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Whirling Nut (Gyrocarpus)

Blowing In The Wind Seeds & Fruits Dispersed By Wind

Seeds provide the vital genetic link and dispersal agent between successive generations of plants. Angiosperm seeds are produced and packaged in botanical structures called fruits which develop from the "female" pistils of flowers. Immature seeds (called ovules) each contain a minute, single-celled egg enclosed within a 7-celled embryo sac. The haploid (1n) egg is fertilized by a haploid (1n) sperm resulting in a diploid (2n) zygote that divides by mitosis into a minute, multicellular embryo within the developing seed. A second sperm unites with 2 haploid polar nuclei inside a binucleate cell called the endosperm mother cell which divides into a mass of nutritive tissue inside the seed. In most seeds the embryo is embedded in this endosperm tissue which provides sustenance to the embryo during germination. In exalbuminous seeds (found in many plants such as the legumes), the endosperm tissue is already absorbed by the time you examine a mature seed within the pod, and the 2 white fleshy halves in the seed are really the cotyledons (components of the embryo). The 2 sperm involved in the double fertilization process originated within the pollen tube that penetrated the embryo sac. The pollen grain (and pollen tube) come from the "male" organs (called anthers) on the same plant or different parental plants in a remarkable process known as pollination. Pollination is also accomplished by the wind (or water), and it may also involve insects in some of nature's most fascinating relationships between a plant and an animal. This is especially true of the amazing fig trees and their symbiotic wasps.

One of the important functions of seeds and fruits is dispersal; a mechanism to establish the embryo-bearing seeds in a suitable place away from their parental plants. There are 3 main mechanisms for seed and fruit dispersal: (1) Hitchhiking on animals, (2) Drifting in ocean or fresh water, and (3) Floating in the wind. This article concerns one of the most remarkable of all seed dispersal methods, riding the wind and air currents of the world.

Gliders include seeds with 2 lateral wings that resemble the wings of an airplane. They become airborne when released from their fruit and sail through the air like a true glider. One of the best examples of this method is Alsomitra macrocarpa, a tropical vine in the Gourd Family (Cucurbitaceae) native to the Sunda Islands of the Malay Archipelago. Football-sized gourds hang from the vine high in the forest canopy, each packed with hundreds of winged seeds. The seeds have two papery, membranous wings, with combined wingspans of up to 5 inches (13 cm). They reportedly inspired the wing design of some early aircraft, gliders and kites. Although the seeds vary in shape, some of the most symmetrical ones superficially resemble the shape of the "flying wing" aircraft or a modern Stealth Bomber. According to Peter Loewer (Seeds: The Definitive Guide to Growing, History, and Lore, 1995), the aerodynamic seeds spiral downward in 20 foot (6 meter) circles, although a gust of wind would probably carry them much farther away.

Parachutes include seeds or achenes (one-seeded fruits) with an elevated, umbrella-like crown of intricately-branched hairs at the top, often produced in globose heads or puff-like clusters. The slightest gust of wind catches the elaborate crown of plumose hairs, raising and propelling the seed into the air like a parachute. This is the classic mechanism of dispersal for the Eurasian dandelion (Taraxacum officinale) and includes numerous weedy and native members of the Sunflower Family (Asteraceae). A giant Eurasian version of the dandelion called salsify or goat's beard (Tragopogon dubius), is one of the most successful wind-travelers in North America. Its seeds have literally blown across mountain ranges, colonizing vast fields of open land in the western United States. Three weedy species of salsify (T. dubius, T. pratensis and T. porrifolius) have been introduced into the western United States, 2 with yellow dandelion-type flowers and one with purple flowers. The latter, purple-flowered species (T. porrifolius) has a large, edible tap root with a flavor resembling oysters, hence the name "oyster plant."

In some parachutes, the crown of silky hairs arises directly from the top of the seed (not on an umbrella-like stalk). Again, the Sunflower Family (world's largest plant family with about 24,000 described species) contains many weedy representatives with this type of parachute seed. One of the most troublesome weeds of farm land in the western United States is wild or thistle artichoke (Cynara cardunculus). The large seed head of this weedy composite releases hundreds of parachute seeds which fly through the air and invade vast areas of grazing land with spiny, perennial bushes that literally take over. The large leaf stalks (resembling giant celery stalks) are edible and are sold under the name of "cardoon." Populations of wild artichoke often contain so much variation between spiny and non-spiny plants, that some experts believe that they belong to one variable species. In fact, some botanists believe that the cultivated artichoke (C. scolymus) may be a cultivated variety of the wild C. cardunculus. Incidentally, the delicious artichoke is really a cooked flower head in which the outer bracts (phyllaries) and central basal portion (receptacle) are dipped in butter and eaten.

