Fig. 31.—Bird’s-nest Orchis (Neottia Nidus-avis). 1/2.
sends up from a mass of fleshy roots a bare brown stem about a foot high, bearing a spike of brown flowers, the whole being so much of the same colour as the dead beech leaves among which the plant is usually found that it may easily be passed over. It is quite incapable of manufacturing its own food, but feeds on the decaying vegetable material which was manufactured by the trees under whose shadow it grows.
It is but a step from saprophytes such as this to parasites, which feed, not on dead, but on living organic matter. In the case of the higher plants, the hosts are always themselves plants, though, as pointed out on p. 78, they are, in the case of the Fungi, sometimes animals. One of the most interesting of these parasites is, like the Bird’s-nest Orchis, found in woods—the Yellow Bird’s-nest (Monotropa Hypopitys). This is, like the last, a leafless plant devoid of chlorophyll, sending up from a tangled root-mass one or more pale yellow stems, each bearing a drooping raceme of flowers of the same colour. The flowers show affinities to the Heath family (Ericaceæ), but the plant differs much from any other member of that Order. The Yellow Bird’s-nest is always found associated with the mycelium, or cobwebby underground portion, of a fungus, on which it appears to be parasitic. The fungus is in turn a saprophyte, and the Seed Plant feeds at second hand, so to speak, on decaying vegetable matter. This parasitism of a seed plant on a fungus is a very exceptional case. A more frequent type is offered by the Broomrapes (Orobanche), which we may find in meadows, etc., growing on Clover, Thyme, Ivy, and so on. These resemble the Bird’s-nest Orchis in sending up a stout leafless stem crowned with a spike of flowers. The different species display almost every colour except green, being red or brown or purple or yellow, and one blue. These plants live by attaching themselves to the roots of their host, and drawing in the nourishment they need for their own growth—robbery pure and simple. The seeds of the Broomrapes are very numerous and very light, and of singularly primitive structure. When they develop, they produce, not a young plant with root and stem, but a delicate spiral filament which grows down into the ground. Should this meet with a root of its host-plant, it adheres to it closely, and grows into a swollen knob at the point of attachment, which when mature sends up the flowering stem already described. Should a suitable root not be met with, the filament withers away and dies as soon as it has exhausted the small amount of reserve food stored in the seed. A parasite of a less sedentary habit, to be found in spring in our copses and hedgerows, is the Toothwort (Lathræa Squamaria). This curious plant has underground creeping stems clothed with whitish, tooth-like, fleshy scales (curiously modified leaves). In autumn and winter the stems lie dormant. In spring they send out delicate roots which attach themselves to the roots of trees of various kinds and suck nourishment from them, with the aid of which the plant sends up into the air fleshy cream-coloured stems bearing many drooping flowers of the same hue, the structure of which shows that the plant is closely allied to the Broomrapes. The Toothwort is a very harmless parasite, and the species of Broomrape also, though sometimes abundant on Clover, etc., do not do much damage; but the same cannot be said for the Dodders (Cuscuta), one of which is parasitic on Flax, another on Clover, and so on. These are little annual plants whose seeds lie dormant in the soil throughout the winter and well into the spring. Then the young plant, which has remained coiled up inside this seed like a spring, pushes forth in the form of a tiny thread. While one extremity fastens itself to the soil, the other rises up into the air, and its point slowly revolves. Should it come in contact with a living stem of a suitable plant, it attaches itself to it by means of disc-like suckers, penetrates the tissues of its victim, draws out nourishment, and, growing rapidly, spreads from plant to plant, taking a couple of close turns round each stem after the manner of a lasso, and then sending in rootlets from the attaching disc, and sucking the life out of each as it goes. It has no roots, no leaves, no chlorophyll, being of a red or yellow tint, and is entirely dependent for its nourishment on the plants which it attacks. In course of time—about August—an abundance of pretty little waxy-white flowers are produced, which produce the next year’s supply of seed. A few seedlings of Dodder, developing under suitable conditions, will form a colony which is capable in its few months of life of sweeping over a large area, wrecking the vegetation on which it has battened.
A parasite of a quite different sort may be studied in the familiar Mistletoe (Viscum album). It is the only parasitic native plant which is shrubby, or which perches itself on trees (the seeds being spread by birds, which devour the white berries). It is not, like some parasites, particular as to the species upon which it grows, flourishing equally upon a number of hosts, and even capable of living upon its own species. It differs from those parasites which we have been considering in possessing an abundance of green leaves, and being therefore capable of manufacturing its own food. At the same time, it has no roots which can penetrate the soil, and is incapable of an independent existence. It seems probable that its relations with its host are to some extent symbiotic—that is, each giving to the other—rather than purely parasitic, where the benefit is entirely on one side. The Mistletoe, retaining its leaves and manufacturing food throughout the year, is clearly capable of aiding its host, which loses its leaves in autumn, and cannot form fresh nourishment until spring is well advanced.
