The Tree (17 page)

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.

B
LACK
P
EPPERS AND
W
HITE
P
EPPERS
: O
RDER
P
IPERALES

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).

7

From Palms and Screw Pines to Yuccas and Bamboos: The Monocot Trees

I
F 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.

J
OSHUA
T
REES AND
D
RAGON
T
REES
: O
RDER
A
SPARAGALES

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.

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.

T
HE
“S
CREW
P
INES
”: O
RDER
P
ANDANALES

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.

T
HE
P
ALMS
: O
RDER
A
RECALES

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.