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. Wherever a cliff or precipice of stone is exposed, it is "weathered." Water gets into the cracks and freezes in winter. But when water is frozen it expands or widens, and as this happens to the water in the crevices and cracks of rocks, pieces of rock are shivered and broken off. Besides frost and wind and rain, these rock plants help to attack the cliff. Their roots get into the crevices, and there widen and expand, tearing off great slabs and splinters of rock which fall down to the foot of the cliff. Down below plants are every year growing over and covering up or "happing up" with green these bare fragments and splinters. A considerable amount falls down every year, so that the ground is always being raised up below the precipice. At the brow or edge above the precipice, there is also always a loss of rock and stone every year. So that every year the bare rock exposed becomes smaller and smaller, until eventually a steep, green, grass-covered slope covers over the entire site of that precipice. Moreover that is not by any means all that plants do in the way of changing the scenery of the country. Look at the outlines of the hills in any part of Great Britain except in the broken, jagged, rocky mountain ranges of Scotland and Wales (also Cumberland, Westmorland, parts of Derbyshire and Dartmoor tors). Everywhere there are smooth, flowing, gently undulating rises and falls. No sharp, abrupt descents break these graceful sweeping curves. If you compare the scenery of a cañon in the rainless deserts of Western America, the contrast is very striking. There the sides of the valleys are steep cliffs; it is all harsh, precipitous, horrible country, which is obviously very unpleasant and very unattractive to civilized people. It is this green covering of plants which makes the difference. The rain that falls is not allowed to cut out ragged ravines; it is intercepted and soaks into the grasses, which so keep a smooth, gentle outline over hill and valley.