Wigandia At Palomar College

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Unusual Plant At Palomar College

Plants With Stinging Trichomes

Waterleaf, Euphorbia & Nettle Families

Waterleaf Family (Hydrophyllaceae)

About ten years ago, a small specimen of Wigandia caracasana was planted by Mr. Wolffia in an unlandscaped area between the Life Sciences Greenhouse and the Staff Building at Palomar College. The hard, sun-baked ground was devoid of topsoil and it was doubtful that this little plant could survive in its new home. But to the astonishment of the Life Sciences staff, survive it did. In fact, it has flourished in this barren area and has rapidly grown into a massive, multi-trunked shrub over 25 feet (8 m) tall, with huge, broad leaves over 2 feet (0.6 m) in length. Although the New York Botanical Garden Encyclopedia of Horticulture (Volume 10, 1982) states that wigandias need "rich, porous soil kept constantly moist," these conditions have not been entirely met at the Palomar College location. In fact, the only available moisture has come from spring rains, with months of drought throughout the summer and fall. The original plant grows adjacent to a concrete ramp (walkway), and it may be that the ground beneath this massive concrete structure remains moist for longer periods of time. But the visible ground around these plants becomes cracked and dry during summer and fall, and is so hard and sun-baked that it is difficult to penetrate with a shovel. During recent years, this plant has received a lot of attention by curious staff members who walk by this massive plant each day. Everyone is amazed by its rapid growth, its tropical-appearing foliage, and its profusion of violet blossoms during spring. [In fact, some people are so fascinated with this plant that they have transplanted small root sprouts to their own gardens.] Whether this plant is a giant weed or a cultivated shrub is up for speculation. But one thing for sure, it is certainly one of the most striking and unusual plants on campus, surviving with practically no care in an area where most other cultivated plants would perish.

Wigandia caracasana near the Staff Building at Palomar College. Approximately ten years ago, this was a small plant in a five gallon container. Today it has grown and spread into a massive, multi-trunked shrub with huge leaves reminiscent of plants from the rain-soaked New World tropics.

Several species of Wigandia are native to Mexico, Central and South America. They are easily recognized along roadsides, with their very large leaves and glistening hairs (trichomes). They belong to the waterleaf family (Hydrophyllaceae) along with several genera of native California wildflowers, including Phacelia and Turricula. The purplish or violet flowers are produced in coiled (scorpioid) clusters and contain five petals which are fused into a bell-shaped or rotate corolla. The attractive bell-shaped corollas fall from the inflorescences intact, with five stamens inserted between the lobes. Wigandias can be rather invasive and need plenty of room. They spread by underground shoots that can literally break through hard, compact ground and asphalt. In the Life Sciences area, it appears to be spreading vegetatively by underground shoots (adventitious stems) and not by germinated seedlings. Wigandia caracasana is also established in the Palomar College Arboretum.

Left: Flowering branch of Wigandia caracasana in the Palomar College Arboretum. Note the dense, glistening hairs (trichomes) on the stem. Right: Detached flowers (corollas) of Wigandia in the Life Sciences area. The petals are united (connate) into a broad, bell-shaped corolla with five spreading lobes (petals) and with five stamens attached to the inner corolla tube, one stamen inserted between each lobe. The middle corolla is inverted to show that the petals are completely fused into a tube.

Like our California phacelias and turriculas, the stiff hairs (trichomes) of wigandias can be irritating to sensitive skin. It is best not to handle wigandia plants without gloves. In fact, they are sometimes called "mala mujer" in Mexico; however, they are not nearly as painful or "bad" as the "mala mujeres" of the euphorbia family (Euphorbiaceae), including species of Cnidoscolus. [See photos of the true mala mujer below.] Like prickly pear and cholla cacti, the general rule of "look but don't touch" also applies to the hairy stems of wigandia. As long as wigandias are pruned back and viewed from a distance, they pose no serious threat to people. They are truly remarkable shrubs with foliage reminiscent of the rain-soaked New World tropics.

