254 THE GEOGRAPHY 



about the same actions, contributes to chain particular 

 plants so much the more firmly, exclusively, to particular 

 soils, or contrarywise, also contributes to conceal or 

 obliterate the connection between plants and the chemical 

 nature of the soil. This consists in the mechanical con- 

 dition and physical peculiarities of the soil. There are 

 plants which will only settle on unbroken roc/cs, which 

 when the other conditions coincide, spring from these rocks 

 over on to our walls, like the Wall Rue Spleenwort,^ a 

 little Fern, the name of which denotes its station. Others 

 occur only where weathering has broken up the solid rock 

 into small fragments, drift plants, which clinging to 

 mankind, select rubbish heaps, which most resemble their 

 natural station ; our great Nettle and Henbane may serve 

 as examples. Lastly, other plants grow only where the 

 rocks have been reduced to fine powder, in sand or in 

 the fine grained clay produced by chemical decomposition. 

 The so-called German Sarsaparilla, the Sea-reed,f is an 

 example of the first condition, but there is no definite 

 condition corresponding to it in the vicinity of human 

 habitations. Clay, on the other hand, stands beside the 

 black substance, humus, resulting from the decomposition 

 of organic matter. Both rich in soluble salts, important 

 to vegetation, both distinguished in regard to their property 

 of absorbing from the atmosphere, and thus conveying to 

 the roots of plants, gases and aqueous vapour, they cause, 

 singly or in combination, the most luxuriant vegetation. 

 We thus obtain three stages in reference to the qualities 

 of the soil ; pure earths, wholly devoid of vegetation 

 mixed earths, without clay or humus, with an arid but 

 characteristic vegetation and lastly, soil rich in clay and 



* Asplenium Ruta muraria. f Ammophila arenaria. 



