ASH. II 97 



have shown that roots branch much more freely in soils which contain abundant 

 food materials than in those which are poor in them. Nobbe (1862 and 1868) 

 has shown this by cultivating clover and maize in a soil which consisted through- 

 out of the same basal material, but with the alternate layers saturated with 

 a nutritive solution. Experiments on this subject were also made by Thiel 

 (quoted by Sachs, 1865, p. 178), and more recently by Hoveler (1892), who 

 employed soils which consisted of alternate layers of sand and humus. 



Having now determined in what condition the plant finds its food-stuffs 

 in the soil, and how it absorbs these with the aid of its roots, there remains for 

 us to study how variations in the chemical composition of the soil influence its 

 occupation by plants. We have already considered how the primal coloniza- 

 tion of a rock takes place, and how it is transformed into a soil under the influ- 

 ence of plants which are easily satisfied. It has further been shown how the 

 humus compounds which arise from the decomposition of not only plant but 

 also animal remains, add to the fertility of the soil. Soils occur in nature, 

 however, which are quite free from vegetation and that for very varied 

 reasons. The great resistance presented by some minerals to decomposition 

 must be considered first, and an example of such is seen in lava, which becomes 

 covered with vegetation only extremely slowly (compare Treub, 1888, on 

 Krakatao, and Schimper, 1898, 200, on GunungGuntur). On the other hand, 

 although it rarely happens, a rock may weather, but does not contain all the 

 elements required by the plant, or, again, it may contain too great a proportion 

 of mineral salts (e. g. sodium chloride, &c. ) which interfere with the introduction of 

 plants, or it may be deficient in water. Such regions of the earth which are desti- 

 tute of vegetation we term deserts. The greater part of the earth's surface is 

 capable of supplying the plant with all the chemical compounds necessary and in 

 quantities sufficient for its growth, and for that reason it is covered with vege- 

 tation, but this vegetative covering takes on very varied characters in different 

 regions. In so far as we are able to understand the causes of this variation, we 

 may refer to climate and soil as the most important determining factors in plant 

 distribution. Here we are naturally concerned with soil only, and we may draw 

 attention to a fact with which we have been long acquainted, that a seashore 

 whose soil contains salt in large quantity has quite as characteristic a flora as 

 inland regions with, say, a lime soil, which exhibit plant societies other than those 

 shown by a sandy soil poor in lime, or by primitive rock. We distinguish in 

 geographical botany between plants which are confined strictly to soils with 

 definite characters, and such as are able to thrive in various kinds of soil ; the 

 former are 'local' in distribution, the latter 'indifferent'. 



Thus there is quite a number of halophytes which in nature occur by prefer- 

 ence or exclusively on a soil which contains a large quantity of sodium chloride, 

 such, for example, as a seashore, where one usually finds as much as 3 per cent, 

 of common salt. So far as we know common salt, however, performs no 

 special function in their metabolism other than it performs in the rest of the 

 vegetable kingdom, for these halophytes can exist in soils containing a mere 

 trace of this salt or none at all. The point where halophytes differ from other 

 plants lies in their capacity for tolerating quantities of sodium chloride which 

 are directly injurious to non-halophytes. In virtue of this power they are able to 

 exist in places from which other plants are debarred, while in ordinary soils they, 

 for the most part, give place to non-halophytic types. The injuries inflicted 

 on ordinary plants owing to the presence of an excessive amount of salt in the 

 soil depend, in the first place, on the osmotic action of the concentrated solutions 

 in the soil, and in the second place, on the difficulty of bringing about absorption 

 of water; further, common salt, when absorbed, has certain, to all appearance, 

 after-effects which are not as yet perfectly understood. Halophytes over- 

 come the greater difficulty of absorption by being extremely economical in 



JOST H 



