A number of plants extract more or less of the organic material they require from other plants, and thus save themselves the labour of building up this material themselves. These are termed parasites; but we must be careful to distinguish between them and certain other plants which, though apparently parasitic, are not really so. One plant may climb on another, perhaps even producing "rootlets" by which it clings to its living support, and yet it may not be a parasite in the proper sense of the term, for it may not absorb the slightest amount of nutritious matter except from the soil and the air. It is not at all uncommon for the Honeysuckle to twine its stems round the trunk and branches of a young tree, with the result that the tree becomes stunted, and assumes a starved appearance, especially in its lower parts; and yet the Honeysuckle is not a parasite. It has withdrawn nothing from the tree which supports it, but has coiled itself so tightly round it as to interfere with the circulation of its sap. The lower part of the tree is especially affected because the strangulating coils of the climber prevent the downward flow of the sap contained in the vessels of the bast or inner bark, and this is the sap which holds the constructive materials that have been built up in the leaves, under the influence of light.

Many of the parasitic plants are of microscopic dimensions, and others are larger species belonging to the Fungi or Mushroom group. Some, however, are flowering plants, and these only fall within the scope of our work.

We shall first deal with parasites which have no green leaves or chlorophyll, and are therefore entirely dependent on outside sources for their supply of organic material, starting with the interesting Dodders (Cuscuta), which coil themselves round herbs, shrubs, or even trees, and produce sucking organs on their stems that come in contact with their host.

These are all smooth plants, with globular clusters of yellowish-pink flowers, the calyx being of the same colour as the corolla. The former is deeply divided into four or five parts, and the corolla has four or five spreading lobes with as many scales inside its broad tube. The ovary has two distinct styles, and the fruit is a globular capsule. The following summary of distinguishing features will enable the reader to identify the British species of the genus:—

1. The Greater Dodder (Cuscuta europÆa).—A plant of a greenish yellow colour, generally more or less tinged with red, with flowers in sessile, globular clusters nearly half an inch in diameter, each individual flower being about a tenth of an inch. This species is not abundant. It may be met with in hop-fields, and is also parasitic on nettles, various shrubs, and trees, including the elder and the ash.

2. The Flax Dodder (C. Epilinum).—Very much like C. europÆa, but the flowers are fewer in number, larger, and more fleshy. The calyx is nearly as long as the corolla, with sharply-pointed segments; and the corolla tube is always globular. This species is not indigenous, but is sometimes met with in flax-fields.

3. The Lesser Dodder (C. Epithymum).—A more slender plant, with thread-like stems, and flowers in small, compact, globular heads, with red calyx and cylindrical corolla. This species occurs principally on sunny heaths, where it is parasitic on shrubby plants, such as thyme and ling. It is much more common than the foregoing.

4. The Clover Dodder (C. Trifolii).—Very much like the Lesser Dodder, of which it is sometimes regarded as a variety. Its calyx is of a very pale colour, and is almost as long as the tube of the corolla, which is cylindrical in form. It is rare, but sometimes appears in undesirable numbers in clover fields.

All the species produce their flowers in August and September, but C. europÆa may often be seen in bloom very early in July.

The seeds of the Dodder fall from the opened capsules during late summer and early autumn, alighting on the soil, or on the decomposing foliage that covers the ground, or on the rough barks of the tree that served as a host for the parasitic plant. The seeds of many other plants fall about the same time, but those of the Dodder do not begin to germinate until about a month later than the majority of these, in the following season, and consequently the young Dodder plants do not appear before their future hosts have had time to grow sufficiently large to support and nourish them. Perennial plants, too, which are attacked by the Dodder, have also produced strong shoots and leaves from their roots or underground stems by the time that the parasite begins its search for ready-made organic food; and it is clear that if the Dodder seeds germinated earlier in the season, the young plants would starve for want of suitable herbs to give them support and nourishment.

When the seed germinates it sends out a filament which penetrates into the soil and fixes the seedling firmly. The other end grows upward, carrying up with it a little swollen mass of food-reserve, sufficient to support the growing seedling until it has had some chance of reaching a suitable host. The upper end of the seedling now sends out a filament which rapidly elongates, and, growing upward, searches for some stem on which to climb.

All this time the little mass of food-reserve is being rapidly exhausted, and if the young seedling fails to reach a suitable plant on which to climb it soon dies, for its lower extremity is unable to absorb sufficient food material from the soil; and the plant itself, having no chlorophyll, cannot decompose carbonic acid gas and build up organic material to add to its substance.

Again, should the young plant fail to reach a favourable support, so that it is of necessity compelled to trail along the ground, the filaments which would soon produce suckers when attached to a living plant have no power to form any structures capable of extracting food material from a damp soil.

Circumstances being more favourable, however, the upper filament eventually finds a stem, and immediately begins to twine itself round it, making a few close coils in a clockwise direction. Should the support prove to be a dead stem, little wartlike swellings are produced at points where the two touch, and these serve as a means of attachment for the climbing filament, but no suckers are formed. If, however, the filament surrounds a living stem, each of the swellings gives rise to suckers that penetrate into the tissues of the latter, and withdraw the organic food necessary for the continued existence of the plant.

