498 DEPENDENCE OF PLANT FORM ON SOIL AND CLIMATE. 



common salt would be injurious to one species, and an abundance of sodium or 

 potassium salts to another. From the present standpoint of our knowledge 

 concerning the absorption of inorganic materials by plants, therefore, Unger's 

 classification, especially the expressions silica-demanding and silica-preferring, is no 

 longer suitable, and it would be more to the purpose to speak of plants which are 

 injured by lime, potash, &c. 



The difference in the vegetation on the closely adjoining limestone and slate 

 mountains met with in so many places in the Al; s, and so well seen in the neigh- 

 bourhood of Kitzbühel, where the climatic influences on the two ranges are identical, 

 can be accounted for most satisfactorily in the following way. Plant-species which 

 demand or prefer a siliceous soil are absent from limestone mountains wherever 

 their roots would be exposed to more free lime than is beneficial; if present they 

 would be weakened, and thus vanquished in the struggle with their fellows, to 

 whom the larger quantitj^ of lime is harmless, and they would eventually perish. 

 These plants flourish luxuriantly, however, on slate mountains, because there the 

 soil does not contain an injurious amount of lime. The absence of species, demand- 

 ing or preferring lime, from slate mountains can be explained in the same way. 

 When seeds are brought thither by the wind from the neighbouring limestone 

 mountains and germination commences, their further development is visibly 

 retarded; they dwindle wherever there is not much lime, and are overgrown 

 and suppressed by the siliceous species which flourish there so luxuriantly. The 

 brown or black mass formed by the decomposition of dead plant residues, known as 

 humus, plays a very important part in the contrasting vegetation on limestone and 

 slate mountains. To obtain a true idea of its significance it must first be pointed 

 out that three distinct stages can be distinguished in the development of a 

 continuous and intricate plant-covering. To the first stage belong the plants 

 which settle down on the bare earth content with a substratum wholly devoid of 

 humus; in the course of time they conquer the most barren rock, the barest 

 boulders, and the dreariest shifting sands. The species of this group belong chiefly 

 to the Lichens, Mosses, Grasses, Pinks, Crucifers, House-leeks, Saxifrages, and 

 Composites, whose spores, seeds, and fruits are exceptionally well adapted for wind 

 distribution, and can be transferred with ease to the steepest slopes and the most 

 uncompromising crags. The second stage includes plants which require a moderate 

 amount of soil mixed with humus: they establish themselves on the ground pre- 

 pared by the first settlers, wresting it from them and taking possession, and then 

 suppressing and overgrowing them entirely. These plants belong to very difterent 

 families, whose distribution and establishment are effected in very many ways to 

 be described subsequently. The third stage of development consists of plants for 

 which the abundant humus stored up successively by the plants of the second stage 

 is absolutely indispensable. Bog-moss, Lycopodiums, Sedges, and Heaths form the 

 chief part of this stage. In the course of years the amount of inorganic materials 

 in the soil which supports the plants of the third stage continuously diminishes. 

 Plants which require a large quantity of inorganic salts languish, and are, moreover, 



