The Tree

by Colin Tudge

  The Aspholadaceae family includes the genera Aloe and Yucca. Both include significant trees. The aloes of Africa, Arabia, and Madagascar may be seen as the Old World equivalent of the American agaves: they look very similar, and turn up in the same gardens worldwide. (There are many pleasing parallels between American and Old World flora and fauna. Animal examples include the iguanid lizards, boas, and condors of the Americas versus the agamid lizards, pythons, and vultures of Africa and Eurasia.) The yuccas include the wonderful Joshua tree, Yucca brevifolia, 10 meters tall, with simple curving branches that have prickly mop-like tops: they enhance and haunt the deserts of the southwest United States and Mexico. Aloe and Yucca (unlike the palms) have reinvented a form of secondary thickening, though without the continuous cylinder of cambium that the conifers and dicot trees have.

  The Ruscaceae is the family of the butcher’s broom—but also of the magnificent and extraordinary dragon trees in the genus Dracaena. In general, the species of Dracaena resemble a yucca or an agave, but some of them are big—up to 20 meters—and hold their own in tropical forests. Some, including D. cinnabari and D. draco (the dragon tree) yield a red resin known as dragon’s blood. Dracaena, too, has reinvented a novel form of secondary thickening.

  Finally—worth mentioning at least as a curiosity—there are the extraordinary grass trees, Xanthorrhoea and their relatives from Australia, in the Xanthorrhoeaceae family. You may well come across them—they are favorite curiosities in botanic gardens all around the world—and very strange they look: like little wooden posts with a big tuft of “grass” at the top. They are not hugely important, perhaps. But they represent yet another way of being a tree, and if a few historical coins had flipped differently, who knows how they might have taken off? It might have been that when we now think of trees we would think not of oaks and beeches but of grass trees, no doubt further evolved into all shapes and sizes.

  THE “SCREW PINES”: ORDER PANDANALES

  So to the Pandanales, with the single family of the Pandanaceae, colloquially known as screw pines. The “screw” is understandable, since the tops of the stems are twisted and the long, narrow leaves, which in reality are in three rows, seem to form a spiral. The “pine” is incomprehensible. “Palm” would have been closer, since many of the Pandanaceae resemble palms—although they are branched, which palms usually are not, and are typically supported by long stilt roots, which is not usual in palms. But screw pines are not closely related to palms, and indeed have no close relatives at all. Not all the screw pines are trees—some are climbers—but members of the biggest genus, Pandanus, certainly are: they grow up to 30 meters tall. The arborescent forms mostly grow around the sea or in marshes—indeed, says Corner, “there are tracts of rivers where the pandans prevail in serried and impenetrable ranks.” Where screw pines flourish, palms can’t get a look in. Some screw pines, however, prefer mountains, at the top of the tree line. They live mostly around the Indian Ocean and the western Pacific. Some have economic importance, not least as ornamentals. It is worth keeping an eye out for them—they turn up in the oddest of places. Some provide good food, too—notably P. leram, whose big spherical fruits are boiled to a mealy mass known as Nicobar breadfruit; and pandanus cake features as a dessert (a delectable one, I am told) on the menus of Thai restaurants. It’s odd that such a significant group of plants—some of them big trees—are so little known outside botanic circles. People probably mistake them for palms.

  THE PALMS: ORDER ARECALES

  There is only one family in the Arecales order—that of the palms, the Arecaceae (formerly known as the Palmae). But that family includes more than 2,600 species in more than 200 genera, and they are among the wonders of the world.

  Most palms are from Asia (especially the southeast), the Americas, and Australia, with surprisingly few species in Africa, although they grow widely there. In the rain forest of Amazonia palms more than hold their own among the dicots, as major players in the canopy. Most are tropical and subtropical, and on the whole they are tender, presumably because the all-important growing tip (the apical bud) is vulnerable. In most species (though not all) the bud is irreplaceable, and once it is killed (by cold or because someone harvests it as a “palm heart”) the whole tree dies. But the bud is typically protected by fibers and leaf bases that presumably help keep out the cold. So it is that some, like the European fan palm (Chamaerops humilis), grow naturally around the Mediterranean, where it can be very cold. Several have drifted north into the United States, including the petticoat palm, Washingtonia filifera. Some flourish in mountains—in tropical latitudes, to be sure, but at altitudes where the dicot trees are stunted. The giant fishtail Caryota in the uplands of Malaysia may be 40 meters tall, towering above the oaks and laurels all around. The wax palm, Ceroxylon, at more than 60 meters, is the tallest palm of all—yet it grows high in the Andes, at 3,000 to 4,000 meters. The Chusan or windmill palm of China, Trachycarpus fortunei, can cope with snow and ice.

