Chap. V] THE SOIL 103 



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

 of soils, that calcicolous plants are xerophilous and silicicolous plants 

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

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

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

 hygrophytes 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 hygrophytes. Instances of decided hygrophytes on calcareous 

 soil are, 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 purely physical conditions. Thus Schultz writes 1 : '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 

 lime-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 

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

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

 classified by Thurmann, on the one condition that these are poor in 

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

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

 soil, the latter to a calcareous soil, yet they are quite indifferent as to 

 the physical characters of the soil. 



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

 lime-flora and the silica-flora must be attributed solely, or at least mainly, 

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

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

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

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

 evidently depends on the fact that, as has been shown above 2 , a plant 

 grown on a substratum inch in lime is an organism of different constitution, 

 and tJiereforc has different physiological qualities and a different oecology 

 from a plant groivn on a substratum that is poor in lime. 



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

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

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

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

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

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



