Chap. V] THE SOIL 103 



The assumption tliat forms the basis of the whole ' physical theory ' 

 of soils, that calcicolous plants are xerophilous and silicicolous plants 

 hygrophiloiis, has no foundation. Waters rich or poor in lime are also, 

 physiologically considered, equally wet ; sphagnum-moors and grass- 

 moors arc equally moist. But even on a substratum of earth, there are 

 hygropliytes on calcareous soil and xerophytes on soils poor in lime. 

 Indeed on basalt the condition is entirely reversed, as silica-plants occupy 

 the slightly disintegrated rock as xerophytes, and lime-plants the fine 

 soils as h}-grophytes. Instances of decided hygrophytes on calcareous 

 soil arc, for instance, Ranunculus lanuginosus, Arabis alpina, Moehringia 

 muscosa, Bellidiastrum Michelii, Campanula pusilla. 



The greatest dependence on the chemical nature of the substratum 

 is frequently most strikingly contrasted with the greatest indifference as 

 regards purel}- ph\-sical conditions. Thus Schultz writes': 'A series of 

 plants are characterized by their ability to live in any habitat, from the 

 driest rocky soil to the swampy peat-meadow. Most of these are peculiarly 

 ime-needing plants ; for example, Polygala comosa, P. amara. Astragalus 

 danicus, Phyteuma orbiculare, Gentiana cruciata, Prunella grandiflora. 

 Orchis militaris, Carex flacca.' According to Boulay, Hypnum chryso- 

 phyllum grows in all stations that are rich in lime, in swamps, on dolomitic 

 >and, on dry stones, and in meadows. On the other hand, Grimmia 

 eucophaea and G. trichophylla grow on nearly all the kinds of soil as 

 dassified bj- Thurmann, on the one condition that these are poor in 

 ime. Achillea moschata and A. atrata, in the regions where they grow 

 :ogether, are markedly confined to one kind of soil, the former to a silicious 

 ■.oil, the latter to a calcareous soil, yet they arc quite indifferent as to 

 he physical characters of the soil. 



From what has been said it follows that the difference between the 

 ime-flora and the silica-flora must be attributed solely, or at least mainh^ 

 o the chemical characters of the soil. Here also must we therefore seek 

 or the key to the enigmatical phenomenon, that, according to the environ- 

 nent, one and the same species of plant shows an inconstant relation 

 the chemical nature of the soil. The cause of this different behaviour 

 :vidently depends on the fact that, as has been shown above ^ a plant 

 'ro'cvn on a substratum rich in lime is an organism of different constitutioti, 

 ind tltcrefore lias different physiological qualities and a different oecology 

 ram a plant grown on a substratum that is poor in lime. 



Different plant-organisms differ in their behaviour in relation to external 

 nfluenees, and the differences in nearly allied plants are as great as, or 

 nay be greater than, they are in plants which are not allied. Whatever 

 nay benefit the lime-form of a species will therefore frequently favour 

 he silica-form to a less degree, or will even injure it. External conditions, 

 ' Schultz, op. cit., p. 43. - See p. 95. 



