SECTION VIII 

 CLASS VI. LITHOPHYTES. FORMATIONS ON ROCKS 



CHAPTER LXI. ROCKY COUNTRY 



THE more uniform a soil, the more uniform and purely typical is 

 the plant-formation growing on it. The soil is generally uniform where it 

 remains flat and horizontal over a wide area. Contrasting with this is a 

 rocky country with numerous declivities, masses of rock, and chasms suc- 

 ceeding one another. A single formation may be composed of many species 

 and many different growth-forms, but it must constitute a single entity, 

 in which one species is more or less dependent upon another or at least 

 stands in some relation to it. In a mountainous, rock-strewn district 

 fragments of various, perfectly independent, formations intermingle in 

 a chaotic manner, which is the more diversified the more uneven and 

 varied the surface. 1 



The exposure of walls of rock or sides of mountains varies extremely, 

 and has a most important influence on vegetation ; neighbouring rocks 

 may differ entirely in their exposure, and consequently may bear radically 

 different plants, and such is the case with mountain-sides respectively 

 exposed to and removed from the sun's rays. 



Slope of the surface of rocks or of the sides of mountains varies in 

 like manner and is of no less significance ; the steeper the general surface 

 the greater the extent to which natural rock will come to the surface, 

 the more flat and horizontal is a rocky tract the more does it favour the 

 accumulation of detritus, of products of weathered rock, and of vegetable 

 fragments, that is to say, the production of a loose covering of soil ; and 

 hand in hand with these distinctions go corresponding ones in the 

 vegetation. 



Of further import is the nature of the rock (if it be primitive, or lime- 

 stone, or slate, and so forth), because its hardness, tendency to split, 

 specific heat, and other properties are of the deepest significance to 

 vegetation) 2 ; and it is a matter of importance whether or no water flows 

 away over the sides of the rock or mountain. 



The vegetation of a rocky tract of country will therefore present an 

 extremely characteristic and kaleidoscopic picture ; xerophytic and 

 mesophytic formations are lodged among one another, usually in small, 

 but none the less typical, parcels ; here may be a bare, vertical rock 

 bathed in burning sunlight, there a humus-laden, sopping, shady gully ; 

 here, a rock over which the water slowly trickles, there a completely dry 

 slope ; here a mountain-side exposed to the prevailing wind, there a mild 

 and sheltered dell ; and so forth. Furthermore, the vegetation in many 

 spots is in the act of changing from one formation to another, and the most 



1 'Mixed vegetation' ; see p. 143. * See Chapters X and XVII. 



