The Tree (19 page)

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.

P
INEAPPLES
, S
EDGES
,
AND
G
RASSES
—I
NCLUDING
B
AMBOOS
: O
RDER
P
OALES

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.

But the family that is most germane to our story, and indeed to our entire existence, is the Poaceae, formerly known as the Gramineae—the 650 genera, with nearly 10,000 species, of grasses. Poaceae is the most successful plant family on earth, alone responsible for vast biotopes and ecosystems (thanks to their hidden apical bud). It includes the cereals—just three of which, wheat, rice, and maize, provide humanity with half of all our calories and more than half of our protein, while the more fleshy grasses feed most of our cattle and sheep. But also, which is what matters here, the Poaceae include the bamboos, in the subfamily Bambusoideae. Truly they are trees, often prominent and sometimes dominant in tropical forests throughout the world. I have gazed up at them in Chinese forests—and up and up and up: some grow to 40 meters, taller than most tropical forest trees. One of the world’s favorite animals, the panda, has turned itself from a perfectly good carnivore (pandas are basically bears) into a dedicated scoffer of bamboo. (Though if you want to catch a panda, lure it into a [bamboo] cage with roast pork. It’s the same with all vegetarians.) Bamboos, like palms, are in many ways eccentric. For example, many flower only at extremely long intervals—every ten to eighty years—and when they do, they all flower simultaneously. How do they manage this? Are they responding to some quirk of light or climate, or to some cryptic message passed between them? And what prompts the message? Some species die after reproducing, sometimes leaving the creatures that depend upon them stranded (like the giant pandas). Binge flowering produces a boom population of young plants over the following years. Many other trees (and animals, such as wildebeest and zebras) practice the same tactic: they produce so many offspring, all at once, that their predators cannot possibly catch them all. A steadier output would produce a steady kill. Perhaps, too, the sudden demise of the adult bamboo plants before the next generation gets going helps to reduce the population of pandas.

Bamboos, too, may compete in the forest as canopy trees.

For the traditional peoples of Asia, bamboos are among their greatest assets. They lend themselves to every purpose, from brushes and pens to food, pots, cutlery, furniture, and musical instruments of all kinds—percussion, wind, and strings. They have created an entire aesthetic of painting and architecture—the swishy calligraphy, the great sagging roofs of palaces and temples. Virtually the whole interior of the headquarters of the International Network for Bamboo and Rattan in Beijing is made from, or veneered with, bamboo. Whole books are devoted to bamboos. They deserve them.

B
ANANAS AND THE
T
RAVELER’S
P
ALM
: O
RDER
Z
INGIBERALES

The Zingiberales order is named for the family Zingiberaceae, which includes ginger, cardamom, and turmeric—beautiful, wonderful, and valuable, but not trees. But two of the other seven families in the order certainly do include trees. The Musaceae are the bananas. They are giant herbs, rather than true trees, for their stems are not woody but fleshy and fibrous. Their stalks, however, are thick and tough enough to give them the form of a tree, and they live like trees. The banana genus,
Musa,
with thirty to forty species, originated in Asia, particularly in Myanmar and New Guinea; Heywood says “they are essentially jungle weeds of disturbed habitats.” Bananas are now grown worldwide, and they grow as wayside weeds throughout the tropics. The cultivated banana originated as a hybrid of two species,
M. acuminata
and
M. balbisiana.
Extraordinarily, the modern cultivars are triploid—meaning they have three sets of chromosomes, which also means that they are sexually sterile (since the parent cells from which gametes are produced must contain an even number of chromosome sets). So the fruits of the cultivated types are seedless and sterile—all form and no reproductive substance. Thus a bunch of bananas emerges botanically as an infructescence of monocot triploid parthenocarps (you never know when such knowledge might come in handy). Growers reproduce the plant itself by cloning from suckers. Other species of
Musa
provide “Manila hemp,” for ropes. Africa has its corresponding genus,
Ensete,
with six species. The exotically decorative
Heliconia
also belong to the Musaceae.

Known as “traveler’s palm”—but
Ravenala
is related to ginger.

The Strelitziaceae are exotic and alluring: they include the bird-of-paradise flower,
Strelitzia reginae,
which is not a tree, and also the traveler’s palm,
Ravenala,
of Madagascar, which very definitely is. This is an extraordinary-looking plant, again beloved of botanic gardens, with its near half circle of ragged, banana-like leaves, all springing from the top of a long, straight trunk that may be 30 meters high. The flowers, encased in tough bracts, produce an abundance of nectar—which attracts the black-and-white ruffed lemur, to whom the prodigious trunk presents no obstacle at all. As the lemurs feed on the nectar they are coated with pollen. Here we see lemur qua hummingbird, bee, or butterfly: the key pollinator.

That ends our rapid recce of the monocot trees. The remaining families of broadleaves all belong to the clade of the modern dicots—known as the eudicots.

