Fig. 542.—Viscum album: A branch with leaves and berries: a scale-leaves; b foliage-leaves; n m n flowers; B seedling, the bark of the branch being removed; C an older embryo which still retains the cotyledons.
Fig. 543.—To the left the Rafflesiaceous Cytinus hypocistus, parasitic on the roots of Cistus. To the right the Balanophoraceous Cynomorium coccineum, parasitic on the roots of Salicornia.
The Mistletoe (Viscum album, Fig. [542]) is a native, evergreen plant which may be found growing on almost any of our trees (sometimes on the Oak), and, like other Loranthaceæ, it produces swellings of the affected branches. Its spherical white berries (Fig. [542] A) enclose (1–) 2–3 green embryos; they are eaten by birds (especially Thrushes), and are partly sown with their excrement, partly struck or brushed off the branches of the trees, the seed being enclosed, at maturity, by viscin, i.e. “bird-lime.” The seeds may also germinate on the branches, without having first passed through the alimentary canal of the birds. On germination, the hypocotyl-axis first appears, as in Fig. [541], and bends towards the branch; the apex of the root then broadens, and forms at the end a disc-like haustorium, from the centre of which a root-like body grows through the bark into the wood, and ramifies between the bark and wood. Suckers are developed on the root like strands which are formed in this manner, without, however, having a rootcap; they are green, and penetrate the wood by the medullary rays (Fig. [542] C). Adventitious buds may also be developed from the root-like strands which break through the bark and emerge as young plants. The young stem quickly ceases its longitudinal growth, and lateral shoots are developed from the axils of its foliage-leaves. These and all following shoots have a similar structure; each of them bears a pair of scale-leaves (Fig. [542] A, a) and a pair of foliage-leaves (Fig. [542] A, b), and then terminates its growth, if it does not produce an inflorescence; new lateral shoots proceed from the axils of the foliage-leaves, and the branching, in consequence, is extremely regular and falsely dichotomous. Only one internode (shoot-generation) is formed each year, so that each fork indicates one year. The foliage-leaves fall off in the second year. The inflorescence is a 3(-5)-flowered dichasium (Fig. [542] A, m is the central flower, n the lateral). The plants are diœcious; the ♂-flower as a rule is 2-merous: perianth 2 + 2, each leaf of which bears on its inner side 6–20 pollen-sacs, each of which opens by a pore; this relationship may be considered to have arisen from the union of the perianth-leaves with the multilocular stamens (2 + 2) placed opposite them. The ♀-flowers always have Pr 2 + 2, G2.—Loranthus is also found in Europe (it has a 3-merous flower), especially in the central and south-eastern districts, on Quercus cerris and Q. pubescens; but the great majority of the 520 species grow in the Tropics on trees which they ornament with their often brightly-coloured flowers, and ultimately kill when present in too great numbers. The pollination in the numerous Loranthaceæ with unisexual flowers, is effected by the wind. In Viscum album this takes place in autumn, the actual fertilisation in the following spring, and the maturity in November or December; in the succeeding month of May the berry is ready to germinate, and falls off.
Uses. Birdlime from Viscum album.
Order 4. Rafflesiaceæ and Order 5. Balanophoraceæ. These orders comprise root-parasites, almost entirely devoid of chlorophyll; they are reddish or yellow, without foliage-leaves (Fig. [543]). As far as our knowledge of these rare tropical plants extends, they have thalloid organs of vegetation resembling the root-like strands of Viscum, or they are filamentous and branched like Fungus-hyphæ; they live in and on the tissues of the host-plant, from which their flowering-shoots, often of mushroom-like form, are subsequently developed (Fig. [543]). In order to unfold they must often break through the tissues of the host-plant.
Of the Rafflesiaceæ, Cytinus hypocistus is found in S. Europe living on roots of Cistus-plants and to some extent resembling Monotropa (Fig. [543]). Rafflesia is the best known; it lives on roots of Cissus-species (belonging to the Ampelidaceæ) in Java; its yellowish-red, stinking flowers attain a gigantic size (one metre or more in diameter), and are borne almost directly on the roots of the host-plant. Besides these there are other genera: Brugmansia, Pilostyles, Hydnora.—To Balanophoraceæ (Fig. [543]) belong: Balanophora, Langsdorffia, Scybalium, Sarcophyte, Helosis, etc., and in S. Europe, Cynomorium coccineum.
Sub-Class 2. Sympetalæ.
The characters which separate this from the first Sub-class, the Choripetalæ, have been described on page [336]. They consist in the following: the flower is always verticillate, generally with 5 sepals, 5 petals, 5 stamens, and 2 carpels (in the median plane), the calyx is generally persistent and gamosepalous, the corolla is gamopetalous and united to the stamens, which are therefore adnate to it, the ovules have only one thick integument and a small nucellus. (The exceptions are noted later.)
This Sub-class is no doubt more recent than the Choripetalæ; it is also peculiar in including fewer trees and shrubby forms than the latter.