Another plant family which has evolved this parachute method of seed dispersal is the Milkweed Family (Asclepiadaceae). Hundreds of parachute seeds (each with a tuft of silky hairs) are produced within large, inflated pods called follicles. So abundant are the silky hairs, that they were actually collected and used as a substitute for kapok during World War II. Kapok comes from masses of silky hairs that line the seed capsules of the kapok tree (Ceiba pentandra), an enormous rain forest tree of Central and South America. Kapok is used primarily as a waterproof filler for mattresses, pillows, upholstery, softballs, and especially for life preservers. The floss silk tree (Chorisia speciosa), another member of the Bombax Family (Bombaceae) also produces large seed capsules lined with masses of silky hairs. This tree with its distinctive thorny trunk and showy pink flowers is commonly planted in southern California. The seeds of kapok and floss silk trees are embedded in these silky masses which aid in their dispersal by wind; however they probably belong in Section 5 below (Cottony Seeds & Fruits).

The Dogbane Family (Apocynaceae) also includes members with seed pods (follicles) and parachute seeds similar to those of milkweeds. One of the best examples is Nerium oleander, a drought-resistant, Mediterranean shrub planted throughout southern California. The foliage contains a powerful cardiac glycoside that can permanently relax the heart muscle.

Helicopters (also called Whirlybirds) include seeds or one-seeded fruits (samaras) with a rigid or membranous wing at one end. The wing typically has a slight pitch (like a propeller or fan blade), causing the seed to spin as it falls. Depending on the wind velocity and distance above the ground, helicopter seeds can be carried considerable distances away from the parent plant. The spinning action is similar to auto-rotation in helicopters, when a helicopter "slowly" descends after a power loss.

Numerous species of flowering trees and shrubs in many diverse and unrelated plant families have evolved this ingenious method of seed dispersal, good examples of convergent evolution. Representative examples of helicopter seeds and one-seeded fruits (called samaras) include the Maple Family (Aceraceae): Maples and box elder (Acer); Olive Family (Oleaceae): Ash (Fraxinus); Legume Family (Fabaceae): Tipu tree (Tipuana tipu); and the Protea Family (Proteaceae): Banksia and Hakea.

Although they are classified as gymnosperms with naked seeds arising from woody cones rather than flowers, the Pine Family (Pinaceae) contains many genera with winged seeds, including Pinus (Pine), Abies (fir), Picea (spruce), Tsuga (hemlock), and many additional genera. When shed from cones high on upper branches, they fly over slopes and across deep canyons. The natural reforestation of conifers following fire is proof of the flying ability of seeds from nearby forested slopes.

Maples have a double or twin samara composed of 2 winged one-seeded fruits (double samara) joined together at their bases. When they break apart, each winged fruit flies like a typical helicopter seed. Although the Legume Family (Fabaceae) is the third largest plant family with over 18,000 described species, the vast majority of legumes do not have winged seeds or fruits. The South American tipu tree (Tipuana tipu) is a notable exception, with beautiful yellow blossoms that give rise to pendant, samara-like legumes, each with a large wing on the lower end. The dried, winged legumes spin so neatly in the air that they could be marketed as a child's toy.

The remarkable Protea Family (Proteaceae) of Australia contains some truly amazing genera with winged seeds, including Banksia and Hakea. Although they are flowering plants, banksias produce a dense flower cluster (inflorescence) that gives rise to a cone-like structure containing many woody carpels. Each carpel bears 2 winged seeds and the entire cone-like structure superficially resembles a pine cone. In fact, some banksias release their seeds following fire and even resprout from subterranean lignotubers like chaparral shrubs.

