Chapter XIX Classification of Dicotyledons (Continued) — Division II — Sympetalæ
The Sympetalæ or Gamopetalæ are at once distinguished from the Choripetalæ by having the petals more or less united, so that the corolla is to some extent tubular. In the last order of the Choripetalæ we found a few examples (Mimosaceæ) where the same thing is true, and these form a transition from the Choripetalæ to the Sympetalæ.
There are two great divisions, Isocarpæ and Anisocarpæ. In the first the carpels are of the same number as the petals and sepals; in the second fewer. In both cases the carpels are completely united, forming a single, compound pistil. In the Isocarpæ there are usually twice as many stamens as petals, occasionally the same number.
There are three orders of the Isocarpæ, viz., Bicornes, Primulinæ, and Diospyrinæ. The first is a large order with six families, including many very beautiful plants, and a few of some economic value. Of the six families, all but one (Epacrideæ) are represented in the United States. Of these the Pyrolaceæ includes the pretty little pyrolas and prince’s-pine (Chimaphila) (Fig. 116, J); the Monotropeæ has as its commonest examples, the curious Indian-pipe (Monotropa uniflora), and pine-sap (M. hypopitys) (Fig. 116, L). These grow on decaying vegetable matter, and are quite devoid of chlorophyll, the former species being pure white throughout (hence a popular name, “ghost flower”); the latter is yellowish. The magnificent rhododendrons and azaleas (Fig. 116, F), and the mountain laurel (Kalmia) (Fig. 116, I), belong to the Rhodoraceæ. The heath family (Ericaceæ), besides the true heaths (Erica, Calluna), includes the pretty trailing-arbutus or may-flower (Epigæa), Andromeda, Oxydendrum (Fig. 116, E), wintergreen (Gaultheria), etc. The last family is represented by the cranberry (Vaccinium) and huckleberry (Gaylussacia).
The second order, the primroses (Primulinæ), is principally represented in the cooler parts of the world by the true primrose family (Primulaceæ), of which several familiar plants may be mentioned. The genus Primula includes the European primrose and cowslip, as well as two or three small American species, and the commonly cultivated Chinese primrose. Other genera are Dodecatheon, of which the beautiful shooting-star (D. Meadia) (Fig. 117, A) is the best known. Something like this is Cyclamen, sometimes cultivated as a house plant. The moneywort (Lysimachia nummularia) (Fig. 117, D), as well as other species, also belongs here.
The sea-rosemary (Statice) and one or two cultivated species of plumbago are the only members of the plumbago family (Plumbagineæ) likely to be met with. The remaining families of the Primulinæ are not represented by any common plants.
The third and last order of the Isocarpous sympetalæ has but a single common representative in the United States; viz., the persimmon (Diospyros) (Fig. 117, E). This belongs to the family Ebenaceæ, to which also belongs the ebony a member of the same genus as the persimmon, and found in Africa and Asia.
The second division of the Sympetalæ (the Anisocarpæ) has usually but two or three carpels, never as many as the petals. The stamens are also never more than five, and very often one or more are abortive.
The first order (Tubifloræ) has, as the name indicates, tubular flowers which show usually perfect, radial symmetry (Actinomorphism). There are five families, all represented by familiar plants. The first (Convolvulaceæ) has as its type the morning-glory (Convolvulus) (Fig. 118, G), and the nearly related Ipomœas of the gardens. The curious dodder (Cuscuta), whose leafless, yellow stems are sometimes very conspicuous, twining over various plants, is a member of this family which has lost its chlorophyll through parasitic habits. The sweet potato (Batatas) is also a member of the morning-glory family. The numerous species, wild and cultivated, of phlox (Fig. 118, A), and the blue valerian (Polemonium) (Fig. 118, D), are examples of the family Polemoniaceæ.
The third family (Hydrophyllaceæ) includes several species of water-leaf (Hydrophyllum) (Fig. 118, E) and Phacelia, among our wild flowers, and species of Nemophila, Whitlavia and others from the western states, but now common in gardens.
The Borage family (Borragineæ) includes the forget-me-not (Myosotis) and a few pretty wild flowers, e.g. the orange-flowered puccoons (Lithospermum); but it also embraces a number of the most troublesome weeds, among which are the hound’s-tongue (Cynoglossum) (Fig. 119, A), and the “beggar’s-ticks” (Echinospermum), whose prickly fruits (Fig. 119, C) become detached on the slightest provocation, and adhere to whatever they touch with great tenacity. The flowers in this family are arranged in one-sided inflorescences which are coiled up at first and straighten as the flowers expand.
