Means of Dispersal. 23 



mean that only the outer individuals of each cluster, 

 presumably on the average those that have already been 

 selected by the dispersing agency, have much chance of 

 propagating themselves. In the case of small-seeded gre- 

 garious plants like the heaths, without highly specialised 

 means of dispersal, this difficulty probably tends to keep 

 the seeds small and chaffy, so as easily to be scattered by 

 the wind. The berry-bearing heath-plants on the other 

 hand, though equally gregarious, have seeds fewer, larger, 

 heavier, and with thicker walls. These latter have been 

 modified for dispersal by birds. The small-seeded heaths 

 without special adaptation for dispersal are often singularly 

 local ; though occurring in profusion, they tend to 

 occupy widely separated areas, and are absent from 

 other districts equally favourable. The berry-bear- 

 ing species are of more general occurrence in suitable 

 localities, though individually they may not be so 

 abundant. * 



Other species have special methods of throwing the 

 seeds beyond the shadow of the parent plant. The Gorse, 

 Wood-sorrel, Geranium, and Spurge forcibly eject their 

 seeds from the ripe pod or capsule. The acorn is attached 

 lightly for some time after it is ripe, and grows at the end 

 of a thin branch which, lashed by the October gales, flings 

 the acorn as boys throw clay-pellets from the end of a 

 switch. Many umbelliferous plants have a similar mode of 

 scattering their seed ; for when ripe the carpophore splits 

 and the seeds hang loosely by their upper ends to the two 

 whip-like filaments. At the same period the withered 

 plant hardens and becomes very elastic, so that any 

 passing animal causes it to spring back and throw off the 

 seeds, which unless thus scattered, tend to hang on till 

 they decay. This process one can study in a patch of 

 these withered umbellifers, part of which is accessible to 