Although their mode of dispersal is similar to single-winged helicopter seeds, the flutterer/spinners include seeds with a papery wing around the entire seed or at each end. When released from their seed capsules they flutter or spin through the air. Whether they spin or merely flutter depends on the size, shape and pitch of the wings, and the wind velocity. This method of wind dispersal is found in numerous species of flowering plants in many different plant families. Some examples of flutterer/spinner seeds include the Quassia Family (Simaroubaceae): Tree of heaven (Ailanthus altissima); Figwort Family (Scrophulariaceae): Empress tree (Paulownia tomentosa); Bignonia Family (Bignoniaceae): Jacaranda (Jacaranda mimosifolia), catalpa (Catalpa speciosa), desert willow (Chilopsis linearis), yellow bells (Tecoma stans), bower vine (Pandorea jasminoides), violet trumpet vine (Clytostoma callistegioides), and the fabulous trumpet trees (Tabebuia serratifolia and T. ipe); Elm Family (Ulmaceae): American and Chinese elms (Ulmus americana and U. parvifolia); Soapberry Family (Sapindaceae): Hop seed (Dodonea viscosa); and the Goosefoot Family (Chenopodiaceae): Four-wing saltbush (Atriplex canescens).

Any discussion of flutterer/spinners would not be complete without mentioning the quipo tree (Cavanillesia platanifolia), a massive rain forest tree in the bombax family (Bombacaeae) native to Panama. The enormous winged fruits of the quipo tree flutter through the air, carpeting the ground beneath the huge canopy of this striking tropical tree.

Some of the most beautiful flowering trees of the New World tropics belong to the Bignonia Family (Bignoniaceae). They typically produce long, slender (cigar-shaped) seed capsules containing masses of flat seeds with papery wings at each end. [The beautiful jacaranda of Argentina has flattened, circular seed capsules.] The lovely yellow bells (Tecoma stans) is native to Mexico and the Caribbean region, and is the official flower of the U.S. Virgin Islands. Some of the South American trumpet trees, including the pink-flowered Tabebuia avellanedae (listed as T. ipe in some references) and the yellow-flowered Tabebuia serratifolia, are also called ironwoods or axe-breakers (quebrachos) because of their dense, hard wood. The latter species is called "pau d'arco" and its wood actually sinks in water, with a specific gravity of 1.20. In South America, trumpet trees drop their leaves during the dry season and produce a profusion of pink or yellow blossoms. The crowns of these huge timber trees resemble gigantic floral bouquets in the midst of the forest. As with so many tropical species, some of the trumpet trees inhabit rain forest areas that are seriously threatened by slash and burn agriculture, large plantations of exportable products, and the general annihilation of the South American rain forests.

Other South American species of Tabebuia are also referred to as pau d'arco, including the pink-flowered T. impetiginosa and T. avellanedae. According to The New York Botanical Garden Encyclopedia of Horticulture Volume 10, 1982, T. avellanedae is a synonym for T. impetiginosa, and T. ipe " is so closely similar to T. impetiginosa that it can scarcely be more than a variety of that species." These attractive pink-flowered species are commonly used as landscape trees in temperate regions.

The powdered inner bark of these pink-flowered species of pau d'arco is sold as a popular herbal remedy that reportedly stimulates the immune system. According to a book by Kenneth Jones (Pau d'Arco: Immune Power From the Rain Forest, Healing Arts Press, 1995), this valuable herb has been proven successful in the treatment of certain cancers, allergies associated with the Candida yeast syndrome, and in disorders involving a weakened immune system.

Cottony seeds and fruits include seeds and minute seed capsules with a tuft (coma) of cottony hairs at one end, or seeds embedded in a cottony mass. Some of the examples in this group are very similar in function to parachute seeds, but probably are not carried as far by the wind. Many plant families have this type of wind dispersal, including the Willow Family (Salicaceae): Willows (Salix) and Cottonwoods (Populus); Cattail Family (Typhaceae): Cattails (Typha); Evening Primrose Family (Onagraceae): Willow-Herb (Epilobium) and California fuchsia (Zauschneria); Bombax Family (Bombaceae): Kapok tree (Ceiba pentandra) and floss silk tree (Chorisia speciosa); and the Sycamore Family (Platanaceae): Sycamore (Platanus).

In the California sycamore (Platanus racemosa), a common riparian (streamside) tree throughout the state, the one-seeded fruits (achenes or nutlets) are produced in dense, globose heads. The spherical heads hang from branches like little balls. Individual achenes have a tuft of hairs at the base which probably helps in their wind dispersal. Seeds of the South American kapok tree (Ceiba pentandra) and floss silk tree (Chorisia speciosa) are embedded in dense masses of silky hairs inside large woody capsules. This undoubtedly helps to disperse the seeds when seed-bearing masses of hair are carried by the wind. In tropical regions of the New World, the kapok grows into an enormous rain forest tree with a massive buttressed trunk. Kapok hairs are coated with a highly water-resistant, waxy cutin layer. The empty lumen (cavity) inside each hair is larger the cotton hairs; hence, the hairs are lighter. Unlike cotton hairs, kapok is difficult to spin and is not made into textiles. It is used primarily as a waterproof filler for mattresses, pillows, upholstery, softballs, and especially for life preservers. A kapok-filled life jacket can support 30 times its own weight in water.

