CHAP. LXII LITHOPHYTES 243 



sharply circumscribed outlines or dovetailing with one another ; and the 

 one species eventually suppresses the other. 1 



The first colonists of bare rock, namely algae and lichens, gradually 

 produce a nutritive substratum suitable for more highly organized species, 

 the earliest of which are mosses and fruticose lichens. While the first 

 colonists have horizontally extended vegetative organs and are appressed 

 to the rock, or even crustaceous, their immediate successors rise above 

 the surface as cushions or miniature shrubs (e.g. species of Cladonia), 

 which overgrow the algae and crustaceous and foliose lichens, retain 

 more water than these, and produce more organic detritus. In such 

 communities of mosses and fruticose lichens space may be provided for 

 an under-vegetation of algae. 



Means of firm attachment are always the first necessity to the invading 

 species ; and just as the first colonists provide a favourable substratum 

 for taller species, so these in turn produce a substratum in which still 

 more highly organized plants, including Pteridophyta and Spermophyta, 

 can fix and nourish themselves. In cushions of moss one notes seeds of 

 Sedum acre, and other Spermophyta, germinating and developing ; and 

 in the cushions of Sedum other species find place for their roots. The 

 less inclined is the slope of a rock the more easily does this successional 

 development take place. Gradually, in this way, a block of rock or a 

 whole slope may become clothed with a colony of Spermophyta and 

 mosses, and in the course of years, as humus accumulates and earth is 

 conveyed thither by wind or water, bush or forest may develop on these 

 spots. Such superficial vegetation is, of course, oecologically greatly 

 influenced by the thickness of the substratum thus deposited. 



In this superficial vegetation we must also include that which develops 

 on all projecting pieces of rock that rise above the surface in rocky country ; 

 on many rocks one sees quite small prominences surmounted by a vegeta- 

 tion of grasses and perennial herbs, or of Calluna, or even of shrubs if the 

 layer of soil be sufficiently thick. These small communities of plants 

 must not be regarded as belonging to true rock- vegetation, of which, on 

 the contrary, chasmophytes are genuine constituents. 



CHAPTER LXIII. CHASMOPHYTES 



CHASMOPHYTES are plants rooted in clefts of rock that are filled with 

 detritus. In these clefts, particles of earth conveyed by wind and water 

 accumulate, and water collects. The amount and rate of accumulation 

 depend upon the width and situation of the clefts. In the soil thus con- 

 stituted plants settle, and their dead fragments further add to the supply 

 of nutritive material in the clefts. Thereafter earthworms appear, as 

 do other animals that burrow in the humus soil and improve it. 2 The 

 flora of the clefts varies with such prevailing factors, as exposure, 

 width of cleft, position of this, and, according to Ottli, with the presence 

 or absence of any covering of snow during winter. 



When a rock has a very steep surface and is devoid of crevices or 



clefts, only lithophytes develop upon it, and rarely is rock entirely bare 



of vegetation (e.g. algae). When, on the contrary, the rock shows cracks 



and clefts, chasmophytes develop in these. But in many cases, in northern 



1 Bitter, 1898. * Ottli, 1903. 



R 2 



