496 DEPENDENCE OF PLANT FORM ON SOIL AND CLIMATE. 



of the geological, climatic, and botanical conditions of his new locality, devoting 

 his fullest attention to the relations between the plants and the rocks forming their 

 substratum. The result of this study was his work, published in 1836, On the 

 Influence of Soil on the Distribution of Plants as shown in the Vegetation of 

 the North-east Tyrol, which marked an epoch in questions of this sort. The ter- 

 minology introduced in the book found rapid entrance into the Botanical works 

 of the time. Unger divided the plants of the district according to their occurrence 

 on one or other of the substratums— in which lime and silica respectively pre- 

 dominated—into (1) those which grow and flourish on limestone only; (2) those 

 which prefer limestone, but which will grow on other soils; (3) those which grow 

 and flourish on silica only; and (4) those which, whilst preferring silica, will grow 

 on other soils. He tabulated his results in such a way as to show clearly how 

 certain species grew on the limestone and others on the silica-containing rock. 

 Naturally these facts elicited a number of speculations. If the species Gentiana 

 Clusii; Hutchinsia alpina, and Juncus monanthos growing on the limestone soil 

 are replaced on the slaty soil by the similar (but yet distinct) species, Gentiana 

 acavlis {excisa), Hutchinsia brevicaulis, and Juncus trifidus, we are justified in 

 assuming that the difference in form is due to the influence of the substratum, i.e. 

 to the influence of the chief materials in the rock — limestone and silica. But it 

 has yet to be ascertained and proved, if possible by experiment, how this influence 

 works; whether limestone and silica, respectively, introduce certain compounds into 

 a plant, thus altering its outward appearance, or whether the difference is due 

 rather to the fact that each plant -species requires so much lime or so much 

 silica, and that when this is lacking in the soil the outward form becomes changed; 

 or again, whether, after all, the physical properties of the substratum, its porosity, 

 •capacity for retaining water, and its specific heat, have not more influence on the 

 form of plants than its chemical constitution. 



Unger and his followers, amongst whom I enroll myself, thought they would 

 obtain an answer to these questions by comparing the chemical composition of the 

 plant-ash with that of the soil in which the plants were grown. But the results of 

 careful investigations were anything but satisfactory. Both the substances named, 

 the presence of which was supposed to be of special importance, could be demon- 

 strated in most of the soils examined. Labrador felspar, hornblende, and other 

 minerals in crystalline slate yield as much lime in the upper soil as is required 

 by plants demanding or preferring limestone (classes 1 and 2, above), whilst the 

 Limestones, which almost all contain clay, have silica enough for the needs of 

 plants which demand or prefer silica (classes 3 and 4, above). Moreover, it was 

 shown that plants have the power of obtaining materials which are valuable to 

 them even when these occur around their roots in hardly appreciable quantity, so 

 that they actually become accumulators of certain materials, and in this way 

 a substance of which there are only minute traces in the underlying rock may 

 be relatively abundant in the superficial layers of soil impregnated with the dead 

 plant-remains (cf vol. i. pp. 70 and 259). 