8

Thoroughly Modern Broadleaves

J
UST TO RECAP:
Flowering plants (angiosperms) are traditionally divided into two great groups, the dicots and the monocots. The trees among the dicots are commonly known as broadleaves, to distinguish them from the monocots, like palms and grasses, which have narrow leaves. That is: broad-leaved trees are dicot trees.

But as outlined in Chapter 6, the dicots are no longer perceived as a single, coherent group. They include a mixed bag of primitive types, such as magnolias, peppers, and waterlilies, which are presumed to resemble the ancestors of all flowering plants; and they also include a particular, more modern group, a true clade, known as the eudicots. What I am now calling “thoroughly modern broadleaves” are the eudicot trees.

The eudicots are the most varied plants of all. They range from tiny floating duckweeds through a host of herbs and scramblers and climbers to some of the world’s mightiest trees. To look at them, you would see no reason to suppose that they all arose from the same ancestor, and that they indeed form a true clade. But as is so often the case in taxonomy, it’s the small, cryptic features that betray true relationships. All the eudicots have a characteristic kind of pollen that has three slits in it and is known as “tricolpate.” The eudicots have other features in common too, of course; and their DNA confirms their general relationship. But the feature that pins them down, shows them all to be of the same broad lineage, is the tricolpate pollen. Different authorities divide up the angiosperms into different numbers of families, but most agree that there are about 450, and that most of these belong among the eudicots. So there are at least several hundred eudicot families, which, you may well feel, is too many to keep track of. Fortunately, however, these families are further grouped into orders of which Judd recognizes thirty-one. This, I hope you will agree, is a manageable number.

Trees? Why not? Big cacti form veritable desert forests.

As you can see from the figure on page 130, fifteen of the thirty-one eudicot orders form a true clade, known as the rosids: they are not all particularly rose-like, but they do include the order of the roses, the Rosales, and the grouping has to be called something. Another ten form the group known as the asterids; they are named after the order Asterales, which includes the family Asteraceae, which was formerly known as the Compositae and includes the daisies. But six of the thirty-one eudicot orders belong neither to the rosids nor to the asterids, and again can reasonably be seen as primitive outliers (“primitive” being a relative term: they are not primitive relative to magnolias, but they are primitive relative to daisies). This chapter deals with the six primitive orders of eudicots. The next chapter looks at the rosids, and the one after that is a rapid survey of the asterids.

Of the six outlying orders (neither rosids nor asterids), two contain no significant trees. The Ranunculales is named after the buttercups. It does include some shrubs—for example, the barberries—and a few trees among the moonseed family, but nothing that need delay us (though one of the moonseed family provides curare, much favored for poisoned arrows and now deployed in medicine as a muscle relaxant). The other order without trees is the Polygonales, which includes the families of rhubarb and dock, and of the insectivorous sundews. The four orders of primitive eudicots that do include trees are as follows.

G
REVILLEAS
, M
ACADAMIAS
, P
LANES, AND
B
OX
: O
RDER
P
ROTEALES

The Proteales, as now defined by Judd, is highly intriguing. Of course the order includes the Proteaceae family, for which it is named. The root of the name is the same as in “protean”; it implies an ability to change shape, and indeed young Proteaceae typically have juvenile leaves that are markedly different in shape than those of the mature plants. The family contains over 1,000 species, in 62 genera, spread throughout the southern continents, with occasional encroachments into the north: South America (whence they have spread to Central America); sub-Saharan Africa; eastern India; all of China and Southeast Asia; and Australia and New Zealand. Many Proteaceae are beautiful, like the bottlebrush trees of Australia that drip with nectar:
Banksia,
and the pincushion-flowered
Hakea.
The
Protea
flowers of South Africa’s upland fynbos, huge and flamboyant and good for drying, alone justify the journey (the beauty of the landscape is a bonus). Quite a few of the Proteaceae, too, are fine trees. Among them are
Macadamia integrifolia
of Australia, source of the egregiously hard but excellent nuts, which are a staple for many traditional Aborigines and are also the only native Australian food plants of significance to world markets. The rewarewa of New Zealand
(Knightia excelsa),
which grows up to 40 meters, has juvenile leaves that are long and thin and mature leaves like a chestnut’s, though thicker and glossier. Australia’s
Grevillea robusta
is a magnificent timber tree confusingly known as the silk oak and sometimes even as the golden pine (while other trees are also sold as “silk oak”). It is also widely grown in India as a shade tree in plantations of tea and coffee, both of which grow best when not too exposed; and its leafy branches when trimmed make fine cattle fodder.

It is a taxonomic novelty to find the plane family, the Platanaceae, in the Proteales order; yet DNA studies show this family to be close to the Proteaceae. There is only one genus,
Platanus,
with about ten species, but it’s enough to leave fine scope for confusion; for to the British
Platanus
are the plane trees, while the Americans call them sycamores (though some are also called buttonwoods). To the British, the sycamore is
Acer pseudoplatanus—
meaning “maple false plane.” (Just to stir the pot a little further, the “sycamore” referred to in the Bible is a species of fig, in the genus
Ficus
.) In any case, planes make fine ornamental trees, especially in cities (they shed their bark and the soot with it), and in North America their timber has been used for everything from barrels and butcher’s blocks to fine veneers; while the Native Americans of the East used planes for dugout canoes, one of which was reputed to be 20 meters long and weighed four tons. Planes grow wild throughout the southeastern United States, and also in the eastern Mediterranean, northern India, and China. Given that Platanaceae is closely related to Proteaceae, and Proteaceae is obviously from Gondwana, the southern supercontinent, where did the Proteales as a whole originate?