  Most palms look as we all imagine palms should: a tall, straight stem with a crown of leaves on the top. But others deviate startlingly from our imagined archetype. Some keep their stems underground, so their leaves seem to shoot straight from the soil surface. The underground stems do not grow horizontally, like the rhizome of an iris. The trunk of Attalea, for example, first grows downward and then, as if realizing its mistake, grows upward again, to form a U. Attalea is also the genus of the American oil palm and is a relative of the coconut—although the coconut palm itself, Cocos nucifera, grows tall and straight, albeit often leaning over. There are many so-called stemless palms in the Cerrado, the dry forest of Brazil. Other palms, notably in the genus Calamus, are climbers; indeed, Calamus provides rattan cane, the stuff of colonial screens and chairs that creak on the veranda. Yet there are around 370 species of Calamus, and many are bona fide trees, suggesting that climbing is a late evolutionary departure and illustrating once more the extreme versatility of nature.

  In many palms the trunk is smooth, in others patterned with the scars of earlier leaves. Many retain the ragged bases of old leaves, like the African oil palm, Elaeis; or, indeed, like America’s Washingtonia filifera, which Hugh Johnson, in The International Book of Trees, compares unkindly to “an Alpine haystack,” even though Californians are so fond of it. Many palms are valued for their timber—like palmyra (Borassus), much favored in India. Palm wood, says Corner, can be “as hard as steel.” Yet the structure is quite different from that of dicots or conifers. The xylem and phloem run from roots to leaves in scattered strands, for there is no cambium to divide the two into neat concentric cylinders. So there can be no secondary thickening. Instead, the stem starts stubby and simply grows taller—although in bottle palms the trunk may swell at intervals, through the general proliferation of tissue and enlargement of cells. These swellings may in part serve as food stores. Bottle palms are not a discrete group: there are various kinds, in unrelated genera. Best known and most magnificent among them is the royal palm, Roystonea, whose tall, pale gray trunks are swollen for much of their length and topped by a huge, shaggy crown of dark green pinnate leaves. Roystonea came originally from Cuba, but it is grown as a street tree throughout the affluent tropics. I am told there is a most amazing avenue of royal palms in the Botanic Garden at Rio. (Note: Always check out botanic gardens!)

  Many palms have vicious spikes. Those on the trunk may be like bodkins, up to a foot in length. Some that grow from near the base originate bizarrely from the tips of adventitious roots, rising directly from the trunk. I have grabbed the leaves of palms to keep my balance along Amazonian forest paths and come close to shredding my hands on barbs as sharp as fishhooks, ranked along the midrib. Not nice. Yet on islands in the Indian Ocean that lack insouciant ramblers and rapacious herbivores, the palms are spikeless—just as birds on islands that traditionally had no humans or cats tended to be tame (and were wont to perch on the rim of sailors’ cooking pots). Armaments, and the emotions of fear and aggression, require energ
y. With no threat, an air of relaxed innocence can prevail. There is an obvious moral in here.

  Royal palms may grow to thirty meters. This is a young one.

  The roots of palms are peculiar. First, they are adventitious, growing directly from the trunk—generally from the base, of course, but also from higher up, occasionally forming significant stilt roots as in the screw pines, so that the whole tree can look like a rocket about to take off (and sometimes remaining dwarf but with hard and sharp tips to the roots, which form some of the most murderous spines). Big dicot trees have extremely thick roots—but they become thick in the same way as dicot trunks do, by secondary thickening. Palm roots and screw pines have no secondary thickening, but have instead the thickest primary roots seen in nature. They can also be, as Corner says, “exceedingly numerous”—so that 8,000 roots each a centimeter thick may emerge from the base of a coconut tree or an African oil palm, and sometimes up to 13,000. The root system begins normally enough in the seed—in the dicot manner, from the radicle (the primary root) of the embryo. But that first effort soon dies, and the adventitious roots take over. Because there is no secondary thickening, the roots remain cylindrical, though they taper off toward the end and finish with a stout root cap. Seldom are there root hairs of the kind that dicots generally rely upon; water is absorbed from a short region just behind the root cap. Palm roots (like screw pine roots) are very rich in xylem. They are highly efficient conduits.

