grow and flourish, and above which 

 they are unprofitable. This more or less cor- 

 responds with the limits of growth of the chief 

 deciduous tree types, as the Oak, &c., or 

 looo ft. above sea level. 



This has a decided effect upon wild plants 

 that are addicted to a mountain habitat. For 

 up to that altitude both soil and vegetation 

 have been more or less disturbed. At the 

 same time also the soil conditions, where culti- 

 vation has been carried on, in many cases have 

 undergone considerable change, making the 

 return of natural vegetation, when the land 

 relapses into an uncultivated stage, less likely, 

 and its substitution to a great extent by the 



followers of man and the plough the more 

 probable. 



Dry and Wet Hills. Mountains upon the 

 older granitic, siliceous, or schistose rocks that 

 reach a considerable altitude are frequently the 

 habitats of true moorland or bog plants. There 

 are, in fact, upland moors and lowland moors. 

 In each case it is a sine qua non that peat of 

 a considerable depth be formed. When the 

 peat is waterlogged there is also a Sphagnum 

 or bog-moss association, and a moss is formed. 

 But since these conditions are really dependent 

 largely upon soil characters, they have been 

 treated separately under bogs. A peaty dry 

 surface gives rise to a moorland with erica- 

 ceous plants, such as Ling, Heath, and Mat- 

 weed, whilst a Cotton-grass moor is largely 

 intermediate in regard to the supply of mois- 

 ture. The heaths that are developed on thin 

 stony or gravelly soil at lower elevations cor- 

 respond to the lowland moors, as do the upland 

 moors to the upland heaths. 



Excluding these special types there are wet 

 mountains or hills and dry ones, and the plants 

 that are found upon the one do not as a rule 

 occur on the other. 



Dry hills furnish such plants as Dyer's 

 Weed, Rock Rose, Hairy Violet, the chalk or 

 limestone on which they grow being naturally 

 well drained. Marsh Mallow also grows on 

 sandy rocky hills, and Hare's Foot Trefoil, 

 Kidney Vetch, Silky Mountain Vetch, Sain- 

 foin, Oak-leaved Mountain Avens, Salad Bur- 

 net, Ploughman's Spikenard, Cotton Thistle, 

 Wild Thyme, Clary, Sheep's Sorrel, Box, Musk 

 Orchis, Sheep's Fescue are found also on dry 

 hills. Yellow Balsam, Gentian, Fehvort, and 

 Fragrant Orchis are found on wet hillsides, 

 and Field Scabious, Dropwort, on damp clay, 

 often, though not invariably, at high altitudes. 



Climate and Hills. Hills have an important 

 bearing upon climate. As one ascends 300 ft. 

 there is a difference of i F., so that the ther- 

 mometer in the lowlands at 56 F. would read 

 at the summit of a hill of 4500 ft. (the limit in 

 the British Isles) at 41 F., a difference of 15 

 degrees. This has naturally a great effect upon 

 plants in performing their various life functions. 

 Germination to begin with is slow, and may 

 never take place in many ill-developed seeds. 

 Growth is maintained slowly by impoverished 

 powers of assimilation, respiration, and trans- 

 piration, for nutrition is scanty, and therefore 

 reproduction must be hazardous, so that it is 

 inevitable that montane plants differ in their 

 seasons of flowering from those within the 

 plains. Fresh types of plants succeed each 

 other at different altitudes owing to these 

 variations in temperature, &c. Thus the trees 

 disappear at 1000-1500 ft., sub-Alpine plants 