Before leaving this question of abnormal methods of procuring food as found among the higher plants, we may return for a few moments to the consideration of carnivorous plants, to which reference was made in [Chapter IV.] Of these the Sundews (Drosera), Butterworts (Pinguicula), and Bladderworts (Utricularia) supply very interesting examples within our own flora, which anyone may study on a holiday spent on the moors or mountains. The Sundews are familiar to all plant lovers—little plants of the bogland, usually growing among Sphagnum, and well distinguished by their leaves decked with spreading red hairs, each of which is tipped with a little drop of sparkling sticky fluid. It is these hairs or tentacles and their movements which place the Sundews among the most interesting of all plants. It is important to note that they are not hairs in the ordinary sense, which are organs of very simple structure arising from the epidermis or skin of the leaf. The tentacles of Drosera have a complicated structure resembling that of leaves, and the tip is occupied by a gland which produces the sticky secretion already mentioned. These glands are exceedingly sensitive, and, moreover, sensitive in a selective way. They are unaffected by the drops of rain which frequently fall on them, but the touch of any solid body, especially of organic material, immediately affects them; most of all nitrogenous substances of any kind. Darwin found that a morsel of human hair weighing only 1/78,740 of a grain was sufficient to set the machinery of Drosera in motion, and that immersion of a leaf in a solution of phosphate of ammonium so weak that each tentacle could absorb only 1/20,000,000 of a grain acted as a strong stimulus. In nature the stimulus is usually given by some unwary insect—a midge or other small flying creature—which, attracted by the bright colour or by the odour of the leaf, ventures too close, and becomes entangled among the sticky hairs. Then a most interesting series of events takes place. Almost at once the tentacles—first the ones actually touched, and then the adjoining ones—bend towards the point of disturbance, closing down one by one on the unfortunate victim till the leaf resembles a closed fist. At the same time the production of secretion increases, so as further to entangle the victim. When it is firmly secured, the secretion changes in character. Digestive ferments, closely resembling those by which animals digest their food, are poured out. These dissolve the animal’s body, all except the horny parts; the digested materials are then absorbed into the plant, which, as experiments show, benefits considerably by the addition to its diet of this animal food. When digestion is completed, the tentacles open again and prepare for a fresh victim. While the details of this remarkable process have been worked out only by careful and minute research in the laboratory, the main movements may be watched by anyone on any British moorland; or, bringing home a few plants in the damp moss in which they grow, we may amuse ourselves by experiments in feeding them.
In comparison with the Sundews, the other insectivorous plants which are included in the British flora are of less interest. The Butterworts (Pinguicula), of which four species are known in these islands, have a rosette of smooth, broad, yellowish leaves covered with glands which exercise the same functions as those of Drosera. To the touch of raindrops, sand-grains, or other inorganic substances they are indifferent; but a tiny insect alighting on the sticky leaf at once provokes an outpouring of secretion, while the leaf rolls inward from the edges till the victim is securely caught; it is then digested as in the Sundew.
The Bladderworts (Utricularia), of which several species may be found floating in boggy pools, are rootless, limp plants with finely divided leaves, among which are numerous little bladders (in reality strangely modified leaflets), and upright stems bearing pretty yellow Snapdragon-like flowers. The bladders do not help the plant to float, and appear to have for their sole function the securing of animal food. In the Common Bladderwort (U. vulgaris) they are about 1/10 inch long. At the upper end is a little hinged door, which is kept closed as by a spring against a thickened rim or door-frame. Outside the door are a few stiff hairs, a convenient perching-place for small aquatic creatures such as the minute Crustaceans known as Water Fleas. Should one of these try to explore the bladder, the door opens easily, but closes at once behind the rash wanderer, imprisoning it. The Bladderworts do not digest the victims which they secure in this manner, but when the bodies are decomposed by means of bacteria, the products of decomposition are absorbed. How fatal this mousetrap arrangement is to Water Fleas can be determined by dissecting the bladders of the plant.
Thus far, then, as regards some of those peculiar members of our flora which make their living by the unusual method of stealing their neighbour’s goods, or which eke out their existence by the capture of animal food. Let us now take another line of exploration and consider the conditions which prevail on the loftiest portions of our islands, and how these affect the vegetation. Mountain-tops are always attractive and interesting places—the keen rarefied air, the freedom and openness of the summits, fill us with exhilaration. Our own mountains are not lofty; nowhere in the British Islands is a height of a mile attained. But we have only to ascend to a couple of thousand feet to note a great change in the vegetation. The plants of the lower grounds to a great extent die out (though some accompany us to our highest summit), and the vegetation takes on a low compact form, which becomes more emphasized as we ascend farther, till in sheltered nooks alone do we find any plants more than a few inches in height. Furthermore, we notice an incoming of new plants unknown at lower levels, which search will show us to be confined to the mountains, each of them having a more or less definite limit below which (also above which, though our mountains are not high enough to render this point well marked) it is not found.
Among the plant formations and associations of the lower grounds which we considered in [Chapter II.] it was noted that the controlling factors were mainly connected with the nature of the soil and the amount of the water-supply. Here on the mountains another factor, the climatic, comes in emphatically, and takes charge. The temperature of the atmosphere falls one degree centigrade for about every 200 feet of elevation, so that a sharp frost on the lowlands may easily mean zero Fahrenheit on a 4,000-foot hill. The rarefaction of the atmosphere, too, tends to produce a much greater range of temperature, both diurnal and seasonal. Again, the velocity of the wind is much higher on the summits than on the plains, where friction is greatly increased by trees and other obstacles. These high winds have a very great cooling effect, as we may notice on our own bodies even in summer. In fact, as regards climatic change, an ascent of a thousand feet is comparable to a journey of several hundred miles northward. Anyone who has, on a winter tramp, been caught in a snowstorm on a 3,000-foot hill is forcibly reminded of what he has read of winter conditions in the Arctic regions. In ascending Ben Nevis we travel, in a sense, to the Arctic Circle. But the analogy is false, for conditions, especially in summer, are very different in the two places. The plants of our mountains have all the advantages of the high summer elevation of the sun, very different from the weak, sloping sunlight of the