The Dodder now grows rapidly, giving off branches which search in all directions for additional supports, sometimes climbing from one plant to another, and producing new suckers whenever a favourable situation has been reached. The plant has now all it requires both in the way of mechanical support and nourishment, and its lower part, thus rendered useless, soon withers, breaking all connexion with the soil on which the seed originally germinated. New branches continue to form, each one producing additional suckers for the extraction of food from the host or hosts, until a tangled mass of clinging stems is the result. Then the globular clusters of little flowers appear, followed by balls of small capsules which throw off their lids when ripe, allowing the seeds to be shaken out by the wind. The Dodder plant now withers, leaving, in the autumn, its dead tangles of climbing filaments still attached to the withered herbs on which it fed, or to the branches of the tree which served as its host.

Other parasitic plants possessing no chlorophyll, and therefore incapable of building up organic compounds for themselves, derive their food from the roots of trees and shrubs.

Among these is the Toothwort (LathrÆa), which is carnivorous as well as parasitic, and is described in our chapter (XXIV) dealing with carnivorous plants, so that we need only refer here to its habit as a parasite.

The seed of this plant germinates on the damp ground to which it falls in early summer. The young root penetrates into the soil, deriving its nourishment entirely from the food reserve that was stored up in the seed, and soon sends out lateral branches in search of the roots of a suitable host. If it fails to attain this end by the time that the reserve is exhausted, it dies; but if it succeeds in reaching the root of an Elm, Hazel, Hornbeam, Ash, Poplar, or other tree, it fastens itself to it, and develops suckers which penetrate into the substance of the root to extract its sap. The parasite now grows very rapidly, producing its underground stems, with their fleshy, overlapping scales, as described on p. 352.

The Broomrapes of the same order (OrobanchaceÆ) are very similar in their parasitic habits to the Toothwort, and, like the latter, they possess no chlorophyll. The seeds germinate on the damp soil, producing a long, narrow embryo that grows downward into the ground until it reaches the root of some herb or shrub. It then gives off suckers which penetrate into the root, and, with the aid of the organic food thus obtained, forms a tuberous swelling on its surface. Flowering stems are afterwards produced, and these, rising above the soil, bear terminal spikes of lipped flowers, followed by capsules containing many seeds.

There are several British species of this genus (Orobanche), and their flowering stems, which are usually unbranched, produce scale-like leaves of the same colour as themselves. Each flower of the spike is in the axil of a bract resembling the scales of the lower part of the stem; and in some species there is a pair of smaller bracts close to the base of the calyx. The corolla is either tubular or bell-shaped, and more or less distinctly lipped. Each flower has four stamens, arranged in pairs, and a two-lobed stigma. The following outline of leading features will serve for the identification of the common Broomrapes:—

1. The Great Broomrape (O. Rapum).—A plant from twelve to eighteen inches high, of a pale yellow colour at first, but afterwards turning to a dull purple brown. Stem thick, especially below, and unbranched. Scales lanceolate. Flowers sessile, whitish, with only one bract, forming a spike from six to nine inches long. This species is moderately common, and is parasitic on the roots of Furze and Broom. Time of flowering—May to July.

2. The Clove Broomrape (O. caryophyllacea).—Very similar to the Great Broomrape in colour, but usually smaller, and easily distinguished by the sweet clove-like scent of its flowers. Spike not so dense as in the last species, and the corolla tube not so broad. The plant is not uncommon in the southern counties of England. It is parasitic on the roots of the Great Hedge Bedstraw, and flowers from May to July.

3. The Tall Broomrape (O. elatior).—Also much like the Great Broomrape, of which it is perhaps a variety. It retains its original yellowish colour for a longer period, and is parasitic on the Great Knapweed, flowering from June to August.

4. The Least Broomrape (O. minor).—A yellow or pale brown plant, from six inches to over a foot in height, more slender than the preceding species, with smaller flowers. The flowers are whitish, but more or less tinged with purple, and bloom from June to October. It is parasitic on a number of different plants, including the Ivy, Clovers, Hawkweed, Wild Carrot, &c., and is found in many districts in South and Central England.

We have now to consider those parasites which bear leaves possessing chlorophyll granules, and are therefore able to build up a portion of the organic compounds necessary for their development. Most of these, at least as far as the British flowering species are concerned, have also true roots which grow into the soil and absorb mineral food, like those of the non-parasitic plants allied to them, so that it is difficult to understand why they should require the additional nourishment stolen from the roots of neighbouring plants. One, however, the well-known Mistletoe, grows on trees at a distance from the ground, and therefore obtains the whole of its food, with the exception of carbonic acid gas, direct from its host.

This plant—the Mistletoe (Viscum album), of the order LoranthaceÆ—is attached to the tree on which it grows by a thick stem that becomes woody when old. Its branches are of a yellowish-green colour, and are repeatedly forked in such a manner as to form a dense tuft that often reaches a diameter of two feet or more. The leaves are of the same colour as the branches, and are rather thick and fleshy. The flowers grow in the forks of the branches, on very short stalks, and are imperfect, the males and females being on separate plants. The former are in clusters of about three or four, in a cuplike, fleshy bract, each flower having four thick, triangular petals with an anther on the middle. The females are either solitary or in clusters of two or three, with a similar bract, and very small petals. The fruit is a white, glutinous berry, almost transparent, with only one seed.

The Mistletoe grows on a variety of trees, including the Apple, Pear, Black Poplar, and Oak; and thrives most luxuriantly on those which have a soft tissue beneath the bark. It is found principally in the southern and western counties of England, and flowers from March to May.

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.