The Tree (23 page)

Many legumes are grown as ornamentals. The suburban favorite is
Laburnum.
The tropical American
Enterolobium
genus includes the huge-leaved
E. cyclocarpum,
known as “monkey ears” or sometimes as “elephant’s ear.” The round-leaved Judas tree is
Cercis siliquastrum.
The forty or so species in the genus
Parkia
that grow widely in the tropics are glorious umbrella-shaped forest trees that hold out their flowers to be pollinated by bats and dangle their bright fruits to be dispersed by birds. The Malaysians eat the seeds of
Parkia
as petai, which has a strong flavor of garlic (which persists in the urine). The 400 or so species of
Mimosa
provide great benison, from thorn fences to their much-admired pom-pom flowers. The rain tree of India is
Samanea saman.
For reasons best known to itself, the rain tree seems to encourage epiphytes—which most trees seem to go to some lengths to get rid of. Indeed, it is often grown for its epiphytes. It also harbors the lac insect, which periodically sprays the ground with water (or so it seems); and this, presumably, gives rise to the tree’s common name. The asoka,
Sarraca indica,
is planted around Buddhist and Hindu temples, where its yellowy-red blossoms are religious offerings. Asokas are said to blossom more vigorously when given a good kicking by young women. Don’t we all.

Many other leguminous trees serve many more workaday functions. The 44 species of
Prosopis
are generally resistant to drought and salt—which makes them promising candidates for the many million acres worldwide now spoiled by salinity, brought about by overzealous irrigation.
Prosopis
includes the mesquites:
P. glandulosa
is the North American kind, favored for its aromatic charcoal that adds flavor to the barbecue.
P. cineraria
of tropical India also provides charcoal, plus firewood, fodder, green manure, and goatproof thorny fences. The extraordinary
P. tamarugo
of Chile is slow-growing but widely planted for its resistance both to salt and to very low rainfall.

Many legumes are toxic and many are medicinal (these are two sides of the same coin). One of several “ordeal” toxins used in Africa in various kinds of initiation rites is the famous red bark from
Erythrophleum.
The fifty-two species in the genus
Sophora
provide hardwoods but also toxins and medicines: the Japanese pagoda tree,
S. japonica,
has been cultivated in China for more than three thousand years for its beauty, and for dye and medicines.

Many leguminous trees provide food. Several species from the huge tropical genus
Inga
have edible fruits and seeds, including the ice-cream bean,
I. laurina.
(The genus
Inga
is huge. Exactly how many kinds there really are, at least in tropical America, is the subject of research discussed in Chapter 12.) Tamarind is the fleshy fruit of
Tamarindus indica,
which adds astringency to curries and pickles, of which the Indians tell a charming story: A man set out on a long journey, but his wife didn’t want him to go. So she asked the local guru how she might hasten his return. “Make him promise,” said the guru, “to sleep every night under a tamarind tree on the outward journey, and to lodge beneath a neem tree every night on the way home.” The man kept his promise. But tamarind trees exude toxic vapors (or so it is claimed) and make you feel ill; while neem trees are restorative. So the farther the man traveled, the worse he felt; and as he got nearer to home again, he felt better and better. There are many morals in this tale, beyond doubt. (The neem or nim is from the mahogany family, Meliaceae, of which more later.)

It would be easy to fill several volumes with leguminous trees. But we should move on.

A
PPLES
, P
LUMS
, E
LMS
, F
IGS
,
AND
C
ECROPIAS
: O
RDER
R
OSALES

The Rosales has been radically reorganized these past few years, thanks mainly to molecular studies. Judd recognizes seven families—all of which have interesting trees and deserve discussion. But the relationships between those families, and the plants that they include, are often surprising—and the modern taxonomy differs enormously from most traditional treatments of only a decade or so ago. The position is still fluid because in many ways the Rosales is especially tricky, with enormous morphological variety on the one hand and a strong tendency to hybridize on the other, which sometimes makes it hard to tell where one group ends and the next begins. But the following reflects the state of play.

Fittingly, the most primitive of all the Rosales families, apparently the sister to all the rest, is the Rosaceae, with its simple round flowers evolved to attract generalist insect pollinators—flies for the smaller kinds, bees for the bigger ones, and long-tongued moths for the biggest. Among the 3,000 species of the Rosaceae are many lovely and useful herbs, like cinquefoil and strawberry. But most (three-quarters) of the 85 genera include woody plants. Some are mainly shrubby, as in the roses
(Rosa),
or the blackberries, raspberries, and loganberries
(Rubus).
But many are very significant trees.
³

