It is interesting to scramble to the top of one of these heaps, especially in summer. One then begins to realize how every plant attends strictly to its own business.
All over the sides of the heap there will be hundreds of a rare groundsel (Senecio viscosus), which is not really a native, and never occurs except on such places. In a grass field close by hundreds of thousands of Ragwort (Senecio jacobæa) make a glorious golden carpet; in the marshy part of the meadow the Water Ragwort (Senecio aquaticus) may be found. In the cottage gardens and here and there along the roadside the groundsel (Senecio vulgaris) is flourishing abundantly.
These plants never interfere with or encroach upon one another's grounds. Every year thousands of ragweed and groundsel seeds must be blown on to the shale-heap, but they never manage to grow there.
It is only the foreigner (S. viscosus), accustomed to a very hot and dry climate, and with sticky leaves which catch atmospheric dust and probably insects, that can exist on the bare shaly sides. These slopes of shale are easily heated by the sun, and at the same time radiate the heat rapidly away, so that the Viscid Groundsel must have a very hard time of it. When its roots have worked up the shale a little, and its dead leaves have covered the surface with mould and organic matter, then possibly others (true British plants) can get a footing and suppress it.
Along railway tracks, also, the ballast forms a very hot, a very dry, and a very barren soil. Many of the regular railway-track plants are foreigners from the far south, even from the sunny shores of the Mediterranean. They are mostly annuals, such as the little Toadflax (Linaria minor), which can just manage to exist under those conditions.
Of course, the sides of the banks and of cuttings on railways are generally formed of good earth or soil, and support a rich and flourishing flora of true Britons.
Besides these slow, laborious lichens, mosses, and others which attack rock, there are other plants which are generally called rock plants, though they behave quite differently.
These are those fine hardy Hawkweeds, Roseroots, Sempervivums, Mew, and others which establish their roots in cracks or crevices of the rocks.
Such cracks are soon full of good soil, for the wind blows decayed leaves and dust into them, and the roots are always burrowing into, eating into, and shattering the rocks. Most of them have a circle of leaves which are pressed flat to the ground. Thus they escape the violent winds and storms always common on such crags and precipices. The flowers, however, supported on tough, strong, and flexible stalks, sway freely to and fro in the wind, and can be seen by insects a long way off.
These rock plants are of some importance as stonebreakers and pioneers in a very interesting process.