The family of the box trees, Buxaceae, has not been easy to place. It has at times been linked to the rubber tree within the Euphorbiales order—although its members do not have latex, as the euphorbias do. Now it seems to have fetched up in the Proteales. Its 100 or so species (in four to six genera) stretch throughout all the tropics (apart from Australia) and through Europe as far as Scandinavia. Common box,
Buxus sempervirens,
is one of only two evergreen hardwood trees that seem to be native to Britain. (The other is the holly.) But I say “seem” because botanists have been arguing for 150 years about whether box really is a native or was simply introduced a long time ago. The box tree is not big—up to 10 meters—but it is highly prized, partly for high-class hedges (the stuff of medieval knot gardens) and also for its close-grained, pale yellow timber, favored for rulers and for wood engraving. Traditionally woodcuts were made with blocks of fruitwood (usually pear), cutting along the grain (as ’twere into a plank), while finer work—engraving—needed metal blocks. But in the late eighteenth century the English artist Thomas Bewick showed that boxwood, cut across the grain, gave results comparable with that of metal; boxwood is now the wood engraver’s staple fare (and extremely expensive).

A
N
O
DDBALL FROM
J
APAN AND
T
AIWAN
: O
RDER
T
ROCHODENDRALES

There is just one family (the Trochondraceae) in this order, which has just one species,
Trochodendron aralioides.
It comes from Japan and Taiwan and is large, with a trunk up to 1.5 meters thick. I regret I have not seen a
Trochodendron
and can throw no further light on it (but it provides yet another reason for getting into the countryside in Japan and Taiwan and checking out the extraordinary flora there).

P
RICKLY
T
REES FROM
M
ADAGASCAR AND
“T
REE
” C
ACTI
: O
RDER
C
ARYOPHYLLALES

A big grouping, with 8,600 species in 18 families, two of them including trees (the others range from carnations to sugar beets). First we have the extraordinary family of the Didiereaceae. They are unique—endemic—to semidesert in Madagascar, that long-isolated island, effectively a small continent, that has so many unique creatures, including all the present-day lemurs (and other prosimians, like the aye-aye); the fossa, a civet that looks for all the world like a slinky cat; and, probably until about the fifth century
A.D.,
the heaviest bird of modern times, the elephant bird—which is probably the roc of Arabian legend. The elephant bird laid the biggest eggs of any known creature, big as an overinflated rugby football (fragments of shell can still be found). For their part, the Didiereaceae offer yet another remarkable example of convergent evolution, for most of their eleven species (in four genera) might reasonably be compared to the swollen baobob tree, which is also at its most various in Madagascar, though is widespread in Africa. More strikingly, the trees of the Didiereaceae resemble columnar cacti, with green, swollen, prickly trunks. One of them, however,
Alluaudia procera,
Heywood describes as “a bent and thorny telegraph pole up to 15 metres high.” It is possible, I suppose, that if the historical coin had flipped differently, Europe and North America might have wound up with the Didiereaceae, and Madagascar with oaks and ashes; and then we would think that oaks were weird. Incidentally, Judd no longer recognizes the Didiereaceae as a discrete family, but includes it within the purslane family, the Portulacaceae. More work has still to be done on the DNA, however. Watch this space.

Also within the Caryophyllales order are the cacti themselves, the Cactaceae. Although they are close relatives of the Didiereaceae, they surely evolved the cactus-like form independently. Almost all cacti are American (though many—notably the prickly pears in the genus
Opuntia—
have been naturalized in warm, dry countries everywhere, and indeed were once a serious pest in Australia). Of course, most of the 1,400 or so species of cacti (in 93 genera) are not tree-like. Many horticultural favorites, like
Mammillaria
and
Notocactus,
are for the most part prickly orbs. But some, including
Opuntia
and many of the columnar
Cereus
and
Carnegia,
have woody trunks and form veritable desert forests. Most tree-like of all are the extraordinary
Pereskias,
almost certainly the most primitive cacti of all; they have green, swollen branches but also have leaves. Within the cacti, indeed, we can see how a family that began as trees (similar to
Pereskia,
and resembling Didiereaceae) has evolved several times, independently, into forms that are very non-tree-like—including the spherical types and the various epiphytes.

All in all, phylogeny never fails to intrigue. It is pleasant to contemplate that cacti and carnations, which seem such poles apart, are not too distant cousins; or that the supremely exotic Didiereaceae and the purslanes that grow as wayside weeds (and feature in traditional European salads) are of the same (extended) family. What does this say of their evolutionary history, and the forces and happenstance that took virtual siblings in such different directions?