2 I 2 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
is continually going on, and slight changes are sufficient to enable 
one of the latter to rise to first place. Over any area of ground there 
is taking place, gradually or quickly, a regular succession of social 
forms. This is easily seen in the peopling of freshly opened earth, 
where each society prepares the way for the next, and is then in turn 
driven out by it. The beginning of such a succession we see in the 
growth of lichens on the bare rock, gradually eroding its surface and 
preparing a place for the germination of moss-spores, then a little 
humus is formed where a seed may sprout; thus one succeeds the 
other until the rock has a carpet of vegetation. 
The late Dr. Buchanan White and others have well exemplified 
this in their studies on the growth of the shingles, islands,- and 
backwaters of the Tay. On the banks of the river, near Ballinluig, 
I have observed one example of the order in which the plants appear 
on the bare shingles. The first scattered herbs, with no definite 
society, are replaced and dominated by the broom (Cytisus scoparius^ 
Link.^. After a few years, however, the broom becomes long and 
woody, and finally dies out, its place being taken by grass associations, 
which thrive on the richer soil it has left. The amount of nitrogen 
collected by the micro-organisms in its root-tubercles has probably 
much to do with the power which the broom has of enriching the 
soil. Amongst the grass one finds seedlings of trees, and above 
Ballinluig a young plantation of self-sown pines has sprung up on one 
of these old river shingles. I give the lists of the broom and grass 
associations of the shingles in parallel columns for the sake of com¬ 
parison. 
SHINGLE BEDS ON THE BANKS OF THE RIVER TAY NEAR BALLINLUIG. 
Association of Cytisus scoparius^ Link. 
Viola canina, L. 
Silene maritima, With. 
Cytisus scoparius. Link. 
(Dominant.) 
Alchemilla alpina, L. 
Association of Grasses. 
Ranunculus acris, L. 
Viola canina, L. 
Cerastium triviale. Link. 
Stellaria graminea, L. 
Cytisus scoparius. Link. 
(Rare.) 
Trifolium repens, L. 
Lotus corniculatus, L. 
Potentilla Fragariastrum, Ehrh. 
Alchemilla vulgaris, L. 
Rosa canina, L. 
Pimpinella Saxifraga, L. 
