Several of our flowering plants are to be seen most frequently on walls and rocks, or in other situations where there is hardly a trace of soil of any kind. Some of these thrive in such dry spots, with often such free exposure to the rays of the spring or summer sun, that it is difficult to understand how they manage to survive the periods of drought through which they live until we become acquainted with certain peculiarities of their form and structure.

In the first place we must recall the fact that plants lose a considerable amount of moisture by evaporation from the transpirating surfaces of their leaves, and that this loss must necessarily be greatest when the air is warm and dry unless there is some means by which the transpiration is automatically regulated according to the requirements of the plant and to the varying conditions under which it has to exist.

The leaves of plants are covered with a thin skin or epidermis which consists of a single layer of cells, and which is practically impermeable to moisture. In this epidermis, however, on one or both sides of the leaf, are minute pores (stomata) through which water vapour is free to pass; and beneath the porous epidermis is a loose, cellular tissue, with air-spaces, from which the moisture can readily pass, in the form of vapour, to these stomata.

Each of the stomata is bordered by a pair of crescent-shaped guard-cells, placed with their concave sides towards each other, and joined at the ends. Further, the guard-cells are capable of changing their form, becoming straighter, and thus reducing or even closing the aperture between them; and becoming again curved, opening or enlarging the pore. The former change takes place during darkness, thus preserving the plant from the cooling effects of evaporation during the chilly nights; and also during dry weather when the plant is in danger of losing much more moisture than the roots can absorb.

So far, however, we have been dealing with a regulating process that is common to green plants in general; but we must look for some additional protection against loss of moisture in the plants which grow in such places that they have to live through longer or shorter periods during which the roots have little or no moisture within reach.

From what has been said concerning the structure of the leaf it will be understood that, as a rule, the larger the surface the greater will be the loss of water in a given time. But when we examine the leaves of the plants that grow on dry walls and rocks, we frequently find that they are more or less thick and fleshy—that the material of the leaf is disposed in such a manner as to reduce the area of the surface as compared with other leaves made up of the same amount of tissue.

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.

We have now to consider a few species of the order CrassulaceÆ, which includes some very interesting succulent plants that are peculiarly adapted to a life in the dryest of situations on walls, roofs, &c. In addition to the thick, fleshy nature of their stems and leaves, these plants are distinguished by terminal cymes or corymbs of flowers with (usually) five sepals, the same number of distinct petals, twice as many stamens arranged in two whorls, and carpels equal in number to the petals. Three of the plants referred to are known as Stonecrops, and may be recognised by the following descriptions:—

1. The English Stonecrop (Sedum anglicum).—A smooth plant, two or three inches high, abundant in rocky and stony places, especially in the West and near the sea, flowering from June to August. Its stems are more or less decumbent, much branched and rooting at the base; and its leaves are small, thick, almost globular, of a pale green colour with, often, a tinge of red. On the small flowerless branches the leaves are very crowded and overlapping; but on the taller, flowering stems they are more scattered and placed alternately. The little starlike flowers are white, frequently tinged with pink or spotted with red, and arranged in a short, two-forked panicle. They have short, green sepals; narrow, sharply-pointed petals about twice the length; and stamens with bright red anthers.

2. The White Stonecrop (S. album).—A somewhat similar plant, from three to seven inches high, sometimes seen in large clusters on rocks, walls, and roofs, bearing white or pinkish flowers during July and August. The whole plant is smooth; and its short creeping stock gives rise to short barren stems with crowded leaves, and erect flowering stems with scattered leaves. The leaves are very thick, of a bright green colour, about a third of an inch long, and oblong or cylindrical in form. The panicles are much branched, with, usually, reddish stems; and each consists of numerous flowers with short, blunt sepals, and narrow, oblong petals about three times as long. This species is not so common as either the last or the following.

3. The Biting Stonecrop or Wall Pepper (S. acre).—A smooth plant, of a yellowish green colour, biting to the taste, very common on rocks, walls and roofs, bearing golden yellow flowers during July and August. It has short, barren stems, covered with closely-overlapping leaves arranged in six rows; and erect flowering branches from two to four inches in height. The leaves are very small, thick, succulent, oval or almost globular in form. The flowers, which are in small, terminal, three-cleft panicles, have very short, blunt sepals; and much longer, narrow, pointed petals.

The same order (CrassulaceÆ) includes the House Leek (Sempervivum tectorum)—a plant which has been introduced into Britain, and is now commonly seen growing wild on rocks and on the roofs of country houses. Its spreading offsets give rise to globular tufts of flowerless shoots, and to thick, succulent, flowering stems that grow to a foot or more in height. The lower leaves are ovate, acute, thick, fleshy, edged with red, and arranged in a dense rosette; and the flowering stem, with its sessile leaves, is covered with a short, sticky down. The flowers are of a dull pink or purple colour, and are sessile along the spreading branches of the stem. They have usually about twelve short sepals; the same number of pointed petals, two or three times the length of the sepals; about twice as many stamens; and an ovary of as many carpels as there are petals and sepals. It is interesting to note that half the stamens—those forming the inner whorl—produce no pollen, and that their anthers are often modified into ovaries, the ovules of which, however, do not mature. This plant flowers in July and August.

