by Colin Tudge
Some traditional classifications list up to twelve genera in the Magnoliaceae.1 But Judd combines eleven of the traditional types into Magnolia; the other genus is Liriodendron. Between them they include 218 species, and they have the kind of distribution that we have already seen among the conifers: many in Southeast Asia, from the Himalayas out to Japan, and many more in the southeastern United States and Central America, with some in South America. Why are they spread out this way? Did they at some point leap the Pacific? Perhaps. But there are fossil Magnoliaceae in Europe and even in Greenland—which suggests that the family may once have spread more or less continuously from Southeast Asia to the Americas before the two continents drifted apart, and that they have simply died out in the middle of their range.
Magnolia is, of course, magnificent, with some of the most stunning flowers of any tree—sometimes star-like, sometimes huge like waterlilies. Their greatest value for humanity is in horticulture; and I commend to you the magnificent botanic gardens in Yunnan, in southwest China, which has a hundred or so species. Some magnolias give serviceable timber. The bark and flower buds of M. officinalis of China are used medicinally and are a valuable export. (China is also one of the world’s greatest centers of biodiversity, and its potential for ecotourism is unsurpassable. It is up there with Africa, Madagascar, and Amazonia.)
Liriodendron includes the two species of tulip trees—one Chinese (L. chinense) and one North American (L. tulipifera). Their leaves are strange and absolutely characteristic—like glossy, dark green versions of maple leaves, but with the pointed tip cut out. Their flowers are tulip-like, although difficult to appreciate since tulip trees can be big (the tallest in England is 36 meters) and the flowers are borne aloft. The American tulip tree yields a creamy timber, with streaks of olive green, black, pinkish brown, or even steely blue, from growing in mineral soils; it is much valued for carving, as well as for doors and suchlike. As a timber tree, Liriodendron is sometimes known as “yellow poplar” and is sold as “American whitewood.” Such names do it scant justice.
You can’t mistake the leaves of the tulip tree, Liriodendron.
The 2,300 species of Annonaceae, arranged in 128 genera, form a glorious family widely spread through lowland tropical and subtropical forest. In many ways they seem wonderfully primitive. Their flowers are pollinated by beetles—which they have evolved to encourage: they have a fruity odor, and they reward their visitors with thick, fleshy petals for feeding on, and extra fleshy tissue that serves no purpose except to provide beetle food. Some flowers of the genus Annona are able to heat up—a fairly common trick among several plant families. This encourages the beetles to stay inside the flowers overnight and mate, so becoming covered in pollen. Various genera, notably Annona and Rollinia, provide marvelous fruits of the kind that seem primitive and in Cretaceous times doubtless were food for dinosaurs: big and pulpy, with many big seeds. Custard apple (Annona cherimola), with its gray, tessellated skin, is the best known of these fruits in the West. Others include the soursop and sweetsop. The fruits of Monodora myristica are sometimes used in place of nutmeg, and some Annonaceae with thick, fibrous bark are grown as ornamentals, at least in warm countries.
The third important family of the Magnoliales order is the Myristicaceae. It includes 370 known species in seventeen genera, which occur across the tropics: in South and Central America, across equatorial Africa, through south India and Southeast Asia, and into Queensland, Australia. The trees are usually dioecious (only one sex per tree) and although their flowers are small and inconspicuous they are pollinated by insects—beetles and thrips. Clearly they are very different from those of the Annonaceae or Magnoliaceae—illustrating that although flowers are one of the main guides to classification, they may nonetheless differ enormously even between closely related families.
The biggest genus is Myristica, with 125 species, centered on New Guinea. Myristica fragrans from the Molucca Islands of Indonesia is the most economically important of all the family. Its big seeds are nutmeg; the fleshy coating of the seed (the “aril”) is scarlet while it remains on the tree (although concealed inside a thick fruit with a pale green skin) but is a pale buff pink after drying and is the stuff of mace. The seeds of the Brazilian genus Virola are ground to make snuff that is hallucinogenic. (Perhaps the world needs more hallucinogens rather than less. The puritanical attitude of the West might more properly be seen as an offense against nature.) V. surinamensis, whose seeds are waxy, are used to make a type of butter for eating and for candles. So too are those of Gymnacranthera farquhariana, from India.
