Torrey Pines Docent Society 2018, rev. 2019 Notes on the Wild Cucumber The wild cucumber (Marah macrocarpa, meaning a bitter big fruit) is an attention grabbing plant. After a long dry season, this plant is among the very first to grow back quickly after light rains in late fall. The following spring, the plant produces large, prickly fruits that are a good conversation piece. These notes expand on the Margaret Fillius plant highlights column in the February 2016 issue of the Torreyana. In the latter half, the notes will cover the fish-stunning experiment led by Christina Bjenning in the November 2005 issue of Torreyana. Key points: • The wild cucumber is well adapted to the short, wet winter in the Mediterranean climate. With food stored in the tuber (the storage root), it can start growing quickly after the onset of a rainy season, and the plant returns to a dormant state underground soon after. In fact, the plant spends most of the year underground. • The plant has separate male and female flowers. The male flowers are the smaller ones clustered along a stalk. The female flowers appear by themselves, and even when very young, one can observe a small swelling, the ovary, behind the flower. • The seeds are huge, as one might guess from the size of the compartments in a dried fruit. The seed germination is adapted to self-planting a tuber deep underground without sunlight. Thus a large food reserve is needed. • The seed dispersal is by rodents with underground burrows. By being hidden underground, the seeds are well protected from wildfires in the dry season. • The seed pods split by uneven drying of the skin, but the molecular biology underlying the drying is complicated. • A tendril tightens its grip by coiling. The coiling is attained by uneven contraction within a tendril, and the contraction is generated by a lignified cell layer. One often sees two coils on a tendril—a tendril starts coiling up separately from each respective end. Charles Darwin had studied the phenomenon in detail. • The entire plant is bitter because it contains saponins, a soapy substance. Chemically, the saponins in the wild cucumber contain cucurbitacins, which can have various medicinal properties. (But too much of these chemicals is toxic.) • Native American groups along the California coast had many uses for plants in the same genus as the wild cucumber. Notable uses are in stunning fish and body or face painting using roasted seeds. A really interesting use is in the treatment of men’s hair loss. 1 What is so unique about the wild cucumber? Charles Darwin studied a wild cucumber, even though it is not exactly the same species as what we have locally. Asa Gray, the renowned American botanist, mailed Darwin some seeds of Echinocystis lobata, which is found east of California, and also called the wild cucumber. Indeed, E. lobata, now classified in its own monotypic genus, shares many general characteristics with our local species, Marah macrocarpa. Darwin studied the E. lobata, and wrote about the unique germination features and the intriguing tendril coiling. The wild cucumber is a geophyte—a plant that spends most of its time underground, and these plants often have an underground storage organ. With the wild cucumber, the aboveground structures die off, and the plant stays as an underground tuber most of the year. However, it can sprout the vines quickly at the onset of a rainy season in late fall or early winter. Pretty white flowers and interesting spiky fruits develop quickly afterward. General characteristics of the wild cucumber The common name “wild cucumber” is used by various sources for several related Marah species, not to mention also for Echinocystis lobata. So is the other common name, manroot, for the enormous tuber or storage root. A large one can weigh well over 200 pounds. Marah macrocarpa, the common species in San Diego, has other common names, including Cucamonga manroot, the Chilicothe, and the bigroot. The genus name Marah means bitter, a reference to the bitter waters of a fountain in Marah in the Bible. The species name macrocarpa means large fruit. So what we have is a bitter big fruit, but the entire plant is bitter. All parts of the plant have various quantities of a bitter chemical (more on this later). The Marah genus, native to Western North America, is in the gourd family, Cucurbitaceae. Plants in this family that are familiar to us include squash, pumpkin, zucchini, watermelon, and of course, the common cucumber (Cucumis sativus). Most of the gourd plants are vines. They are either monoecious or dioecious, meaning that they have separate male and female flowers either on the same plant or on