  Palm roots are lignified through the middle and have huge tensile strength. The roots of the coconut spread up to eight meters in any direction, branching more or less at right angles two to four times as they advance. Their spread far exceeds that of the crown, and they provide prodigious anchorage. Coconuts grow on windblown islands by the edge of the sea. The waves dig out the sandy soil around them until the trunks collapse to the horizontal, held out like the bowsprit of a ship to a length of 20 meters, with a huge mop of leaves and a few dozen coconuts at the end. The strain is fierce. Yet the trees remain stable. As Corner says, nothing less than a hurricane can dislodge them. Perhaps it’s the roots of palms, above all, that have made them so successful. They can hang on through storms where dicots and conifers are toppled. Perhaps palms established themselves first as swamp and riverside trees and then spread to the surrounding forests.

  The apical bud at the top of the palm, from which the crown arises, is the biggest bud in all of nature. The buds are harvested from many species as “palm hearts” or “palm cabbage,” common on Brazilian menus. Few palms can grow a replacement bud, so harvesting kills the tree: a huge loss if they are taken carelessly from the wild. But some species, like the Brazilian Euterpe, can be coppiced like a European chestnut or a hazel. When a stem is cut, more stems grow to replace it. Euterpe also yields a fine fruit with the common name of acai (pronounced ass-ay-ee). Its deep-purple juice is a delightful drink, makes excellent ice cream, and is one of the most valuable of the nontimber forest products from Amazonia (behind rubber, cocoa, and Brazil nuts). For small farmers, a copse of Euterpe, constantly harvested but then regenerating, can be a fine source of income. For good measure, it has lovely, fern-like leaves.

  In general, the leaves of palms are either pinnate (feather-like), as in the coconut, or palmate (hand-shaped), as in the European fan palm. For some reason the palmate types include most of the hardiest species. The flowers may be of one sex or both. If they are unisexual, the tree itself may be of one sex only (dioecious), as in the American oil palm; or it may carry both sexes (monoecious). The inflorescences may be huge—up to 250,000 separate flowers. In some species, like the date palm (Phoenix), the inflorescence grows from between the leaves and hangs out from the side of the tree, typically below the leaves. In others, the inflorescence erupts from the top like a slow firework. Sometimes this is the tree’s last fling, for some apical flowerers die after reproduction. The flowers are mostly pollinated by insects—commonly beetles, bees, or flies. They often have nectar to reward the pollinators.

  The fruits of palms are immensely variable. Most are “drupes”—a fleshy fruit with a stone, enclosing one or two seeds. Often they are small and berry-like. But often they are huge, sometimes warty and sometimes fibrous—like the coconut. The seeds of Lodoicea, endemic to the Seychelles, are the biggest of any plant: like a great pair of brown wooden buttocks, and therefore also known as the double coconut, occasionally as the bum seed, and more felicitously as coco de mer. Lodoicea seeds are dispersed by water (like those of coconut and nipa palms) and can stay at sea for many months.

  Many palm fruits and seeds are rich in fat, including oil and wax. The coconut is a staple in many countries and a source of valuable oil. The oil palm (Elaeis guineensis) from West Africa provides an oil that’s used for many purposes, including soap. Oil palm has long been Malaysia’s second-biggest earner, after rubber, and is now being grown on a larger and larger scale throughout the tropics in the get-rich-quick manner of modern agribusiness, to the huge detriment of the traditional farming that actually feeds people. Attalea is the American oil palm. Copernicia cerifera provides carnauba wax.

  The coco de mer is the world’s largest seed and a prodigious mariner.

  Other palm fruits, rich in sugar, are delectable. Besides the acai and the date, there is the betel nut (from the betel palm, Areca catechu). The 200 species of Bactris include the peach palms. The pulp of the palmyra fruit yields a pleasant jelly; in India the germinated nuts are eaten as vegetables, and the palmyra’s sap, drawn by cutting through the great bract that envelops the base of the inflorescence, yields a sugar called “jaggery” and is also distilled to make arrack or toddy. The Chilean wine palm (Jubaea chilensis) is the hardiest of the pinnate-leaved palms and perhaps the sturdiest of all palms, with a trunk a meter thick. From South America, too, comes the ivory palm (Phytelephas, literally “elephant plant”): its big and immensely hard seeds are “vegetable ivory,” once favored for billiard balls.

