CHAPTER VIII

SOME INTERESTING BRITISH PLANT GROUPS

In the preceding chapters glimpses have been obtained of some of the wider aspects of plant life, particularly as seen on the hills and plains of our own country. The species composing our flora have been seen mostly, not as individuals, but as portions of regiments and armies, particular plants being mentioned but seldom, where required for purposes of illustration. In the final chapter it will be well to abandon this collective treatment, and glance at a few individual species or genera or small natural groups which possess features of interest of one sort or another. No systematic arrangement need be attempted: it will be pleasanter to ramble on, allowing our points of inquiry to turn up as they might on a country walk.

A consideration of abnormalities in the manner in which plants obtain their food-supply--irregular nutrition, as it has been called--will raise some interesting questions, and will bring us up against some of the most remarkable species which are found in the British flora. The outlines of the method by which plants manufacture their food are familiar to all, and have been referred to already (pp. 75, 132). The roots absorb from the soil water containing dissolved salts, which is passed up by the stems into the leaves. The leaves extract from the air carbon dioxide. The chlorophyll, or green colouring-matter of the leaves, possesses the remarkable power in the presence of sunlight of breaking up and recombining these substances into the compounds which go to build up the plant-body. As has been pointed out, it is this power of forming organic out of inorganic matter that especially distinguishes plants from animals. But not all plants manufacture their food in this way. A large number feed like animals, finding their sustenance sometimes in living, more often in dead, organic material, either animal or vegetable. The whole enormous group of the Fungi do not possess chlorophyll, and in consequence are dependent on organic materials for their food. Some of the most familiar of the lower Fungi live on cheese, leather, bread, or any other damp animal or vegetable material. The higher forms, which decorate our woods and pastures, find their sustenance largely in leaf-mould. The groups of the Mosses, Hepatics, and Ferns, which are more highly organized than the Fungi, possess chlorophyll, and manufacture their own food; and it is with some little surprise, therefore, that when we come to the Seed Plants, the highest group of all, we find, though in relatively few cases, a reversion to the animal trait of using organic food. Some of our woodland plants have taken so entirely to a diet of leaf-mould that they have discarded the apparatus which would enable them to manufacture their own food. Chlorophyll, the magic wand by means of which the inorganic is transformed into the organic, and also leaves, the mills wherein the transformation takes place, are absent from these plants. For instance, the Bird’s-nest Orchis (_Neottia Nidus-avis_),

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