Vinca (Periwinkle) has a salver-shaped corolla, which is twisted to the left in æstivation (i.e. the left edge of the petals is free); nectaries 2, alternating with the carpels; the summit of the style is hairy. Follicles; seeds without hairs. Mostly creeping, perennial, evergreen plants, whose large flowers are apparently axillary; in reality they are terminal, but by the development of the bud in the axil of one of the two uppermost leaves, they are thus displaced over the other leaf of the pair (a helicoid sympodium being formed).—Plumeria, Tabernæmontana, Cerbera (drupe). Aspidosperma.
Nerium (Oleander). The leaves are in whorls of 3. Corolla funnel-shaped, in æstivation twisted to the right, and with a corona resembling that of Lychnis. The anthers are prolonged at the base and each also bears at the apex a long, linear, hairy appendage; these finally become spirally twisted. Follicles; seeds hairy. Apocynum, Echites, etc. Epigynum is epigynous.
124 genera, 1,000 species; principally in the Tropics. Only 2 species of Vinca are natives of this country; the following are cultivated as ornamental plants:—Vinca minor, V. major, V. (Lochnera) rosea, Amsonia salicifolia, Nerium oleander (Eastern Mediterranean). The latex of some is poisonous (Tanghinia venenifera, Cerbera). Caoutchouc is obtained from others (Hankornia, Landolphia, Vahea, etc.). Tough bast is frequently developed. The bark of Aspidosperma quebracho and the seeds of Strophanthus hispidus are used in medicine (also for African arrow-poison), the latter is officinal.
Order 3. Asclepiadaceæ. A natural and easily recognised order, closely allied to the Apocynaceæ, having, like it, frequently a poisonous latex, opposite, single, entire leaves and fundamentally the same floral diagram and floral structure (S5, P5, A5, G2); but in some the æstivation of the corolla is valvate. The carpels here also have free ovaries, but are united for some distance above into a large, shield-like, 5-angular head, having on its underside the true stigmas, and the fruit always consists of 2 follicles; seeds most frequently numerous and hairy at the micropyle (“vegetable silk”); endosperm scanty.—The order is distinguished from the Apocynaceæ and from all other plants also, except the Orchids, by having all the pollen-grains in each of the 2 loculi of the anthers (true 2-locular anthers) united into one waxy, club-shaped pollen-mass (“pollinium”), for the purpose of pollination by insects. These heavy masses, in order to secure pollination (as in the case of the Orchids), must be attached to sticky discs (corpuscula); there are 5 corpuscula, one at each of the corners of the 5-angular stylar-head (alternating with the anthers), and to each of these are attached 2 pollinia, one from each of the anthers situated on either side (thus each anther gives its right pollinium to one corpusculum and its left to another). The stamens are frequently united at the base, and each bears on the back a variously formed, petaloid appendage, termed a “cucullus.”
Fig. 582.—Asclepias cornuti. A An open flower with the calyx (k) and corolla (c) turned down; the stamens are bent together and surround the gynœceum. B The andrœcium after removal of the sterile part (cucullus) of the anther, which functions as a nectary: e the lateral expansions of the fertile portion of the anthers; f the slit between the expansions of two contiguous anthers, through which the insect’s foot, and later a pollinium which is caught by it, is dragged, and behind which the only receptive part (stigma) is hidden; above the slit f is the gland (r), which secretes the horny corpusculum, which is split at its base and joined on either side with a pollinium (this is more distinctly seen in D and E). When the foot of the insect is caught in the slit (f) and is drawn upwards, it becomes entrapped in the slit of the corpusculum, which is then pulled out together with the pollinia firmly attached to it. In walking over the flowers the insect will draw its foot through other slits (f) and so leave the pollinia on the stigmas. C, D The gynœceum with the pollinia hanging freely. E A corpusculum and two pollinia.
A peculiar relative position (and therefore a good, distinctive characteristic) is often found in the inflorescence, which is cymose; it is placed between the two leaves of a whorl, nearer to one than to the other. The leaf-pairs are placed obliquely in the floral region, at acute and obtuse angles, and not at right angles (as in the purely vegetative parts); the inflorescences are placed in two rows only which are nearly 90° from each other, and the two contiguous to one another are antidromous; they are in reality terminal, each on its own axis, and the entire floral portion of the shoot is a unipared scorpioid cymose sympodium; in addition, complications also arise through individual parts becoming united.—Herbs and shrubs, some twining or climbing.
In Asclepias the corolla is bent back and there is a cup-like cucullus, from the base of which protrudes a horn-shaped body, bent inwards.—Vincetoxicum has a rotate corolla and a ring-like, 5-lobed cucullus, without internal prominences.—Stapelia (especially from S. Africa) is remarkable on account of its Cactus-like, leafless stems and large, brownish flowers, often with carrion-like smell. Periploca has more powdery pollinia (S. Eur., etc.); Hoya carnosa (Wax-flower; Trop. Asia) is a climber, and has small, annual, flower-bearing dwarf-branches. Ceropegia.
201 genera with 1700 species, distributed over all tropical countries; few outside these limits: no native species. Several are used in medicine on account of the pungent properties of the latex. Condurango-bark of Gonolobus condurango is medicinal. Caoutchouc is obtained from the latex of some (e.g. from Cynanchum). The seed-hairs, which are most frequently shining, silk-like, and white, are not sufficiently pliant to be of much value. Ornamental plants in our gardens: Asclepias-species, etc.
Order 4. Loganiaceæ. Ovary single, with two loculi, in structure resembling the Rubiaceæ, but superior. 360 species are included in this order; the majority are tree-like, some lianes which climb by tendril-like branches. The interpetiolar stipules of some species are very characteristic (as in Rubiaceæ, to which they maybe considered to be closely related). The fruit is a capsule or berry. The most familiar genus is Strychnos, which has spherical berries with an often firm external layer, and compressed seeds with shield-like attachments; endosperm abundant. The leaves have 3–5 strong, curved nerves proceeding from the base.—Spigelia.—They have no latex, as in the two preceding orders, but many are very poisonous (containing the alkaloid “strychnine,” etc.); the South American arrow-poison, urare or curare, is made from various species of Strychnos, also an arrow-poison in the East Indian Islands (Java, etc.). Officinal, the seeds of Strychnos nux vomica (“Vomic nut,” Ind.). The seeds of Strychnos ignatii (Ignatius-beans, medicinal), and others are poisonous.