different plants. Their flowers are funnel-shaped with generally 5 petals. The female flowers have inferior ovaries, and the fused carpels are separated by septa into compartments (locules). There are at least five species of Marah in California. The most notable and studied ones are Marah oregana and Marah fabacea. Their natural ranges are north of San Diego County, especially M. oregana as suggested by its species name. The range of Marah macrocarpa is concentrated in Southern California, and this species is particularly well adapted to a long, dry summer. Where ranges of different Marah species overlap, they can hybridize. The vines of Marah macrocarpa can reach well over 20 feet (6 m). Spaced intermittently along a stem are nodes. At each node one finds the petiole of a leaf, a short pedicel of a single female flower, a longer peduncle leading to a raceme of male flowers, 2 and a small branch leading toward a tendril. If this tendril-bearing branch keeps growing long enough in search of an anchor, it may bear small leaves and also branch off to make multiple tendrils. The number of male flowers in an inflorescence increases with time into the season. There may be three to five at the beginning, and eventually increasing to well over a dozen. The Marah species is monoecious. Both the male and female flowers have five white petals that together take a shallow cup shape (cupulate) and the appearance of a starfish. The petals bear white, soft hairs (pubescence). At the center of the smaller male (staminate) flowers, about 2/3-inch (17 mm) in diameter, is a ring of fuzzy yellow anthers. The female (pistillate) flower, about 1¼-inch (32 mm) in diameter, has a yellowish, cushion-like stigma at the top of a compound pistil from fused carpels. A small, spiky inferior ovary is already evident when the flower is very young. The sepals are fused in both the male and female flowers. (See photos in Fillius 4th edition, p. 167.) The wild cucumber is pollinated by insects, but the plant is self-fertile, meaning that pollens from the male flowers can fertilize the female flowers on the same plant. The leaves are palmately lobed with generally five irregular lobes, and the two lobes near the base have smaller sub-lobes. The main veins are also palmate, but the secondary venation is cross-venulate within a lobe. The leaves on a given plant can exhibit large variability in shape and size. The fruit is a pepo. Even though it is not as fleshy as other gourd plants, the fruit is made from a fused carpel and has a tough skin or rind. The fruit is oblong, and easily over 4 inches (100 mm) long and 3.5 inches (90 mm) in diameter. The fruit capsule bears spines that are soft when young, but they stiffen up quickly. The spines are of uneven lengths, with short bristles (spinules) interspersed among some long ones over an inch in length. Inside the fruit are four compartments separated by septa. Each chamber can hold as many as six seeds. The number varies. As many as 20 seeds from one fruit have been observed. The oblong-elliptic seeds are hard, and large, easily 0.6 inch (15 mm) in length. The seed is olive green or dark tan, and turns to a dark gray with time after the fruit has split. The Western scrub jay is known to eat the seeds, but the seed dispersal is carried out mainly by burrow-dwelling rodents such as deer mice, kangaroo rats, and especially the bigger ground squirrels. In an experiment by Borchert (2006), he found that about 90% of the seeds were moved to underground burrows. Because the seeds are below ground, they are protected from wildfires, and this way, the wild cucumber is well adapted to the fire ecology of the chaparral habitat. 3 Fig. 1. Germination photos 12 and 13 of Marah oreganus (the old synonym of Marah oregana) from Schlising (1969). Photo 12 on the left shows a seed (S) germinating several inches underground. The petiole tube (P) grows from the seed and extends deeper into the soil, being led by the hypocotyl (H) and the radicle (R). Photo 13 on the right shows that roughly six inches farther down from the seed, the root forms a small tuber (T). From the tuber, the epicotyl (E) grows back up in the petiole tube and eventually breaks through and continues toward the soil surface. Fig. 2. Epicotyl growth photos 10 and 11 of Marah oreganus from Schlising (1969). Photo 10 on the left shows a young epicotyl (E) in the middle of the petiole tube, the radicle (R) below it, and in between them a developing tuber. (The labels E and R are not very clear in this old photo.) Photo 11 on the right shows an older epicotyl continuing its growth up the petiole tube, above a slightly bigger tuber above the radicle.