58 OECOLOGICAL FACTORS AND THEIR ACTION sect, i 



Although the characteristics of the Hme-flora are clear and distinct, 

 yet in the past the influence of lime upon vegetation has been over- 

 estimated. Indeed, a distinction has been made between calciphilous 

 and calciphobus plants.^ Recently it has been definitely estabhshed that 

 the amount of lime in itself, in so far as it does not operate physically, 

 cannot be the cause of differences in the flora, for not only can calci- 

 colous plants be cultivated in soil that is poor in lime, but sihcicolous 

 plants, and even bog-mosses, which are regarded as pre-eminently calci- 

 phobous, can grow vigorously in pure lime-water - if the aqueous solution 

 be otherwise poor in dissolved salts. It has been overlooked that nearly 

 all lime soils are rich in soluble mineral substances, and this wealth ex- 

 cludes plants belonging to poorer soils ; beyond this the important physical 

 characters of calcareous soil, compared with granite soil, come into play. 



Geographical significance of nutriment in soil. The nutritive substances 

 indispensable to the higher plants occur in nearly all soils certain ones, 

 such as quartz-sand, being excepted in quantities so considerable that 

 in this respect there is no obstacle to prevent any plant growing almost 

 anywhere on earth. It must be remembered that even when a nutritive 

 substance is present in the substratum in very small quantity, the plants 

 requiring it can yet absorb it in large quantities ; for instance, species of 

 Fucus accumulate a great deal of iodine, though extremely little of this 

 is contained in sea-water. The plant has a certain power of quantitative 

 selection, in that it absorbs various substances in proportions other than 

 those in which they occur within the substratum. There are, however, 

 substances which exert a poisonous action on certain plants and exclude 

 these from soils that contain a large amount of them. This is perfectly 

 comprehensible when it is remembered that the plant can select its 

 nutriment only to a certain extent. The larger the amount of a substance 

 in the soil, the more of it, as a rule, is absorbed by the plant ; and in all 

 cases substances that are useful or even essential in small quantities, 

 may be absorbed to excess or act as poisons. Substances of this nature 

 are common salt and ferrous salts. But a certain amount of latitude 

 prevails in this matter, for one and the same species absorbs the various 

 nutritive substances from various soils in different proportions. Indi- 

 viduals of the same species on granite-soil contain much silica, and on 

 calcareous soil much lime. Finally, it may be noted that certain sub- 

 stances, for instance lime and magnesia, can replace each other to some 

 extent. 



It is of profound significance to communities of plants that each 

 species has its own peculiar economy, the nature of which is almost 

 unknown to us ; for in virtue of its metabolic activity and the attributes 

 of its root-system each absorbs substances in proportions different from 

 those prevaihng in other species. For the communal life of species it 

 is also of importance that substances are not absorbed at the same rate 

 and time, or at the same ontogenetic stage. This renders it possible for 

 many species to live side by side on the same soil without entering upon 

 a struggle for food. Partially dependent upon this is also the system of 

 ' rotation of crops '. 



^ Sendtner, i860; Contejean, 1881. 



^ See C. A. Weber, 1900 ; Griibner, 1901 ; and the critique of this by F. E. 

 Clements, 1904. 



