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THE MISTLETOE AND PARASITIC PLANTS.
BY MRS. LANKESTER.
RUT few botanists would expect to prosecute their researches
in the midst of Christmas fêtes and gambols, and even on specimens rescued from the lurid flame of a burning plumpudding. Yet we can assure those of our readers whose botanical thirst is impatient for a supply of genuine spring flowers that no more interesting subjects for study can be found in the fields than the holly and the mistletoe—both, perhaps, associated more with the idea of indoor merrymakings than outdoor rambles. Neither are we limited in our botanical studies during the hard winter months, when all Nature seems hiding herself from observation, to these familiar and remarkable plants,—the coloured patches on the barks of trees, on old walls, and broken palings afford a whole world of interesting study; and beautiful collections may be made of these variegated lichens. We, however, select the Mistletoe as the subject for our “winter's tale,” not only on account of its grcat botanical interest, but from its ancient and curious associations with the history of our country
The Mistletoe, Viscum album (Plate VIII., fig. 1), may be regarded as the type of parasitic plants generally—of those plants which establish a habitation for themselves in the substance of some other plant, and live and flourish on nutriment drawn from this source.
The order Loranthaceae, to which the Mistletoe belongs, and of which it is the only British representative, consists almost entirely of parasitic plants. But three genera are known in Europe. Nuytsia floribunda, an Australian plant, forms an exception to the parasitic nature of the species, and is a very beautiful shrub, with large bright orange-coloured flowers, which gain for it the name of fire-tire.
In describing the order Loranthacea, we find that some botanists have separated a number of plants once included therein, and established another order, Viscacce, of which Viscum is the type; Loranthus being that of the original order, Loranthacea. This distinction is not very generally adopted, and we are safe in regarding our mistletoe as belonging to the family
Loronthacece. The plants of this order have articulated opposite branches, leaves without stipules, and flowers in which the pistils and stamens are usually separated. The calyx is adherent to the ovary, with two bracts at its base, and the corolla consists of four or eight more or less united petals. The stamens are four or eight, opposite the petals. The ovary onecelled, with one erect ovule.
On examining a branch of mistletoe in the hand, we observe the long spreading stems of a yellowish-green colour, with somewhat thick, fleshy, long-pointed leaves, having veins running evenly along their entire length, most evident on the under surface. In the axils of these leaves, or the angle from which they spring, as also at the point where the spreading stems meet, we see, in the autumn of the year, a minute flowerbud-insignificant enough to look at, but of such interest, when examined botanically and carefully, as to have afforded matter for elaborate and beautifully-illustrated papers, which are preserved by the learned societies of this and other countries.
These flowers are divided into staminiferous and pistiliferous ones (figs.2 and 3). The structure of these organs is very curious, and can scarcely be fully described in a few words, for authors differ as to the appropriate names and real functions of the parts. Schleiden, the great German botanist, supposes that the so-called male flowers consist of nothing but anthers; while Decaisne, of whose beautiful drawings in the Transactions of the Royal Society of Brussels we have, in a measure, availed ourselves, contends that the male flowers consist of four anthers grown to the surface of four calycine sepals. They grow on distinct plants from the female flowers, which are botanically the most interesting, as well as popularly, from the fact of their producing the beautiful white berries so characteristic of the mistletoe. The ovule at the base of the female flower does not appear at all until after the pollen has been shed from the anther in the male flower and has entered the little canal leading to the ovarium. Some weeks, or even months, after this has taken place, the little ovule or future seed makes its appearance, and, meeting with the pollen, commences to develop. At the same time, the viscous tissue, of which the fruit chiefly consists, begins to appear, and the tiny seeds are produced, which remain inclosed in their sticky, glutinous cradle until fully ripened and ready to reproduce their own kind. The fact of the ovule being produced after the pollen has exercised its influence is a curious deviation from the usual mode of development in plants. The structure of the woody tissue of the plant is also singular (fig. 4). Decaisne describes it, when young, as consisting of eight woody bundles surrounding a green pith (fig. 5); in these bundles are no spiral vessels, but instead, and in the place where they usually occur, are found some ringed tubes (fig. 6), together with some dotted or reticulated cells and fibres. On the outside of these bundles of woody matter, which form the longitudinal tissue of the plant, are found others, similar in number but smaller, and composed exclusively of fibres of the inner layer of the bark.
To trace these appearances a careful microscopic examination is required, and some of the more delicate observations cannot be conducted without first submitting the portion of stem to a saturation of spirits of wine.
The manner in which this curious plant establishes itself in the tissue of other plants is very remarkable, and has been well described by De Candolle in his excellent “Physiologie Végétale.” Old botanists believed that birds, feeding upon the berries, and getting their beaks surrounded with the viscous matter they contain, rubbed their beaks against the branches to get rid of it, and thus introduced the seeds to their restingplace. Paley, in his “Natural Theology,” gives at great length his views of the subject, and says :
Of no other plant can it be said that the roots refuse to shoot in the ground, and no other is known to possess this adhesive generative quality when rubbed on the branches of trees.
Careful botanists who have examined the process of growth in these plants from their earliest stage, tell us that from whatever cause the seeds are brought in contact with the wood of the tree on which they establish themselves, they adhere by means of the glutinous substance in which they have been embedded, and which hardens into a sort of transparent glue. Then two or three days after application the tiny radicle may be seen pushing towards the support, whether it be on the under or upper surface; reaching this point it becomes enlarged and flattened. It now has the appearance of a sucker, and by degrees penetrates the bark. This operation requires some time, and is not completed until the plumule begins to be developed. By the time the young plant has a pair or two of leaves, the attachment will be found tolerably firm.
Mr. Grifiths, who has written a paper on the Orders of Loranthus and Viscum in the Transactions of the Linnan Society, tells us that on cutting away a portion of the branch
included portion of the parasite, he found that the union had taken place entirely between the ligneous systems of both; the fibres of the sucker-like root of the parasite expanding on the wood of the support in the form of a web foot. There was, however, no interchange of structure between them, neither at this period was there any intermixture of ligneous