GREENHEART, STINKWOOD, AND THE SWEET BAY TREE: ORDER LAURALES
The Laurales order includes around 3,400 species in seven families, of which the most significant is the Lauraceae (named, incidentally, by Antoine-Laurent de Jussieu, who first made clear the monocot-dicot distinction).
The members of the Lauraceae family are mostly trees or shrubs. In all, there are around 2,500 species in 50 genera, a huge presence in tropical wet forests worldwide, and in the subtropics. They serve humanity in many ways: with extraordinarily nutritious fruits, many fine timbers, and a host of medicines and other drugs. The Lauraceae illustrate beautifully how biochemistry runs in botanical families.
Best known of all Lauraceae fruits is the avocado, Persea americana, native to Central America, and P. drymifolia from Mexico. It has more protein than any other fruit and is 25 percent fat. It also has a wonderful strategy to prevent inbreeding. As with other Lauraceae, it is pollinated by insects. It has two kinds of flowers, inventively dubbed A and B; some individuals have A flowers and some have B flowers. The stigmas of A flowers are receptive to pollen only on one particular morning, while the anthers of A flowers do not release their pollen until the afternoon of the same day. In B flowers, the stigmas are receptive on one afternoon, while pollen is not released until the following morning. So A flowers can be pollinated only by B flowers, and B flowers can be pollinated only by A flowers.
Many Lauraceae have oil cavities in their leaves and elsewhere, and many are aromatic. Thus the family includes the bay trees (Laurus nobilis), sassafras (Sassafras albidum), cinnamon (Cinnamomum verum), and camphor (C. camphora), once much used in mothballs (though modern mothballs are made of naphthalene)—one of many examples of a tree’s own insect repellent turned to human use.
Among the many valuable timber trees in the Lauraceae family is the Queensland “walnut,” Endiandra palmerstonii, which grows to a magnificent 40 meters or more and whose timber resembles that of the European walnut (which comes from the quite different Juglandaceae family). It has pinkish sapwood and pale to dark brown heartwood, streaked with pink or purple-black—much prized for furniture of the boardroom kind. The mangeao of New Zealand’s North Island (Litsea calicaris) is another giant (40 meters or more), giving cream to pale brown timber that is favored for everything from turnery to dance-hall floors to pit props in mines—and also for excellent veneers for export. Imbuia (Phoebe porosa) from Brazil is yet another forest giant (up to 40 meters) with a dark brown, fine-grained, lustrous timber that again is much coveted for high-class joinery.
The many fine trees of the genus Ocotea are prized both for their beautiful timber and for their broader biochemistry. South Africa’s stinkwood (O. bullata) is 18 to 24 meters tall in the forest, yielding a dark timber with a fine grain that is indeed malodorous when freshly worked but settles down when dried. Ocotea usambarensis, from Kenya, is camphorwood. It grows up to 45 meters tall, yields a greenish-brown timber that matures to deep brown, smells of camphor, and is much favored for making wardrobes, effectively with the mothballs built in. (I have found, in travels with tropical loggers, that many trees are surprisingly smelly when first cut, and not always pleasantly so.)
But the most famous of all the Lauraceae timbers comes from the greenheart (O. rodiaei), the pride of Guyana. It, too, reaches up to 40 meters, with a long cylindrical trunk that may be 25 meters tall and a meter in diameter. The sapwood is pale yel
lowy-green, the heartwood light olive to dark brown, often streaked with black. Greenheart is highly versatile: much favored in maritime circles for jetties and groins, and in ships for planking and stern posts, but also for turnery and the butts of billiard cues. It is used to make longbows, too, the technology of which has come on apace since the English first juxtaposed the heartwood and sapwood of yews. Modern longbows are laminated, and greenheart often forms the central layer.
