316 PROTECTIVE ARRANGEMENTS OX THE EPIDERMIS. 



tomentosa, stellata), of the grey-leaved Ragwort (Senecio incanus and Camiolicus), 

 of the magnificent silky Cinquefoil (Potentilla nitida), and of the white-leaved 

 bitter Milfoil (Achillea Clavennce); especially is this the habitat of the most 

 celebrated Alpine plants, of the scented Edelraut and the beautiful Edelweiss — the 

 former (Artemisia Miitellina) with a grey shimmering silky coat, the latter 

 (Gnaphalium Leontopodwwm) wrapped in dull white flannel. On looking at the 

 vertical section of the Edelweiss leaf (see fig. 77 1 ), one sees that the epidermal 

 cells with their thin outer walls would be unable to regulate exhalation and drying 

 in the sun, and that a powerful protection is afforded against too rapid evaporation, 

 in case of extraordinary dryness, by the possession of a layer of sapless, air-filled, 

 interwoven hair-structures. The Edelraut, Ragwort, and the other plants named, 

 which grow on the sunny rocks of the Alps, show these same characters of leaf 

 structure, and what has just been said about the Edelweiss applies fully to them 

 also. It should be mentioned that on the heights of the Pyrenees, Abruzzi. and 

 Carpathians, as well as on the Caucasus and Himalayas, the plants growing on 

 sunny ridges of rock, where they are exposed to the wind, are covered with silk and 

 wool exactly after the model of the Edelraut and Edelweiss, and that there is on the 

 Himalayas an Edelweiss which is wonderfully similar to that of the European Alps. 

 In the far north, on the other hand, where the flora in other respects has so much in 

 common with that of the Alps, these plants are absent, and generally a search over 

 the rocky crags for herbs and shrubs, whose leaves are furnished with silk}- or felt- 

 like coverings on the upper surface, is futile. The genera which grow on these 

 places and form a characteristic feature of the vegetation in consequence of their 

 great abundance — as, for example, Diapensia Laj'ponica, Andromeda hypnoides, 

 MerteTisia maritima, Draba aVpinna, and others, possess remarkably smooth green 

 leaves. When hairy coverings are present, they are restricted to the under leaf- 

 surface, especialty to that of rolled leaves. They are never found on the plants of 

 rocky slopes, but only on those of damp marshy ground, or by the side of water 

 which is for a short time free from ice. Here, however, they certainly do not help 

 to lessen transpiration, but function in the way described above in the discussion on 

 rolled leaves. It is indeed not too much to connect these facts with the conditions 

 of the climate, and especially to explain the absence of plants whose foliage is silky 

 or felt-like on the upper surface, by saying that a drying up of the soil and a 

 limiting of the water supply never occurs on the narrow terraces of steep rocky 

 declivities in Arctic regions, and that therefore there is no danger of over-evapora- 

 tion to plants growing in those regions. 



It is in keeping with this explanation that on Central and South European 

 mountains, on whose heights an Alpine vegetation is to be found, the number of 

 forms having silky and felted foliage increases as these mountains are situated 

 further south, and the more they are exposed to temporary dryness. Plants of the 

 Edelweiss type are still wholly foreign to the Riesen-Gebirge; in the Northern 

 Alps their number is comparatively small, in the Southern Alps they increase 

 in a surprising manner, and the summits of the Magellastock. the ridges of 



