In some species this diminution of surface is carried to the extreme, and the leaves have become very thick, assuming a cylindrical or almost globular form; and such leaves are capable of absorbing and retaining large supplies of water that serve to maintain the plant during those periods in which the roots have no moisture within their reach.
We also find that many of the plants in question are further protected from a dangerous loss of moisture by the peculiar arrangement of their leaves, which are often so closely applied to the stem, or so closely overlapping one over another, that the total area of exposed surface is considerably reduced; and it frequently happens that the stem of the plant becomes thick and succulent, as well as the leaves, thus adding to the store of moisture kept in reserve for the rainless days.
While some plants are almost invariably found in dry, stony places, others are very diverse in their habitats, sometimes growing in moist and shady places, and sometimes on cliffs or other rocky situations. In the latter we often find considerable modifications of size, form and structure, the same species being more or less luxuriant and thin-leaved when in damp soils, while in rocky places it becomes more or less stunted, with a tendency to produce thick and succulent leaves.
A few of the plants that we include in the present chapter are to be found only on wet rocks, and are therefore of a nature very different from that of the species growing in dry places. They are always well supplied with moisture; and, being usually surrounded by a damp atmosphere, they lose but little water by evaporation, and thus require no reserves within their leaves or stems.
Our first species is the well-known Wallflower (Cheiranthus cheiri), of the order Cruciferæ. It is a rather shrubby plant, frequent on old walls and ruins, where it flowers during April and May. Though too familiar to need any description, we may note that in the wild state it varies from six to twelve inches high, and bears sweetly-scented, yellow or orange flowers. The plant is not indigenous, but has now become naturalised as a wild flower in most parts of Britain.
The Wall Rocket (Diplotaxis tenuifolia or Brassica tenuifolia), of the same order, is a very similar plant, growing in similar situations, but it does not commence to flower till the summer is somewhat advanced. Its stem is leafy, branched, smooth, woody towards the base, but more slender than that of the Wallflower; and its very variable leaves are generally three or four inches long, deeply divided pinnately into narrow segments with irregularly-toothed margins, and emit a rather unpleasant odour when rubbed. The flowers are of a pale yellow colour, fragrant, about three-quarters of an inch in diameter, in terminal racemes, with sepals more or less spreading; and the fruits are narrow, flattened siliquas, with membranous valves, about an inch and a half long. The plant is to be found principally in the southern counties of England, and flowers from July to September or early October.
On dry rocks, chiefly in the hilly and mountainous districts of North and West Britain, we meet with the Vernal Sandwort (Arenaria verna) of the order Caryophyllaceæ. This is a little tufted plant, only from two to four inches high, with branched stems more or less decumbent at the base; and small, sessile, opposite, very narrow leaves, each with three veins. The starlike, white flowers are about a third of an inch across, on slender stalks, and grouped in terminal, loose, few-flowered cymes. They have five pointed sepals, less than a quarter of an inch long, each with three prominent veins; five spreading petals, a little longer than the sepals; ten stamens; and a superior ovary with three narrow styles. The fruit is a short ovate capsule which opens, when ripe, by three valves.
One of the Geraniums—the Shining Crane's-bill (Geranium lucidum)—is almost essentially a plant of walls and rocks. It is a beautiful species, smooth and shining in all its parts, with a tendency to turn red, like the Herb Robert; and, as in other plants of its order (Geraniaceæ), distinguished by the swollen joints of its stem. The leaves are almost round in general outline, but are deeply divided into five, broad, coarsely-toothed segments. The flowers are small, rose-coloured, and generally grow in pairs on axillary stalks. They have five, erect, wrinkled sepals, with long points; and five short, rather broad, entire petals. This species is common in most parts of Britain. It varies in height from six to eighteen inches, and flowers from May to September.
The Biting Stonecrop or Wall Pepper.