Wishbone Bush

When the heat of the day is over and the morning-glories are folding together their faded chalices, the bright little four o’clock begin to open their myriad magenta-colored eyes upon the closing day, and they, together with the evening primroses, will keep the vigils of the night. ³⁹⁹

This magenta-eyed night-watcher described in 1897 is wishbone bush (Mirabilis laevis), a low, native perennial in the four o’clock family. As the family name suggests, wishbone bush blooms at night into early morning.

The common name comes from the repeated “V” shaped branching structure; each juncture is the shape of a wishbone.

Description ^{4,23,26,34,59,306}

Wishbone Bush is a small, sometimes vine-like, perennial that is usually less than three feet (1 m) high and somewhat woody at the base. Stems are numerous, brittle and repeatedly branched. At each juncture, the main stem bends to one side and the lateral branch mirrors that angle on the other side, forming “V”s along the stem like a series of wishbones. Leaves are opposite, rounded to cordate, with smooth margins, usually one inch (2.5 cm) or less in length. Leaves and stems are covered with glandular hairs which make them soft and somewhat sticky to touch.

Wishbone bush blooms primarily at night; flowers open mid to late afternoon and close again the following morning. One to several flowers appear in clusters at the ends of lateral branches, usually opening one at a time. What appears to be a normal flower actually lacks petals. The bright magenta “petals” are actually colored sepals while the base of a flower, the involucre, consists of five bracts that look like sepals. Bracts are fused at the base, with five green lobes that are edged with purple. The flowers are radial, 1/2-1 inch (1.2-2.2 cm) across. In the Reserve, the five petal-like sepals are bright pink to magenta, but pale pink and white forms occur elsewhere. Sepals are fused at the base, expanding into five lobes that are themselves bi-lobed at the ends. The throat of the flower is green, surrounded by a dark magenta band. There are five stamens with slim, pale filaments that extend beyond the sepals; anthers are bright yellow. The pistil has a one-chambered, superior ovary that is surrounded by the tissue of the flower base; the style extends beyond the anthers and bears a hemispherical purple stigma. Wishbone bush blooms between Dec. and June,¹ although the peak bloom in the Reserve is usually March and April.

The fruit is a one-seeded, spherical, dry capsule partially enclosed in the tube of the fused bracts and the hardened base of the flower (a”diclesium” or “anthocarp”). The old sepals collapse on top of the ovary and persist on the developing fruit like a little beanie. The fruit is mottled green when young, aging to dull, dark brown which is sometimes marked with pale stripes.

Classification ^2,44,306

Wishbone bush is a dicot angiosperm in the four-o’clock family (Nyctaginaceae). This is a small family but classification within the family is difficult and several taxonomic systems have been proposed. Members of the four-o’clock family lack petals. The sepals are united into a tube and are often brightly colored, taking the role of petals; modified leaves, or bracts, are often associated with a flower cluster. The structure of the fruit is unique to this family, and the seed may produce mucilage when wetted.

The best-known member of the four-o’clock family is the ornamental Bougainvillea, in which the bracts are the most showy part of the flower and surround a small, white, calyx.

The genus Mirabilis is the largest genus in the family, with ten species in California. These are set apart from other genera in this family in part by the capitate stigma and in part by characteristics of the bracts below each flower (involucral bracts).

The systematics of this species is difficult. Currently, the species is an amalgam of poorly differentiated forms, some of which were previously described as individual species. The wishbone bush in the Reserve is var. crassifolia.⁴⁸ It was named for its thick leaves and is distinguished by the magenta color of the petals. Variety crassifolia is most common in coastal southern California.

Ecology  

Wishbone bush is a member of a small subset of flowering plants that bloom mainly at night.^2,11,23,34 Other nocturnal flowers in the Reserve include Hooker’s evening primrose (Oenothera elata) and sacred Datura (Datura wrightii). The wishbone bush opens during mid to late afternoon and closes again the following morning, depending on what time the sun emerges. It is primarily pollinated by night-flying insects. The white-lined sphinx moth (Hyles lineata), a common sphinx moth in the Reserve, has been specifically mentioned.⁴⁰¹ Diurnal insects and hummingbirds also pollinate wishbone bush to some extent.⁴⁰²

NOTE: My (EV) first encounter with this plant occurred in Holmwood Canyon on an overcast morning while I was with the monthly bird count team. Just above the eucalyptus was an eye-catching display of bright magenta flowers. Similar magenta patches could be seen on the west-facing hill above the trail. Anxious to identify and photograph this plant, I returned as soon as possible, shortly after lunch on a sunny day three days later. There was not a single magenta flower in sight!

Human Uses  

The Kumeyaay made a tea from “the roots, flower and the whole plant” to drink for a stomach ache,¹⁶ while the Luisaño made an infusion for a purgative.¹⁷ (NOTE: one source ³⁴ reports the root and seeds are toxic). The numerous systematic changes make it difficult to accurately determine many historical uses of this plant.⁴⁰⁰

Interesting Facts ⁴¹

Botanists tell us that flowers in the four o’clock family have no petals and have sepals that look like petals and bracts that look like sepals. But how do they know that such flowers aren’t like many flowers having sepals that look like sepals and petals that look like petals – and no bracts?

This is a subject that very quickly gets complicated and way over my head. Basically, evolutionary developmental biologists have identified three small groups of genes (“A”, “B”, and “C”) that regulate the development of the four flower components: sepals, petals, stamens and pistils. Expression of group A characterizes sepals while a combination of groups A and C are necessary for petals.By determining which group or combination of groups is active, scientists can identify the structures of a flower.

Understand? I’m not sure I do. But at least I am satisfied that there is a scientific basis behind this determination.