96 ROCK PLANTS OF THE PASTURES 



their survival are very small indeed. It need hardly 

 be pointed out that in any case there is always a 

 "high mortality" among seeds. A much larger 

 number are always produced than can ever possibly 

 sm'vive, and the whole rationale of seed production is 

 that, while many are certainly doomed to perish, there 

 is a distinct chance that a few, perhaps only one, may 

 survive, and so the species will continue in being. 



We must, therefore, study the formation of a 

 primitive soil, if we wish to understand all the stages 

 in colonisation. Let us consider a large slab of rock 

 recently laid bare. The surface of the slab will 

 probably not be quite smooth. Smaller or larger 

 uTCgularities in the surface will exist, and further, 

 the rock, under the influence of what the geologist 

 terms "weathering," will soon begin to crack in 

 various directions, and at the same time the small 

 irregularities of the surface will be accentuated. 



It is around these irregularities and in the cracks 

 that the primitive soil accumulates. It may be 

 formed initially by those lowly plants known as 

 Lichens (p. 291), especially the Crustaceous Lichens, 

 forming the yellowish- or greenish-white crust on the 

 surface of the rocks, so commonly seen in the Alps. 

 These Lichens are really composed of two plants — one 

 an Alga or pond- weed, and the other a Fungus, living 

 together. After a time the body of the Lichen dies, 

 but it continues to remain attached to the rock. 

 The dead Lichens tend to hold any rain-water which 

 falls on them for a time, and small wind-blown vege- 



