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.
