Ocotillo and the Boojum Tree

Out in the Sonoran and Chihuahuan deserts of North America live some peculiar and highly remarkable spine-covered plants of the Fouquieriaceae, the ocotillo family. This family consists of eleven species that are native only to deserts and other dry habitats of Mexico and the southwestern United States. Two of the distinctive forms can be viewed within 300 miles from Los Angeles: the ocotillo (Fouquieria splendens) and the boojum tree (F. columnaris, formerly known as Idria columnaris). You can observe the ocotillo growing in the southern desert along Interstate 8 between San Diego eastward to Arizona, or beyond or near Anza Borrego State Park. And if you drive southward from San Diego into the desert of Baja California, you'll encounter the boojum. On the other hand, you can also examine both species--along with four other relatives--right here in Westwood, in the desert garden at MEMBG. In fact, we have living specimens of ten out of all eleven species, soon to form a complete outdoor collection.

The ocotillo plant consists of a set of ascending, wand-like stems, each studded with long, helically arranged, light-gray spines. During growth spurts each spiny stem is bedecked with obovate leaves and topped with a stalk of showy red flowers. So common is ocotillo around Tucson, Arizona that even children in elementary school know this plant by sight. They know that hummingbirds pollinate the terminal clusters of scarlet tubular flowers, seeking thin nectar as a reward.

An entire book has been devoted to the boojum, also called cirio (R. R. Humphrey, The Boojum and Its Home, University of Arizona Press, 1974). This bizarre but picturesque tree has a thickened, central trunk that grows up to 16 meters tall with no major branches, and looks like an upside down, light gray-yellow carrot. Leaves of F. columnaris are smaller than those of the ocotillo but can persist for months, and the tubular flowers--way out of reach--are cream-yellow and typically pollinated by insects. Overall, the external features of boojum and ocotillo are so similar that nobody would doubt that they are very close relatives.

Fouquierias are stem succulents, capable of storing enough water within their stem tissues to remain dormant but healthy, in extremely dry habitats. The wand-like ocotillo, the least succulent species, has wood that is relatively normal by dicotyledonous standards, storing water in the soft stem center (pith) and peripheral tissue (cortex). Boojum, the most highly derived succulent of the family, has a pith that may be several centimeters in diameter and also forms patches of soft, water-storing cells within the wood and bark. In fact, so much succulence may form in the boojum that its trunk base may become nearly three-quarters of a meter in diameter! The trunk surface of boojum has a rock-like texture that appears to deter rodents from eating the juicy inner tissues.

Leaf arrangement on shoots of Fouquieriaceae is helically alternate. On the rapidly elongating young shoots, each leaf has an unusual type of petiole, in that tissues of the stem cortex become elongated and form the lower portion of the petiole. The tough, fiber-like cells of this stem cortex reinforce this tissue, so that when the leaf abscises (that is, the leaf falls away) the upper side of the petiole peels off and leaves behind a rigid, conical "fouquieriaceous spine." No other plant family makes spines in this way. The spine usually is the length of the petiole, but may extend a short distance along the midvein of the leaf blade. One might imagine that the spine evolved to protect the stems from being eaten or defoliated by browsing mammals of old, but please don't ask biologists which ones, because there is no way to know what herbivore living millions of years ago could have been responsible for shaping this adaptation!

One reason to study Fouquieria is that this family is a splendid example of long shoot/short shoot organization. The initial young shoots, termed "long shoots," grow rapidly when the soil is thoroughly wetted by a heavy rain. During subsequent growth seasons, leaves lacking petioles are formed from the axillary bud. From the axillary bud, located just above the base of the spine, arises a dwarf "short shoot" that never grows into a branch because no internodes are formed. Each short shoot forms two to eight leaves at a time. These leaves, in miniature, are hidden in the bud, but from a dormant stage F. splendens can produce leaves within forty-eight hours following a substantial rainfall, expanding rapidly as soon as water is provided. As the plant enters drought, the leaves abscise when the stems become too dry.

The canopies of the fouquierias are open so that sunlight, allowing sunlight to easily penetrate and reach all leaves covering the stems. Hence, in part the growth habit appears to be a light-harvesting strategy. But some authors have also suggested that the wand-like stems of the ocotillo growth habit, are an adaptation to channel more rainwater and dew to the root zone beneath the canopy. Taller species, such as F. formosa and F. ochoterenae have higher canopies that compete for sunlight with other drought-deciduous trees in native tropical woodland and scrubland.

Currently growing in our desert garden are four other species of Fouquieria. Often in leaf are F. diguetii (pronounced di gay´ eye), a shrub or short tree of southern Baja California; F. macdougalii, a similar short tree occurring across the Gulf of California on the western mainland of Mexico; and a small tree, F. formosa, from dry tropical scrub and woodlands in southern Mexico. During springtime, when these three species are flowering, they are fairly easy to tell apart, and all are visited by our hummingbirds. The orange-red flowers of F. formosa, borne in upright spikes, are almost three centimeters in length but look much longer because the many stamens are far-exserted. Flowers of F. macdougalii are rose-red, narrowly tubular, and spreading or nearly pendent because they are borne on long, thin pedicels. The glossy, narrowly tubuluar scarlet flowers of F. diguetii are formed on a narrow panicle. Most distinctive, however, is the white- and pink-tinged flowers of F. burragei, a narrow endemic of coastal southern Baja California. If you are hunting one in nature, you'll easily discover F. burragei on hills overlooking the car ferry terminal in La Paz--which is also a great place to watch brown pelicans.

Ocotillo and its relatives have other interesting stem adaptations. Stems are lobed with hard ridges that are separated by deep furrows. Beneath each of these furrows, a network of water cells occurs. Cork fills the furrows as the stems age, but it is relatively translucent, permitting sunlight to enter the green tissues beneath. For years, these green strips were thought to be important for stem photosynthesis, but measurements have not shown this to be the case. Eventually, the basal stems of ocotillos develop beautiful reddish, papery bark.

The subgenus Bronnia consists of two white-flowered species that have green, bottle-like (swollen), succulent basal trunks. For their protection at MEMBG, we have maintained plants of these narrowly restricted species behind closed doors. One might reasonably assume that the green stem covering of F. fasciculata (from Hidalgo, Mexico) and F. purpusii (from Puebla and Oaxaca, Mexico) is a photosynthetic organ, but no scientist has ever attached a gas chamber to this plant for quantifying its photosynthetic role. In fact, even the green stem tissue of the boojum tree, which has been studied, does not contribute significantly to making sugar from atmospheric carbon dioxide. Thus, the stem is not regarded as an important photosynthetic organ relative to the leaves.

When animals effectively pollinate these plants, the ovary develops into a capsule that splits open with three valves to release a number of small, light-weight, winged seeds that are easily dispersed by the wind. If you are lucky enough to have seeds of fouquierias, it's likely that you will have exceedingly good germination, because the thin seed coat allows for rapid absorption of moisture.

Recent molecular studies of chloroplast DNA now tell us that the Fouquieriaceae family is probably most closely related to North American species of phlox (Polemoniaceae).

Fouquieriaceae, especially the species of ocotillo and boojum, are landmarks of our deserts. How frustrating it is, therefore, for a botanist, trained on the biology of fouquierias, to see a film about Field Marshal Rommel fighting the Allies in North Africa, showing his panzers speeding past ocotillos! How gullible do Hollywood directors think we are? For this endemic family of North America...Tubac, Arizona, not Tobruk, Libya.

ARTHUR C. GIBSON< MEMBG Director

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