ROCK COLONISATION 95 



space is found to be available. But all plants are not 

 capable of taking advantage of the fact that a new 

 area of bare rock has recently become exposed in 

 their neighbourhood. We know that if we transfer 

 a plant of a damp meadow species to, or sow its seeds 

 on, some bare and dry rocky ledge, the chances are 

 very greatly against the survival of the species in its 

 strange habitat. The plants which are most likely to 

 survive on the new ground are those which grew 

 formerly under conditions as nearly as possible similar 

 to those which prevail in the area in which colonisa- 

 tion is being begun afresh. These are the rock plants 

 of the pastures. They are the advance-guard of 

 vegetation in its march from the normal pasture to 

 the bare untenanted rocks, exposed from time to 

 time by geological agencies. 



The colonisation of new ground is effected in the 

 great majority of cases by seeds which, in a very 

 large proportion of Alpine plants, are distributed by 

 the agency of the wind. Where outposts on some 

 bare ledge have become established, the advance of 

 vegetation may be furthered by some asexual method 

 such as the formation of runners and offsets, which 

 tend to distribute the species still further. But 

 initially it is the wind-blown seed which is the 

 coloniser in nine cases out of ten. 



It is doubtful if a seed falhng on absolutely bare 

 rock will survive in any instance. Certainly it often 

 happens that seeds perish in this way, for without 

 some kind of soil, however primitive, the chances of 