The last family (Solaneæ) includes the nightshades (Solanum) (Fig. 119, D), to which genus the potato (S. tuberosum) and the egg-plant (S. Melongena) also belong. Many of the family contain a poisonous principle, e.g. the deadly nightshade (Atropa), tobacco (Nicotiana), stramonium (Datura), and others. Of the cultivated plants, besides those already mentioned, the tomato (Lycopersicum), and various species of Petunia (Fig. 119, H), Solanum, and Datura are the commonest.
The second order of the Anisocarpæ consists of plants whose flowers usually exhibit very marked, bilateral symmetry (Zygomorphism). From the flower often being two-lipped (see Fig. 120), the name of the order (Labiatifloræ) is derived.
Of the nine families constituting the order, all but one are represented within our limits, but the great majority belong to two families, the mints (Labiatæ) and the figworts (Scrophularineæ). The mints are very common and easily recognizable on account of their square stems, opposite leaves, strongly bilabiate flowers, and the ovary splitting into four seed-like fruits (Fig. 120, D, F).
The great majority of them, too, have the surface covered with glandular hairs secreting a strong-scented volatile oil, giving the peculiar odor to these plants. The dead nettle (Lamium) (Fig. 120, A) is a thoroughly typical example. The sage, mints, catnip, thyme, lavender, etc., will recall the peculiarities of the family.
The stamens are usually four in number through the abortion of one of them, but sometimes only two perfect stamens are present.
The Scrophularineæ differ mainly from the Labiatæ in having round stems, and the ovary not splitting into separate one-seeded fruits. The leaves are also sometimes alternate. There are generally four stamens, two long and two short, as in the labiates, but in the mullein (Verbascum) (Fig. 120, M), where the flower is only slightly zygomorphic, there is a fifth rudimentary stamen, while in others (e.g. Veronica) (Fig. 120, J) there are but two stamens. Many have large, showy flowers, as in the cultivated foxglove (Digitalis), and the native species of Gerardia, mullein, Mimulus, etc., while a few like the figwort, Scrophularia (Fig. 120, H), and speedwells (Veronica) have duller-colored or smaller flowers.
The curious bladder-weed (Utricularia) is the type of the family Lentibulariaceæ, aquatic or semi-aquatic plants which possess special contrivances for capturing insects or small water animals. These in the bladder-weed are little sacs (Fig. 120, P) which act as traps from which the animals cannot escape after being captured. There does not appear to be here any actual digestion, but simply an absorption of the products of decomposition, as in the pitcher-plant. In the nearly related land form, Pinguicula, however, there is much the same arrangement as in the sundew.
The family Gesneraceæ is mainly a tropical one, represented in the greenhouses by the magnificent Gloxinia and Achimenes, but of native plants there are only a few parasitic forms destitute of chlorophyll and with small, inconspicuous flowers. The commonest of these is Epiphegus, a much-branched, brownish plant, common in autumn about the roots of beech-trees upon which it is parasitic, and whence it derives its common name, “beech-drops.”
The bignonia family (Bignoniaceæ) is mainly tropical, but in our southern states is represented by the showy trumpet-creeper (Tecoma) (Fig. 121, A), the catalpa, and Martynia.
The other plants likely to be met with by the student belong either to the Verbenaceæ, represented by the showy verbenas of the gardens, and our much less showy wild vervains, also belonging to the genus Verbena (Fig. 121, E); or to the plantain family (Plantagineæ), of which the various species of plantain (Plantago) are familiar to every one (Fig. 121, G, I). The latter seem to be forms in which the flowers have become inconspicuous, and are wind fertilized, while probably all of its showy-flowered relatives are dependent on insects for fertilization.
The third order (Contortæ) of the Anisocarpæ includes five families, all represented by familiar forms. The first, the olive family (Oleaceæ), besides the olive, contains the lilac and jasmine among cultivated plants, and the various species of ash (Fraxinus), and the pretty fringe-tree (Chionanthus) (Fig. 122, A), often cultivated for its abundant white flowers. The other families are the Gentianaceæ including the true gentians (Gentiana) (Fig. 122, F), the buck-bean (Menyanthes), the centauries (Erythræa and Sabbatia), and several other less familiar genera; Loganiaceæ, with the pink-root (Spigelia) (Fig. 122, D), as the best-known example; Apocynaceæ including the dog-bane (Apocynum) (Fig. 122, H), and in the gardens the oleander and periwinkle (Vinca).