Two species of post-burn (fire-follower) wildflowers in southern California. They belong to the waterleaf family (Hydrophyllaceae) and their floral structure is similar to Wigandia. Left: Wild canterbury-bell (Phacelia minor). Right: Parry phacelia (P. parryi). The seeds of these species often germinate during the rainy months of winter and early spring, following brush fires of the previous year. During brush fires of the summer and fall drought season, the dense chaparral and coastal sage scrub is reduced to rich, ashy soil, the perfect germination requirements for these species. By late spring, entire hillsides become a profusion of violet blossoms, as these wildflowers produce a new crop of seeds for the next brush fire.

Large-flowered phacelia (Phacelia grandiflora), a conspicuous post-burn wildflower in northern San Diego County. It is especially abundant in burned or cleared coastal sage scrub and chaparral.

See Article About Ashes To Wildflowers

Euphorbia Family (Euphorbiaceae)

The undisputed "mala mujer" of Baja California, southern Arizona and mainland Mexico is Cnidoscolus angustidens, a member of the large and diverse euphorbia family (Euphorbiaceae). This species is a herbaceous plant with broad, palmately-lobed leaves. The stems and leaves are covered with stinging trichomes that readily penetrate leather gloves and discourage the most avid plant collector. Species in the related genus Tragia inject a painful crystal of calcium oxalate into the skin. Like nettles of the genus Urtica (Urticaceae), the trichomes of mala mujer are sharp, silicified epidermal cells with a bulbous base embedded in a stalk (pedestal) of smaller epidermal cells.

The true "mala mujer" of Baja California, southern Arizona and mainland Mexico (Cnidoscolus angustidens). The stems and leaves of this species are literally covered with stinging hairs called trichomes, which undoubtedly discourage herbivores and avid botanical collectors. The magnified (400x) trichomes shown in right photo are modified epidermal cells embedded in a pedestal-like base composed of smaller epidermal cells.

Stinging trichome of Tragia volubilis, a twining South American vine in the Euphorbiaceae. The stinging mechanism of Tragia is unusual because the calcium carbonate crystal is ejected from the trichome upon contact, where it pierces and enters the skin. The intial stinging sensation is mechanical; however, a secondary stinging sensation is caused by the crystal within the skin.

Read About Stinging Hairs Of Mucuna Pods In Tropical Rain Forest

Nettle Family (Urticaceae)

The trichomes of true nettles in the genera Urtica and Urera inject your skin with several stinging chemicals, including histamine, acetylcholine and 5-hydroxytryptamine. Some tropical nettles can be absolutely excruciating and cause numbness that may persist for weeks. There are reports of the incapacitation and death of horses from the Australian bush nettle Dendrocnide moroides. In general, the Australian bush and tree nettles of the genus Dendrocnide appear to be considerably more potent than the herbaceous North American nettles (Urtica and Urera). The chemical mechanism responsible for the extreme pain from contact with these tree nettles is apparently different from North American nettles. There are unconfirmed reports of human fatalities from a nettle called "devil-leaf" (Laportea or Dendrocnide) in Papua, New Guinea.

The stinging hair of the common North American nettle (Urtica dioica) is actually a sharp-pointed cell called a trichome. This nucleated cell is embedded in a pedestal-like base composed of smaller epidermal cells. The slender shaft of the trichome is composed of silica, and the rounded apex breaks off with the slightest touch creating a sharp, beveled tip similar to a hypodermic needle. The hollow trichome readily penetrates the skin and toxin from the enlarged, bulbous base is injected into the skin tissue. The stinging toxin from this species of nettle is a combination of chemicals, including histamine, acetylcholine and 5-hydroxytryptamine.

Left: Young leaf of the dwarf nettle Hesperocnide tenella, a native herb in the coastal sage scrub and chaparral of southern California. Right: Close-up view of the stem of Hesperocnide tenella showing several trichomes with bulbous bases.

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