RUBIACEAE (Madder Family) Contributed by George Yatskievych and Charlotte M. Taylor

Plants annual or perennial herbs, shrubs, or small trees. Leaves opposite or whorled. Stipules present or absent (the leaves then whorled), when present often interpetiolar (see discussion below), persistent or shed early, of various shapes, most often small and triangular, but frequently bearing 2 to several marginal lobes or bristles. Leaf blades simple, entire. Inflorescences terminal or axillary, of solitary flowers or more commonly clusters or panicles of several to numerous flowers, dense, globose, and headlike in Cephalanthus. Flowers perfect (occasionally some of the flowers imperfect in Galium), epigynous or nearly so (occasionally with the tip of the ovary slightly protruding above the fused portion of the calyx), actinomorphic, sometimes subtended by bracts. Calyces 4- or 5-lobed or sometimes with an entire margin, occasionally reduced to 2 or 3 lobes or apparently absent in Galium. Corollas (3)4- or 5-lobed, saucer-shaped to bowl-shaped, or funnelform (nearly cylindric in Cephalanthus), white, blue, purple, or pink, the lobes overlapping or not in bud. Stamens (3)4 or 5, alternating with the corolla lobes, the filaments attached in the corolla tube, the anthers attached near the midpoint or the base, exserted or not, variously colored. Style 1 or 2, the stigmas 1–2, of various forms, included or exserted. Pistil 1 per flower, of 2 fused carpels. Ovaries inferior or nearly so (occasionally with the tip of the ovary slightly protruding above the fused portion of the calyx in Houstonia and Oldenlandia), 2-locular, the placentation axile or occasionally nearly apical, the ovules 1 to numerous in each locule. Fruits capsules, schizocarps (with 2 indehiscent mericarps), achenelike (2-lobed and 2-seeded, but not splitting into mericarps or dehiscing), or fleshy and drupelike (the ovaries of 2 closely adjacent flowers becoming fused into a single drupelike fruit in Mitchella). Seeds small, variously shaped. About 563 genera, 10,000–11,000 species, worldwide.

The Rubiaceae are a well-delimited family, and can be recognized by the combination of their simple, entire, opposite (or whorled) leaves, interpetiolar stipules (in most genera; see below), corollas of fused petals, and inferior ovaries. The vast majority of the genera and species are shrubs and small trees of wet tropical regions. All our genera range into tropical regions, and Galium additionally is found to the northern and southern extremes, respectively, of the Americas, Eurasia, and Africa.

In spite of its large number of species (currently fourth in size among flowering plants), the family includes relatively few plants of economic importance. The principal commercially useful genera include Cinchona L., the source of the antimalarial drug quinine, and Coffea L., the source of the popular beverage coffee. The alkaloid caffeine, which acts as a psychoactive stimulant in the nervous system of humans, was first isolated and characterized chemically from Coffea (although it was later determined that the earlier-described compound theeine from tea leaves represents the same compound). Various species in other genera are used for their wood, medicinally, as dye plants, and in tanning leathers. A number of species are cultivated as ornamentals, mostly in warmer regions.

The interpetiolar stipules found in most opposite-leaved species of Rubiaceae are formed by fusion of adjacent stipules between the two leaves at each node and are unusual among the flowering plants and therefore distinctive. Stipules normally are produced in pairs, one on each side of the base of the petiole of an individual leaf; thus each leaf normally bears two stipules and the two stipules of each leaf are not fused to those of any other leaves. However, in many Rubiaceae, each stipule is fused to the stipule next to it that belongs to the other leaf at that node. This fused structure looks like a single stipule and is connected to the petioles of two different leaves. Consequently, it stretches between them across the stem node, thus its technical name interpetiolar. Interpetiolar stipules are rarely found in other families of flowering plants, but these other families can all be separated from the Rubiaceae in having leaf blades with toothed margins. In Missouri, aside from the Rubiaceae, interpetiolar stipules are found only in some members of the Urticaceae. In using this feature to diagnose the family, it should be noted that in several species of Rubiaceae the stipules are shed quickly, leaving only a scar in the form of a line between the petioles as a sign of their former presence. This becomes problematic, because several other plant families with opposite entire leaves lack interpetiolar stipules but do develop a low ridge or line that runs across the stem at each node and connects the leaves. Missouri plant families that include such species are Acanthaceae, Caprifoliaceae, Gentianaceae, and Loganiaceae. In such cases, botanists should take care to observe young growth where any stipules produced are still attached.

Another characteristic of many Rubiaceae, including some of the Missouri species of Houstonia, is the production of distylous (heterostylous) flowers. In such species, two different kinds of flowers are produced: a so-called pin flower that has a relatively long style and short stamens such that the stigma is positioned above the anthers; a so-called thrum flower that has a relatively short style and long stamens such that the stigma is positioned below the anthers. Each individual plant bears only one kind of flower. The different flower forms function to promote cross-pollination in the species, and additionally they restrict the plant to breeding with only about half of the other plants in the population. This is because even though they belong to the same species, flowers of the same form mostly cannot successfully cross-pollinate other flowers of the same form, for two reasons. First, because the pollen of a short-styled flower is deposited in a different position on the body of a visiting insect than that from a long-styled flower, so that when this insect visits another flower, the pollen will be transferred successfully to the stigma of only the other kind of flower. Additionally, Rubiaceae species with heterostylous flowers have been found to have a chemical incompatibility system that prevents the pollen of one type of flower from germinating on the stigmas of that same flower type. Thus, even accidental or artificial pollinations involving the same flower type are unsuccessful. Species that lack heterostyly (all flowers have stamens and styles of the same relative lengths) are called homostylous. At least half of the species in the Rubiaceae are distylous.