One fuzzy brown cattail spike may contain a million tiny seeds. Each seed has a tuft of silky white hairs and is small enough to pass through the "eye" of an ordinary sewing needle. They are shed in clouds of white fluff and float through the air like miniature parachutes. A cattail marsh covering one acre may produce a trillion seeds, more than 200 times the number of people in the world. The fluffy seeds have been used for waterproof insulation and the buoyant filling of life jackets. In addition, each plant produces billions of wind-borne pollen grains; in fact, so much pollen that it was used as flour by North American Indians and made into bread. Cottonwoods and willows also produce masses of seeds, each with a tuft of soft, white hairs. Since they are dioecious, with pollen-bearing male and seed-bearing female trees in the population, only female trees produce the actual cotton. During late spring and summer in the western United States, the cottony fluff from cottonwoods resembles newly fallen snow. Because the wind-blown fluff can be quite messy in cultivated parks and gardens, male trees are generally planted. The discriminatory label of "cottonless cottonwood" refers to a male tree.

The common tumbleweed or Russian thistle is a rounded, bushy annual introduced into the western United States from the plains of southeastern Russia and western Siberia in the late 1800s. The name "thistle" comes from the stiff, sharp-pointed, awl-shaped leaves. Although it is depicted in songs of the old west, this species is a naturalized weed in North America. It is listed in most older references as Salsola kali or S. pestifer; however, the Jepson Flora of California (1993) lists it as S. tragus. Russian thistle belongs to the goosefoot family (Chenopodiaceae), along with many weedy species and some valuable vegetables, including beets (Beta vulgaris), goosefoot (Chenopodium album) and spinach (Spinacia oleracea).

Tumbleweed is a prolific seeder and rapid seed germination and seedling establishment occurs after only a brief and limited rainy season. A single plant may produce 20,000 to 50,000 seeds within numerous small fruits, each surrounded by a circular, papery border. Mature plants readily break off at the ground level and are pushed along by strong gusts of wind. As they roll along hillsides and valleys, the seeds are scartered across the landscape. Tumbleweeds often pile up in wind rows along fences and buildings. This is a troublesome weed in agricultural areas because it literally covers the farm land with bushy, prickly shrubs. One interesting use for this plant in arid regions of the American southwest is for a "snowman" at Christmas time. Three proportionally sized tumbleweeds are used to make the head, thorax and main body of a "snowman." Another suggested use is to compress tumbleweeds into logs and use them for firewood.

This miscellaneous category of wind-blown seeds and fruits includes plants that really don't fit the above 5 categories. The Grass Family (Poaceae) includes a number of species with plumose flower stalks that fragment into seed-bearing spikelets that blow into the wind. Some of these species have become troublesome weeds in southern California, including the South African fountain grass (Pennisetum setaceum). Although this tufted perennial makes an attractive, drought-resistant landscaping plant along walkways and roads, it is becoming a widespread weed in disturbed areas of San Diego County. Another species, called squirrel-tail grass (Elymus elymoides), resembles a weedy introduced grass, but it is actually a native perennial of dry, rocky mountains and open land in the western United States. To appreciate its airborne seeds, you really must see this grass during a strong gust of wind on the eastern slopes of the Sierra Nevada during late summer.

Mountain mahogany (Cercocarpus minutiflorus), a native shrub in the chaparral of southern California, produces a rather unique wind-blown fruit. The one-seeded fruit (achene) has a persistent, feathery style that glistens in the sunlight. Although they usually don't travel very far, the achenes are blown into the air by strong gusts of wind during the dry, fire season of late summer and fall. This species is not related to the West Indian mahogany (Swietenia mahagoni) or the Honduran mahogany (S. macrophylla), members of the true Mahogany Family (Meliaceae). Mountain mahogany actually belongs to the Rose Family (Rosaceae) and produces very hard wood that sinks in water when dry. In fact, the wood of a montane species (C. ledifolius), has a specific gravity of 1.12, as heavy and dense as ebony (Diospyros ebenum).

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