SECTION VJII 



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.^ 



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. 



Slo-pe 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) '^ ; 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 sunhght, 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 



* 'Mixed vegetation' ; sec p. 143. * See Chapters X and XVII, 