Our last selection from this order is the Wall Pennywort or Navelwort (Cotyledon umbilicus)—a peculiar plant, common on rocks and walls in the South and West of England. It has a hard stock, producing an abundance of fleshy leaves early in the year, and flowering stems, from six to eighteen inches high, from June to August. The lower leaves are round, wavy, smooth, very succulent and brittle, and depressed in the centre where the long fleshy stalks are attached. Those of the stem have shorter stalks which are more and more removed from the centre from below upwards. The stem is thick and succulent, and bears a long raceme of pendulous yellow-green flowers on short stalks. Each flower has a very small calyx of five sepals; a cylindrical corolla, about a quarter of an inch long, with five short teeth; ten stamens, attached to the tube of the corolla; and a superior ovary.

Several of the Saxifrages grow in rocky and stony places, and four or five species are sufficiently common to demand a notice here. The flowers of this group have a calyx of five sepals that is either quite free or more or less adherent to the ovary; a corolla of five petals; ten stamens, attached with the petals at the base of the calyx; and a two-celled ovary, with two distinct styles, containing several seeds.

Our first species is the London Pride, None-so-Pretty, or St. Patrick's Cabbage (Saxifraga umbrosa), a native of Irish mountains which has been introduced into Britain as a garden flower, and has now become established as a wild flower in many parts. Its flowering stem grows from six to twelve inches high; and the small white or pink flowers bloom during June and July.

The Starry Saxifrage (S. stellaris) is a somewhat similar plant, but much smaller, rarely exceeding six inches in height. It is frequent on wet rocks in the North, flowering in July and August. Its leaves are sessile, oblong or obovate, tapering towards the base, thin, and arranged in spreading rosettes; and the stem is leafless with the exception of little bracts at the base of the pedicels. The starlike flowers, larger than those of the last species, are white, with two yellow spots on each petal, and are arranged in a loose panicle on spreading pedicels. The calyx is adherent to the ovary only at the base, with its segments turned down on the pedicels; and the petals are narrow and spreading.

Another Northern species—the Yellow Mountain Saxifrage (S. aizoides)—is abundant on the wet rocks of mountainous districts, flowering from June to September. It is a tufted plant, with branched, decumbent, leafy stems, about six inches long; and crowded, narrow, fleshy leaves, about half an inch long, fringed with hairs at the base. The flowers are yellow, in a loose panicle. The calyx is yellow, like the petals, but much shorter, and erect; and the ovary is adherent to the short tube of the calyx to about half way up.

The Rue-leaved or Three-fingered Saxifrage (S. tridactylites) is a small species, rarely exceeding four or five inches in height, common on walls in most parts of Britain, flowering from April to July. The whole plant is usually more or less tinged with red, and its erect stem is covered with fine glandular hairs. The radical leaves are very small, stalked, and undivided; those of the upper part of the stem are also small and entire, but sessile; and the intermediate leaves, lower on the stem, are palmately divided into three or five narrow segments. The small white flowers are placed singly on rather long terminal and axillary stalks; and the hairy calyx, which adheres to the ovary, has five blunt lobes less than half the length of the petals.

Our last example of this order is the Cut-leaved or Mossy Saxifrage (S. hypnoides), a very variable plant, from three to ten inches high, rather rare in South England, but much more common in the rocky parts of North England and Scotland. It has numerous procumbent, barren stems with tufted leaves; and erect flowering stems bearing a few small leaves and a loose cyme of a few white flowers. Most of the leaves are narrow, pointed, entire, about a quarter of an inch long; but the larger ones, at the base of the plant, are about twice as long, and divided into three or five narrow lobes. The calyx adheres to the ovary to about two-thirds of the length of the latter, and has five lobes about one-third as long as the petals. This species flowers from May to July.

Old walls, ruins, and limestone cliffs are frequently adorned with the pretty flowers of the Snapdragon (Antirrhinum majus—order ScrophulariaceÆ) which bloom from July to September. The plant varies from one to two feet in height, is tufted and leafy at the base, and has erect stems which bear racemes of large flowers. The leaves are very narrow and entire; and the flowers, which are usually white, pink or crimson, are shortly stalked in the axils of the small upper leaves. The calyx is deeply divided into broad lobes very much shorter than the corolla; and the latter consists of a broad tube and two lips, the whole being over an inch in length. The mouth of the flower is closed by a projecting 'palate,' but is easily opened by pressing the flower at the sides between finger and thumb. There are four stamens on the corolla, two longer than the others; and the fruit is an unsymmetrical capsule that opens when ripe by a few holes near the top.

The Ivy-leaved Toadflax or Mother of Thousands (Linaria Cymbalaria), of the same order, is a pretty little trailing plant very commonly seen on old walls in many parts of Britain, particularly in the South-West. It will grow luxuriantly in places where there is no soil other than that afforded by the crumbling mortar, and will often establish itself even on new walls so compactly built that it is difficult to see how the plant can find the necessary moisture or how its roots can penetrate the hard material to which it is attached. Its slender stems vary from a few inches to two feet in length, often rooting at the nodes; and its little leaves are smooth, with three or five lobes, and generally of a purplish colour on the under side. The little flowers, which bloom from May to September, are of a pale blue or lilac colour. The lipped corolla is very similar to that of the last species, with a yellowish palate closing the mouth, but it has a short spur at the base.

The one remaining flower of this chapter is the Wall Pellitory (Parietaria officinalis), which belongs to the Nettle family (UrticaceÆ). It is a somewhat bushy plant, varying from six inches to two feet in height, bearing axillary clusters of small, sessile, green flowers from June to September, and is common on walls and stony banks, more especially in the South of England. Most of the flowers are usually imperfect, and the clusters are surrounded by a whorl of a few divided bracts. The males are few in number, each consisting of a hairy perianth, and four stamens which are jointed and very elastic, springing suddenly and shedding their pollen when touched; the females have a tubular, hairy perianth of four lobes, and a single tufted stigma.