2. Rhinantheæ, Yellow-rattle Group. Herbs, all of which (with the exception of Lathræa) are annual parasites with green foliage-leaves. They attach themselves by haustoria to the roots of other plants and draw nourishment from them. The majority turn black when dried. Racemose inflorescences. In many the calyx is 4-partite, the posterior sepal being absent, or very small. The corolla is distinctly bilabiate (Fig. [568]), with most frequently ascending æstiration; in the majority it does not become detached at the base, but by means of a ring-like cut some distance up the tube; 4 didynamous stamens; pollen-grains dry, easily falling out; the anthers are often furnished at the base with bristles or hairs (Fig. [568]) which play a part in the pollination, the probosces of the insects, being forcibly pushed against them, agitate the anthers and shake out the pollen-grains. Capsule with loculicidal dehiscence.—Euphrasia (Eye-bright), Melampyrum (Cow-wheat), Rhinanthus (Yellow-rattle), Odontites (Bartsia), Pedicularis (Louse-wort), and Lathrœa (Tooth-wort) all have native species. The last named is pale yellow, or reddish (without chlorophyll); it is a parasite on the roots of the Hazel, Beech and other shrubs, having an aerial stem, and an underground, perennial rhizome, covered with opposite, scale-like, more or less fleshy leaves with a number of internal glandular, labyrinthine cavities. The inflorescence is a unilateral raceme. It approaches Gesneriaceæ in having a unilocular ovary with two parietal placentæ.
Fig. 568.—Euphrasia officinalis. Flower of the large and the small-flowered forms; showing the anthers and stigmas.
The mechanical contrivances for POLLINATION are so numerous that no general principle can be laid down. Personate flowers, like those of Antirrhinum are only accessible to strong insects, such as humble-bees, which can force themselves between the two lips, and so become dusted with pollen on the back. In Euphrasia and other Rhinantheæ the insects become covered with smooth, powdery pollen when they shake the anther-apparatus in touching the hairs and bristles mentioned above. Scrophularia nodosa is protogynous (Fig. [565]). Digitalis purpurea, however, is protandrous. Mimulus luteus and some others have sensitive stigmatic lobes, which shut up on being touched. The Veronica-species constitute a series, from large-flowered down to small-flowered forms, and parallel with them are found various gradations from insect-to self-pollination. In some (as Euphrasia officinalis, Rhinanthus crista galli) there are two kinds of flowers: large, which are pollinated by insects, and small, which are self-pollinated (Fig. [568]). Lathræa squamaria (Tooth-wort) is a protogynous spring-flowering plant, largely visited by humble-bees. Others have cleistogamic flowers. Nycterinia capensis opens its flowers at night.
2,000 species; chiefly from the Temp. Officinal: Digitalis purpurea (the leaves; Europe), a poisonous plant. Verbascum thapsus and thapsiforme, Veronica officinalis (“Herba V.”), Gratiola officinalis (“Herba”) have medicinal uses. The whole of the Scrophulariaceæ are more or less suspicious, if not actually poisonous, and none serve as food. Many are ORNAMENTAL PLANTS: Mimulus luteus (N. America), Paulownia imperialis (the only species; in Japan; a tree), Antirrhinum vulgare (S. Eur.), Linaria, Pentstemon, Veronica, Calceolaria (Peru, Chili, etc.).
Fig. 569.—Leaf of Utricularia vulgaris, with bladder. Median longitudinal section through a bladder containing a Cyclops. At a a hair of the upper-lip, at i 2 bristles of the under-lip of the entrance (a, b); in the latter are placed 4 bristles h; k stalk of the bladder, in which is seen a vascular bundle. (After Cohn.)
Order 4. Utriculariaceæ. To this order belong only perennial, insectivorous, aquatic, and marsh-plants (200 species) with a more or less characteristic appearance. They differ from the Scrophulariaceæ, especially in having 2 stamens (the anterior) and a unilocular ovary, with free, central placenta (like that of the Primulaceæ). For the rest the flower is distinctly bilabiate, both in the calyx and corolla. Two-valved capsule; no endosperm.
Pinguicula (Butter-wort) has a rosette of leaves close to the ground; these are sticky, covered with glandular hairs, and roll round any small insects which may be caught upon them; flowers solitary, terminal on a long scape; calyx, 5-partite; corolla with spur. The embryo germinates with 1 cotyledon.—Utricularia (Bladder-wort). Our native species are floating, without roots, with hair-like, divided leaves, studded with peculiar bladders (in the Tropics there are terrestrial species, with ordinary foliage). The bladders (Fig. [569]) have an aperture, closed by a valve opening inwards, so that small aquatic animals are allowed to enter, but are not able to escape; they are thus entrapped in the bladders, and are probably used as food. Calyx bipartite; corolla personate with spur.
The embryo of Utricularia is very imperfect, scarcely more than a spherical, cellular mass, with a few slight leaf-rudiments. On the germination of U. vulgaris, several bristle-like leaves develop into a compact rosette; the stem then develops, and also the finely-divided, bladder-bearing leaves. A primary root is not developed. The stems branch copiously and in a very peculiar manner. The growing-point of the stem is rolled spirally.—The stigmatic lobes are sensitive and close on being touched; self-pollination often takes place, however, in Pinguicula.