In addition, the nut of the greenheart yields a material called “tipir,” which the native people of Guyana have long employed as a medicine. The Wapishana tribe grates the nuts and uses the extract to stop hemorrhages, prevent infections, and as a contraceptive. In the late 1990s, however, an American entrepreneur, after spending time with the Wapishana, tried to patent tipir as an antipyretic useful in preventing flare-ups of malaria, and perhaps for treating cancer and AIDS. The tribe accused him of theft: “biopiracy.” The dispute rumbles on. Meanwhile greenheart, precisely because it is wonderful in so many ways, is being seriously overlogged. But still it is outstanding even among the distinguished family of the Lauraceae, which are huge players in tropical forests and economies worldwide.
WINTER’S BARK AND WHITE CINNAMON: ORDER CANELLALES
There are only two families in the Canellales order. The Winteraceae family includes up to 120 species (no one is quite sure) in seven or eight genera, largely from around the coasts of South and Central America, eastern Australia, and New Guinea, plus one in Madagascar. Several have peppery leaves and bark that are said to be medicinal; best known is Winter’s bark (Drimys winteri), which was once used to prevent scurvy. The wood of the Winteraceae is strange: the conducting tissue of the xylem contains only tracheids—rows of cells with perforated ends—as in a conifer. They do not lose the cell walls at the ends to form the continuous tube-like vessels that are more typical of broadleaves. But it is not clear whether this arrangement is primitive or secondary; the ancestors of the modern Winteraceae may have had vessels, which may subsequently have been lost. As we have seen, evolution often leads to simplification, and several other groups of flowering trees, unrelated to the Winteraceae, have wood with tracheids rather than vessels.
The second family, the Canellaceae, includes sixteen or seventeen species of aromatic trees, in five genera. The bark of Canella winterana is also known as “white cinnamon” and is a tonic and a condiment; it is also used to poison fish in Puerto Rico (so they can be scooped out of the water). The aromatic wood of Cinnamosma fragrans of Madagascar is exported via Zanzibar to Bombay, where it is used in religious ceremonies. The bark of Warburgia ugandensis of Uganda serves as a purgative, while its leaves are used in curries and its resin is handy for mending tools.
BLACK PEPPERS AND WHITE PEPPERS: ORDER PIPERALES
The last order of primitive dicots is the Piperales. As defined by Judd it includes five families, of which the most significant is the Piperaceae, mainly of rain forests throughout the tropics. Its 2,000 or so species include some small trees, but its best known members are the woody vines Piper nigrum, the source both of black and white pepper, and P. betle, whose peppery leaves are chewed with various spices and betel nut (the fruits of the palm Areca catechu) to provide what the herbal manuals call “a mild stimulant.” The red juice of this mixture dribbles down many a beard on the Indian subcontinent (including mine, from time to time).
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From Palms and Screw Pines to Yuccas and Bamboos: The Monocot Trees
IF SOME EARTHLY FIRM of engineers had designed the magnolia or the bay tree, they would be pretty pleased with themselves. Here, they would conclude, is the finished article: root, trunk, branches, leaves, fruits, and well-protected seeds. Everything taken care of, the whole structure beautifully integrated, beyond improvement—the apotheosis of the plant.
But nature is never satisfied. However fine its inventions may be, evolution powers on. Sometime after it had produced the first magnificent, magnolia-like tree, the mighty laurels, and the ancestral peppers, nature came up with something completely different: the monocots. They are easily discerned (usually) by the kinds of features that John Ray noted. They tend to have long, narrow leaves. The veins in the leaves run in parallel from base to tip, whereas those of dicots generally form a branching network. The parts of the flowers (petals, sepals, stamens, carpels) are typically arranged in multiples of three, while those of dicots (both primitives and eudicots) more usually occur in multiples of four or five. But the differences run far deeper. The monocots represent a new and different way of being a plant.