  Palms yield many fibers. The coir of coconuts, which wraps around the nut, is immensely valuable for ropes, matting, and potting compost. It helps the wild seeds float, as they set out across the oceans from island to island. But the cultivated types have less fiber and more fleshy nut, and are more inclined to sink. The leaves of many palms are used to make mats or walls. Those of Borassus and of the thatch palms, Thrinax and Coccothrinax, are used for thatching. Raffia (Raphia ruffia) is used for weaving mats and baskets. Rattan (Calamus) is stiffer and is used to make furniture.

  Then there are many ornamentals, raised in botanic gardens almost worldwide (including the west of Scotland, anomalously warmed by the Gulf Stream) and along main streets in rich, warm cities from Florida to the Mediterranean to Melbourne. These include the fishtail palm (Caryota), the European fan palm, the cabbage palm (Sabal), and the queen palm (Syagrus romanzoffianum). The ugly-attractive Washingtonia, sometimes known as the petticoat palm, is the California fan palm. There are the hundred different parlor palms (Chamaedorea species), the needle palm (Raphidophyllum), the foxtail palm (Wodyetia), and the graceful, fan-leaved Livistona, from Australia and Asia. The king palm of Australia (Archonthophoenix alexandrae) is magnificent, with its 20-meter-tall trunk. Vast, too, is the Indian talipot (Corypha), with a trunk up to a meter across.

  Finally, out on its own, an eccentric even among the bohemian palm family, is the nipa palm (Nypa). Molecular studies suggest that it may be the sister of all other palms—closest to the common ancestor. The most ancient palm fossils known are nipa palms, which lends some support to that notion: they date from the (fairly) early Cretaceous, around 112 million years ago. There are many more from the early Tertiary, around 60 million years ago, not least from the depths of London clay (for London from time to time has been tropical and swampy, and with global warming might soon be again).

  But nowadays the nipa palm grows among the mangroves of Asia and the western Pacific, sometimes as the dominant species—with its roots in the sea (or at least in very salty water) and its stems
prostrate (meaning not upright) and, unusually among true palms, branching. It holds its feathery leaves vertically. Many palms that may live with their feet in water, including the coconut, the oil palm, and the date palm, have loosely spaced cells through the core of their roots that allow air to circulate. But the nipa palm goes one step further. It has large air spaces within its roots that connect with cavities in the base of the floating leaves—rather as the lungs of birds connect with further spaces in their bones. Corner suggests that “the rise and fall of tide may with slow strokes pump the air around” inside the nipa palm. It’s as if it breathes, but using tidal energy—a common trick among mangrove trees. Sometimes entire clusters of nipa palms float away on little islands, perhaps to take root somewhere else, sometimes simply to perish at sea. Sometimes, doubtless, animals ride on these floating islands and populate new continents. The present distribution of animals suggests that such events have taken place many times in the past. Nipa seeds, too, like coconuts and cocos de mer, are dispersed by ocean currents. Truly, says Corner, the nipa is “the swamp palm par excellence.”

  All authorities agree that far more research is needed on palms, but it is not easy. A detail, though an important one, is that botanists rely quite heavily on material kept in herbaria—but it is extremely difficult to store the often enormous leaves and inflorescences of palms. Thus, ubiquitous as they are, the palms remain largely elusive. Yet if humanity were allowed to retain only one family of trees from all the several hundreds, the Arecaceae would surely be on the short list.

  PINEAPPLES, SEDGES, AND GRASSES—INCLUDING BAMBOOS: ORDER POALES

  Some of the eighteen families within the Poales order are hugely important to our story. The Bromeliaceae includes fifty-one genera and 1,520 species and is best known for the pineapple. But it does have some tree-like forms (including some relatives of the pineapple) and a great many epiphytes—which again include relatives of the pineapple and also Tillandsia, so-called Spanish moss, which festoons trees in the swamps of the southern United States and is very much part of its scenery and folklore. No movie from the Deep South is complete without it. The Cyperaceae, too, the reeds and sedges, include the paper reed, or papyrus (Cyperus papyrus), which, growing along the banks of the Nile, is distinctly tree-like.