Cotton on a Tree

A particular tree species in West Los Angeles evokes some of the strangest conversation. "Is that a giant avocado tree?" "It looks like cotton coming from that tree!" "Look at the sharp spines on that trunk!" "The flowers are so beautiful--what is that tree?" The tree is Chorisia speciosa, or floss-silk tree. Many call it the silk-floss tree, but the way to remember it is alphabetically, f before s. Better yet, use the botanical binomial and impress your friends!

Chorisia speciosa is a member of the Bombacaceae, or cotton-tree family, which includes 30 genera and approximately 180 species, mostly large trees that grow in seasonal dry forests and grassy woodlands of the tropics and subtropics, especially in the Americas. Most photographed among the Bombacaceae, however, is the famous baobab or dead rat tree, Adansonia digitata, an elephantine tree of African savanna woodland with a massively enlarged, bottle-shaped, gray trunk and short, dumpy branches sticking into the air like thick roots. To explain its monstrous appearance, Kenyans say that the devil planted this tree upside down. One baobab has been reported to have a circumference of sixty-two feet, and some Africans have speculated that several of these trees may be up to 5,000 years old! In some cases, the tree develops a hollow center, wherein bats can roost--which is convenient because its white flowers, about six inches across, are pollinated by bats. Moreover, the fuzzy fruits, which may become up to eighteen inches long, are pendent on long pedicels, giving the appearance of hanging dead rats! MEMBG has one small baobab, which will be planted out this coming spring.

But let's return to the floss-silk tree, a native of southern Brazil and Argentina, and a popular L.A. tree since the 1970s. This plant grows rapidly during its early years, and then growth slows after the branched canopy has formed. The leaves are alternate and palmately compound, in some Southland locations totally deciduous and in others semi-deciduous, retaining branches of leaves. But what catches the eye are the grass-green bark and branches and the trunk studded with persistent, stout, gray "spines." Botanically speaking, these sharp projections are termed trunk prickles, bark prickles, or stem emergences--that is, outgrowths from internal stem tissues. (Spine is a term now reserved for a modified stem, leaf, or root primordium.) In C. speciosa, stem emergences are absent on new stems but erupt from cortex on two-year-old stems at the internodes. Thereafter, as cells divide at the base, each emergence gets taller and wider, eventually becoming one to one-and-one-half inches high and often one inch wide. As the trunk becomes slightly inflated with water-storing cells ("a bottle tree"), the bark stretches. On stretched bark, green stem patches become furrows separated by newer gray bark. Circular scars on the trunk bark show where emergences have been pried off or shed.

Locally, you can view average specimens of C. speciosa at the corner of Sunset and Beverly Glen, and at Hilgard and Le Conte, where a pair forms an arborescent entrance to MEMBG. But if you really want to see a prime specimen, come to the Malesian Rhododendron section and look upward. At the edge of the collection, opposite our Blakea gracilis, is a sixty-foot giant, resembling a mature individual in nature with its bottle-like trunk unbranched for forty feet and supporting a symmetrical crown. This plant is best appreciated during winter, when it is totally leafless, so your attention focuses on the green- and gray-striped bark and the immature green fruits--like gigantic avocados--suspended overhead.

Obviously, to view flowers up close you will need to use one of the shorter street trees! Flowers of C. speciosa, which appear in late summer and early fall, are extremely showy and have been compared with those of certain lilies and orchids. A fully open flower may be six inches across, consisting of five distinct (not fused), thick petals. On a single tree, the color tone of petals is light orchid-pink, purplish rose, dark purple, or burgundy, typically marked with ivory or white and spotted brown on the lower half. One grafted clone called 'Los Angeles Beautiful' has wine-red flowers, and 'Majestic Beauty' has rich pink flowers. The pistil consists of a superior ovary with five chambers (locules) having many ovules, a long white style, and topped by a hemispherical, rose-colored stigma. A tubular column formed by the fusion of five stamens (filaments) surrounds the style. The filaments bear the massive anthers, which are loaded with pollen. The features of large size, thick parts to avoid mechanical damage, and copious pollen indicate that bats would be appropriate pollinators for these trees in their natural habitats.

The less commonly cultivated tree C. insignis has similar floral structure, but its petals are white with yellow at the base. This species flowers later than its better-known cousin, often in December and January, but C. insignis is equally magnificent (even if its common name is drunken tree!) and should be more widely grown.

The fruit of both species of Chorisia, in fact of most Bombacaceae, is a capsule. It is oblong or somewhat pear-shaped and may grow to six inches in length. While it is developing and even later when it is large and green, one can often observe a persistent style at the end of the fruit. At maturity, five valves of the capsule fall away (the fruit is dehiscent) to reveal the locules, now filled with five elongate masses of silky white hairs. Floss-silk resembles cotton but, unlike cotton, is attached just below the seeds, which are hidden by the silk. When released, these silky hairs help to disperse the seeds in strong winds in the canopy, but in Los Angeles, instead, they can litter a lawn or planting bed.

Among Bombacaceae, the most famous economically important fruit hair has been harvested from Ceiba pentandra, the kapok or silk-cotton tree. Probably few people in California have ever heard of kapok, a towering emergent of lowland neotropical forests, often reaching fifty meters in height and forming enormous, flaring root buttresses that prevent the tree from snapping at the base. Unlike cotton, kapok cannot be woven into cloth, but formerly it was widely used for stuffing pillows, bases and balls for baseball and softball, mattresses, and, especially, life jackets. In fact, during World War II, a U.S. sailor would commonly refer to his life jacket as a "kapok." Since the war, however, synthetic fibers have replaced kapok for these traditional uses.

Along the main service road at MEMBG, located just south of The Nest, is a recently transplanted small specimen of Ceiba aesculifolia, a native of western Mexico. The gardeners had to hastily relocate this tree from its former bed when equipment repairs were required near the corner of the Botany Building, and fortunately it is thriving in its sunny new home. Like chorisias, this plant has formidable stem emergences and palmately compound leaves, which look like leaves of Aesculus (buckeye).

Also growing above the rhododendrons are two prized specimens of the shaving-brush tree, Pseudobombax grandiflorum. This species has the green and gray stripes on its bark but totally lacks the stem emergences. Its flowers are unusual in having a pincushion of showy stamens rather than showy petals.

There are beautiful flowering trees elsewhere in this family, as well as other species that are useful to humans. Bombax, which includes the red silk-cotton tree, is the largest genus (about sixty species). Ochroma pyramidale is not only a key pioneer tree species of disturbed neotropical rain forests, but also the source of balsa, the lightest of all commercial woods. Durio includes the tree that produces the durian, a large, highly prized fruit that is famous for its delicate flavor but disagreeable odor. Like related families Malvaceae, Sterculiaceae, Urticaceae, and Tiliaceae, Bombacaceae have high-quality "bast" fibers in the inner bark, which preserve the integrity of the bark but can be stripped off in ribbons and twisted into cord for fishing nets.

Little scientific research has been done to understand the adaptations of these tropical tree species, which often live in communities with unrelated plants that also are thorny and have green bark. Yet this is a prime family to study for learning about the importance for survival, if any, of bark photosynthesis; the possible role of stem emergences in protecting plants from herbivores; and the influence that stem water storage has on surviving drought and initiating growth after the dry season.

Anyway, if you are looking to add a tree to your garden, why not consider one from the showy-flowered Bombacaceae? Keep in mind, however, that Chorisia wood is soft, and branches may break off in strong winds, and also that its roots can heave pavement. Still, these plants add great color to a neighborhood at the end of the calendar year.

Rebecca Bonney, MEMBG Docent

[Return to Volume 2(1) Menu]