What really matters, what is truly profoundly different, is the way the monocot grows, particularly as manifest in the leaves and roots. Notably, the leaf of a dicot grows out from the edges. The youngest bits are farthest from the twig. The strap-like leaf of the monocot grows from the bottom up, typically from a bud at the tip of a stem, known as an apical bud. The youngest part of the monocot leaf is at the bottom and the oldest at the top, so that grass leaves die from the tip downward, and an onion leaf is white and immature at the base and green or even senescent at the top. (The great tactic of the grasses is to keep their growing tip, the apical bud, below the surface of the ground so that it is not destroyed by grazing animals—and, in fact, grasses, in contrast to almost all other plants, positively gain from being grazed and grow rank if they are left alone. To find a way of benefiting from the attentions of the predators who come to eat you is a trick indeed; and it explains why the world’s grasslands, basically created by members of the single family Poaceae, are almost as extensive worldwide as the world’s forests, which contain many thousands of species in scores of families.) The roots in monocots are different too: in general they are much more likely to grow straight from the stem (the technical term for this is “adventitious”), rather than from other roots.
An arborescent relative of asparagus: the dragon tree.
As you can see from the chart, there are ten orders of monocots—five of which contain significant trees, and five of which are predominantly herby. This is in contrast to the dicots (both the primitives and the eudicots), in which most of the orders contain trees.
This difference can be explained on evolutionary grounds. We can assume that the first flowering plants of all were primitive dicots—and that these ancestral types were trees. Then we merely have to suggest that the dicots that are herby, like dandelions and waterlilies, have simply lost their woodiness and their arborescence. But it seems very likely that the very first monocot was itself an herb. So each modern order of monocots that contains trees must have reinvented the form of the tree afresh. Dicots as a whole seem to have stayed with the timber of the original angiosperm ancestor. All their timber is basically very similar—and similar to that of the conifers, with whom they probably shared a common ancestor about 300 million years ago. But the timber of monocot trees is highly variable, and in general is nothing like that of dicots at all.
Most obviously, most monocot trees do not undergo secondary thickening of any kind. Palms, which are the most various and ecologically significant of all, may be very big—up to 60 meters tall—and their trunks may be up to 2 meters in diameter. But in general they are just as thick when the tree is young as they are when it is at full height (although the stems of some of them do sometimes thicken, sometimes along only part of the length as in bottle palms; but they do this just by accumulating more tissue—there is no regular secondary thickening from a sheath-like cambium). Some other monocot trees, such as the dragon tree, Dracaena, do undergo some secondary thickening. But the mechanism is quite different from what we see in oaks or magnolias (or, indeed, in pines). In particular, there is no continuous sheath of cambium regularly turning out new tissue. The dragon tree’s form of secondary thickening is another reinvention.
I will not dwell on the five monocot orders that do not contain significant trees, but a brief mention is called for just to provide context. The Acorales are sweet-smelling herbs found both in North America and
Eurasia. They may be the most primitive of all the living monocots: closest to the ancestral form. The Alismatales include some extremely interesting plants of huge ecological importance, including pondweeds, sea grasses (mentioned again in the context of mangroves), a great many epiphytes (of great significance in tropical forests), and the important staple food crop taro. The Liliales are of course the lilies, as well as the autumn crocuses. You will find trees included in the Liliales in some traditional classifications, including yucca and aloe; but these two genera have now been repositioned, as will become apparent. The Dioscereales are the bryonies, and the Commelinales are more water plants, including the spiderworts and the water hyacinth, which has become such a pest in many tropical waterways. The five monocot orders that do include trees—including some extremely significant trees—are as follows.
JOSHUA TREES AND DRAGON TREES: ORDER ASPARAGALES
This order was, of course, named after the asparagus, in the Asparagaceae family—which also includes some shrubby species and a few woody vines. Also in the Asparagales are the families of the daffodils, the hyacinths, the irises, the onions, and the orchids. But more directly to the point is the Agavaceae family. The genus for which the family is named, Agave, includes about 300 species of prickly succulents, visually like pineapple tops, that are native to the Americas but feature in warm gardens worldwide. Several are big with woody trunks: bona fide trees. A. americana provides the Mexicans with pulque, which is sometimes distilled to make mescal. A. sisalana and A. fourcroydes provide strong fibers—sisal hemp—for ropes and fishing nets. A. americana was imported to Europe soon after Europeans became aware that the New World existed and is now grown everywhere that is not too cold. But the genus of Agavaceae most familiar to gardeners in temperate regions is Hosta—another homely link to a more exotic world.