The last family is the milk-weeds (Asclepiadaceæ), which have extremely complicated flowers. Our numerous milk-weeds (Fig. 122, K) are familiar representatives, and exhibit perfectly the peculiarities of the family. Like the dog-banes, the plants contain a milky juice which is often poisonous. Besides the true milk-weeds (Asclepias), there are several other genera within the United States, but mostly southern in their distribution. Many of them are twining plants and occasionally cultivated for their showy flowers. Of the cultivated forms, the wax-plant (Hoya), and Physianthus are the commonest.
The fourth order (Campanulinæ) also embraces five families, but of these only three are represented among our wild plants. The bell-flowers (Campanula) (Fig. 123, A, D) are examples of the family Campanulaceæ, and numerous species are common, both wild and cultivated.
The various species of Lobelia, of which the splendid cardinal-flower (L. Cardinalis) (Fig. 123, E) is one of the most beautiful, represent the very characteristic family Lobeliaceæ. Their milky juice contains more or less marked poisonous properties. The last family of the order is the gourd family (Cucurbitaceæ), represented by a few wild species, but best known by the many cultivated varieties of melons, cucumbers, squashes, etc. They are climbing or running plants, and provided with tendrils. The flowers are usually unisexual, sometimes diœcious, but oftener monœcious (Fig. 123, I).
The last and highest order of the Sympetalæ, and hence of the dicotyledons, is known as Aggregatæ, from the tendency to have the flowers densely crowded into a head, which not infrequently is closely surrounded by bracts so that the whole inflorescence resembles a single flower. There are six families, five of which have common representatives, but the last family (Calycereæ) has no members within our limits.
The lower members of the order, e.g. various Rubiaceæ (Fig. 124, A, E), have the flowers in loose inflorescences, but as we examine the higher families, the tendency for the flowers to become crowded becomes more and more evident, and in the highest of our native forms Dipsaceæ (Fig. 124, P) and Compositæ (Fig. 125) this is very marked indeed. In the latter family, which is by far the largest of all the angiosperms, including about ten thousand species, the differentiation is carried still further. Among our native Compositæ there are three well-marked types. The first of these may be represented by the thistles (Fig. 125, A). The so-called flower of the thistle is in reality a close head of small, tubular flowers (Fig. 125, C), each perfect in all respects, having an inferior one-celled ovary, five stamens with the anthers united, and a five-parted corolla. The sepals (here called the “pappus”) (p) have the form of fine hairs. These little flowers are attached to the enlarged upper end of the flower stalk (receptacle, r), and are surrounded by closely overlapping bracts or scale leaves which look like a calyx; the flowers, on superficial examination, appear as single petals. In other forms like the daisy and may-weed (Fig. 125, F), only the central flowers are perfect, and the edge of the inflorescence is composed of flowers whose corollas are split and flattened out, but the stamens and sometimes the pistils are wanting in these so-called “ray-flowers.” In the third group, of which the dandelion (Fig. 125, H), chicory, lettuce, etc., are examples, all of the flowers have strap-shaped, split corollas, and contain both stamens and pistils.
The families of the Aggregatæ are the following: I. Rubiaceæ of which Houstonia (Fig. 124, A), Galium (E), Cephalanthus (button-bush), and Mitchella (partridge-berry) are examples; II. Caprifoliaceæ, containing the honeysuckles (Lonicera) (Fig. 124, I), Viburnum (G), snowberry (Symphoricarpus), and elder (Sambucus); III. Valerianeæ, represented by the common valerian (Valeriana) (Fig. 124, L); IV. Dipsaceæ, of which the teasel (Dipsacus) (Fig. 124, P), is the type, and also species of scabious (Scabiosa); V. Compositæ to which the innumerable, so-called compound flowers, asters, golden-rods, daisies, sunflowers, etc. belong; VI. Calycereæ.
Besides the groups already mentioned, there are several families of dicotyledons whose affinities are very doubtful. They are largely parasitic, e.g. mistletoe; or water plants, as the horned pond-weed (Ceratophyllum). One family, the Aristolochiaceæ, represented by the curious “Dutchman’s pipe” (Aristolochia sipho), a woody twiner with very large leaves, and the common wild ginger (Asarum) (Fig. 126), do not appear to be in any wise parasitic, but the structure of their curious flowers differs widely from any other group of plants.