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 
