Part 54

There is no doubt but that the seeds of the Mistletoe are distributed from tree to tree by the agency of birds, especially the thrushes, which devour the berries in large numbers. The seed of the berry is protected by a covering which remains quite untouched by the digestive fluids of the bird, and consequently it is expelled intact with the excrement, and frequently drops to a branch of the tree, where it lodges in a crevice of the bark, and is securely fixed in its place by the slimy excrement in which it is embedded.

Here the seed germinates, sending out a little rootlet that always turns towards the bark on which it rests, and subsists for a time on the food-reserve that it contains. When the young root reaches the bark it becomes flattened against the surface, and spreads out, forming a disc that holds the seedling firmly to the tree.

A projection (the sinker) is then sent inwards from the disc, and this penetrates the bark, reaching the wood beneath, but does not enter the latter. This terminates the growth of the seedling for the first year, but as soon as the warm weather of the following spring commences, the sinker begins to spread over the surface of the outer ring of wood, while at the same time a new annual ring of wood begins to form outside, thus surrounding and banking in the sinker. It would appear, on making a section of the tree, as if the sinker had actually pushed its growth through the outer ring of wood, whereas it does not penetrate the wood at all, but is only banked up by the new wood that grows round it. This is repeated year by year, until the sinker is at last quite deeply set in the branch, being surrounded by the wood of several annual rings.

During the second year's growth the sinker sends out little roots which run up and down the stem, beneath the bark, and these give rise to new sinkers that grow down to the surface of the wood, and become, in turn, embedded in the new layers of wood that form round them. And while the young Mistletoe plant is thus securing a firm hold on its host, and withdrawing ready-made organic compounds from its sap, the outer green stem develops, and soon gives rise to the first pair of leaves.

If food is obtained in abundance, as is the case when the host is a tree of a soft and sappy nature, the growth is rather rapid; but otherwise the development is comparatively slow. In any case the age of the parasite may be ascertained by counting the number of annual rings of wood that lie outside the deepest sinker; and by this means it has been found that the Mistletoe may attain an age of over thirty years.

We have now to consider a group of plants, the parasitic habits of which would scarcely be suspected by an ordinary observer. They are green plants, with well-developed foliage leaves, and true roots which absorb mineral food from the soil. Their seedlings grow in the same way as those of non-parasitic species, deriving no nourishment from neighbouring plants, but obtaining all their food from the air and the soil, and building up all the organic compounds required for their growth by the agency of their own chlorophyll.

It is difficult to understand why these plants should afterwards produce suckers on their roots in order to obtain nourishment from other species, but they do this, and experiments have proved that the food thus obtained is more or less essential to their development. Some of them die while still young if grown apart from other species, and the others, under similar conditions, though they reach what we may term the adult stage, remain somewhat weak and stunted, and produce but few flowers and fruits.

Most of the plants referred to belong to the order Scrophulariaceæ, and among them we may mention the Eyebright (Euphrasia), the Yellow Rattle (Rhinanthus), the Cow-wheat (Melampyrum), and the Lousewort (Pedicularis). They generally appear in large numbers close together, often in such abundance as to determine the general colour of the ground on which they grow, and yet they do not apparently cause much damage to the grass and other plants which they rob.

These green parasites are described in various chapters, according to their habitats and their flowering seasons; so we shall do no more here than to briefly refer to their parasitic habits.

The Eyebright (p. 274) grows on heaths and downs, where it derives organic food from the roots of the neighbouring grasses. The Lousewort, too (p. 118), which grows in marshes and moist meadows, is parasitic principally on the roots of grasses, apparently without affecting the latter. The last-named species is a perennial, the roots of which have to find hosts that are capable of supporting it year by year. If the host of the present year should happen to die in the autumn, the suckers that were attached to its roots soon die, and the parasite has to seek a new source of supply. This it does by extending its roots until it reaches a new host, and then producing new suckers. Thus we are able to understand the origin of the long roots so often seen on the Lousewort, and also the reason why these roots never grow downwards into the soil, but always horizontally, just beneath the surface. Further, since the roots extend themselves in search of food at times when the supply is temporarily diminished or stopped, it is clear that some reserve is necessary for the elongation referred to. Such a reserve exists in the older, thick portion of the perennial root, near the base of the stem.

In the case of the Cow-wheat (p. 146) no suckers are produced until the lateral branches of the root of the seedling reach a moderate length; but in order to increase the chances of finding a suitable host these branches are developed in large numbers, and extend themselves in all directions. The suckers produced on them cling very firmly to the root-fibres of the host, which they almost completely embrace.

The suckers of the Yellow Rattle (p. 118) are globular, often nearly one-eighth of an inch in diameter, and partly surround the root-fibres of the plants to which they are attached.

XXIV

CARNIVOROUS PLANTS

Quite a number of plants, belonging to different orders, are provided with the means of capturing small animals, and of digesting their prey and absorbing the nutrient matter thus obtained into their own systems. In this way they are enabled to obtain nitrogenous material which, in the ordinary way, is absorbed in the form of mineral solutions, from the soil, by the agency of the roots. The greater number of these carnivorous plants are to be found in tropical lands; but a few are British, and are of such an interesting nature that we propose to devote a short chapter to a description of their peculiar structure and habits.

The plants to which we refer are often spoken of as insectivorous species; but although in nearly all cases the animal food consists almost entirely of insects, it is not entirely derived from this one group of animal life, and therefore the term carnivorous is rather more appropriate.

In pools we sometimes meet with floating plants that have no true roots, at least at the time of flowering, but consist of a tuft of long, rootlike, submerged branches, bearing much-divided leaves, and sending leafless stalks of yellow flowers above the surface of the water. These plants are the Bladderworts (Utricularia), of the order Lentibulaceæ, and are so called because they have little air-bladders either attached to the leaves or supported on leafless branches.

The leaves are divided into numerous very narrow segments, thus presenting a proportionately large amount of surface to the water for the absorption of dissolved gases required by the plant; and the flowers consist of a deeply two-lobed calyx; a spurred corolla, with its mouth closed or nearly closed by means of a convex 'palate'; two stamens; and a one-celled ovary that ripens into a globular fruit.

As to the little air-bladders mentioned above, they form, perhaps, the most interesting feature of the plant, for they are the traps by means of which small aquatic creatures are caught, and also the organs concerned in the absorption of nutritive products derived from the prey. Each bladder has an opening, guarded by a kind of valve which allows easy ingress, but no exit. It does not seem to produce any secretion which would hasten the death of the creatures entrapped, nor does it appear to produce any kind of digestive fluid, as is the case with other carnivorous plants; but small aquatic creatures, such as water-fleas, cyclops, very small larvæ, &c., entering the bladders for shelter or some other purpose, are securely imprisoned until they die of starvation or suffocation; and their bodies then decay, giving rise to soluble gases and other products which are absorbed into the plant by special cells within the bladder.

There are three British species of these plants--the Greater, the Lesser, and the Intermediate Bladder-worts. The first of these--Utricularia vulgaris--is rather local in its distribution, and is easily distinguished from the other two by its superior size, having floating branches from a few inches to a foot in length. The second (U. minor) is much more common. Its floating branches are only two or three inches long at the time of flowering, but they grow longer after; and the flowers are pale yellow, with a short, broad spur. The third (U. intermedia), which is very local, has also pale yellow flowers, but with a much longer spur; and the bladders are at the ends of leafless branches.

In the preceding chapter we gave an account of certain plants which are parasitic on other plants and trees, deriving more or less of their nutriment from their vegetable hosts. One of these--the Tooth-wort (Lathræa squamaria), of the order Orobanchaceæ--is not only a parasite, deriving nourishment from the roots of trees, but is also a carnivorous species, feeding on minute animals which are captured and digested by its peculiar leaves; and therefore it may be conveniently considered here.

The whole plant is of a fleshy character, and lives entirely underground, attached to the roots of the Hazel, Elm, or other tree, except during April and May, when it sends up thick flowering stems, from four to ten inches high, bearing a few broad, fleshy scales which gradually pass into bracts, and a one-sided spike or raceme of flowers. The stem and scales above ground are of a pale rose colour, and the flowers are either brown, flesh-colour or slightly bluish. The latter are numerous, closely placed, and either sessile or shortly stalked. The calyx is bell-shaped, nearly half an inch long, with four broad lobes; and the corolla, which is about half as long again as the calyx, is distinctly lipped.

The whole plant is devoid of chlorophyll, and consequently has not the power of building up organic compounds after the manner of green plants; and, being parasitic on the roots of trees, it derives but little organic material from its host. To compensate for this the underground portion is so constructed that it can capture minute animals which exist in the soil, and has the power of digesting them and of absorbing the products of digestion.

The underground stems are quite white, and are thickly covered with broad, cordate, fleshy leaves that closely overlap one another. There appears to be nothing very remarkable in these underground leaves until one has been removed from the stem and closely examined; and then we find that what appears to be the apex of the leaf is really its middle; and that what seems to be, at first sight, the under surface, is really an extension of the upper side; for the leaf is bent backwards in such a manner as to bring its apex close to the stem, immediately below its base. This peculiar folding of the leaf results in the formation of an irregular cavity, and the tip of the leaf, brought close to its base, is curled upward, close to the stem, in such a way as to form a little canal, with several small openings by which the cavity may be reached. It will not be easy to make out this strange folding of the leaf by an examination of the exterior only, but a longitudinal section, made with a sharp knife or razor, will show it clearly.

When minute animals enter the cavity of the leaf through the little openings above mentioned, they are seized by means of small filaments that protrude from the lining cells; and although no special digestive secretion has been discovered in the leaves, it appears certain that the creatures entrapped are really dissolved, for nothing remains of them after a time except the harder, indigestible portions. Also, there is every reason to believe that the products of digestion are absorbed, probably by the same filaments that are concerned in the capture of the microscopic prey.

Perhaps the most interesting of the carnivorous plants are those which exhibit distinct movements in connexion with the capture of their prey, and among these are the British Butterworts and Sundews, which grow in bogs and other wet places.

There are three British species of Butterwort (Pinguicula), similar in structure and habit, all growing in bogs and on wet rocks. They have each a rosette of entire, radical leaves, the lowest of which lie close against the soil or rock on which the plant grows; and violet or yellow flowers on leafless peduncles. The calyx has four or five teeth, arranged in two lips; and the corolla, which is also lipped, has a broad, open throat, and a spur.

The commonest species is the Common Butterwort (P. vulgaris), which is found in bogs and wet places, principally in the hilly, humid districts of the West of Britain and Ireland, flowering from May to July. Its leaves are succulent and clammy, of a pale green colour, and covered all over with little glistening spots. The flower stems are three or four inches high, each bearing a single violet flower. In this species the throat of the corolla is bell-shaped, and the spur is as long as the rest of the corolla.