All of the most important temperate tree fruits are from the Rosaceae family. The apples are the genus
Malus.
All the hundreds (actually, thousands) of varieties that are generally eaten are variations on a theme of
M. domestica,
which, so recent studies in Oxford confirm, has been selected over many a century from the wild Asian
M. sieversii.
The various wild ancestors had small fruits, like modern crab apples, but as cultivation spread westward they became bigger, until by the Middle Ages we had recognizably modern fruit. Domestic varieties of apples stay constant from generation to generation (or century to century) because they are reproduced as cuttings and grown by grafting desirable varieties onto robust rootstock. Thus all the Cox’s Orange Pippins in the world, for instance, are a clone of the first ever Cox that was bred in the nineteenth century. Several other
Malus
species are kept as ornamentals. The 76 species of pears
(Pyrus),
are close relatives of apples; some species are grown for fruit, some for ornament (some have pleasantly silvery foliage, for instance, sometimes weeping), and valuable smooth pale-golden timber for much-prized kitchen furniture and for woodcuts. Pear trees can grow big: 18 meters tall, 5 meters in girth (about 1.5 meters in diameter).
Cydonia
is the quince and
Eriobotrya
is the loquat.

The genus
Prunus
abounds with good things.
P. dulcis
is the almond.
P. armeniaca
is apricot.
P. avium
is the sweet cherry (known as the bird cherry when wild) and
P. cerasus
the sour cherry.
P. persica
is the peach and
P. domestica
the plum. Many
Prunus
species are grown as ornamentals—notably the Japanese flowering cherries—and several are noted for their fine reddish timber. In the wild,
Prunus
and
Crataegus,
the hawthorn, are early to put in an appearance as new forests establish themselves, although
P. serotina,
the black cherry, grows on in mature deciduous forest.
P. avium,
the bird cherry, grows widely in Europe, including Britain, where it is apparently native.

Other ornamental trees (or near trees) include
Amelanchier,
which the Americans call the serviceberry or shadbush; the flowering quince
(Chaenomeles); Cotoneaster,
always known as cotoneaster; the hawthorn,
Crataegus,
otherwise known as the quickthorn or may tree; the firethorn
(Pyracantha);
the roses, of course, in the genus
Rosa—
many thousands of cultivars hybridized from about nine wild ancestors;
Sorbus,
which includes the mountain ash, or rowan, and also the whitebeam; and the florists’ favorite,
Spiraea.
Some have other uses, too. Hawthorn in particular is Britain’s favorite hedging plant, layered—the branches cut halfway across, then laid sideways—to form an impenetrable thorny barricade—or sometimes left to grow into a big mature tree in the hedge, as elms often were.

Five closely related genera within the Rosaceae (though none of the important ones mentioned above) have developed symbiotic relationships with nitrogen-fixing bacteria that live in nodules in their roots. The bacteria are not
Rhizobium,
as in the Fabaceae, but
Frankia.
In all, plants from about ten families harbor nitrogen-fixing
Frankia
in nodules in their roots (one of the chief of which is the alder,
Alnus
). It would be tempting to suggest that the Fabaceae also started out with
Frankia
in their roots but that these were later displaced by
Rhizobium.
Only if this were so, we would expect the most primitive Fabaceae to harbor one or other of the two bacteria. In fact, the most primitive Fabaceae do not have nitrogen-fixing bacteria at all. Thus it seems that nodules to harbor
Frankia
and nodules to harbor
Rhizobia
are independent, parallel inventions—yet another, stunning case of convergent evolution. We also see, yet again, the propensity of organisms—one might almost say their eagerness—to cooperate.

Then there are the Rhamnaceae, the family of the buckthorns: 850 species in 45 genera: often thorny; some trees, some shrubs, some climbers; and, again, sometimes with nitrogen-fixing bacteria in their roots. We are familiar with a few Rhamnaceae in the temperate north: buckthorn is
Rhamnus; Ceanothus
is a highly fashionable ornamental. But the Rhamnaceae come mainly from the tropics, where many are useful.
Hovenia dulcis
is the raisin tree.
Ziziphus jujuba
is the Indian jujube. The jujube grows fast on dry, poor land to form red-flowered trees that are often scrubby but can reach 24 meters. Its timber is good, and it burns well; its prickly branches make serviceable fences; its leaves and twigs are fodder for camels and goats; its wild green fruits make sherbet, sold in the markets and (it’s said) much loved by students; and it is cultivated for its fruit, used for seasoning, cooked with sugar, or stored in oil or sugar syrup. Perhaps most of all, though, the jujube is a fine host for the lac insect, which sucks its sap and exudes a reddish resin over the whole surface of the twigs—which yields a dye and also becomes shellac, once used for gramophone records and still favored for polishes and for lacquer. The jujube illustrates a general principle: how much use is made of plants that outsiders would scarcely notice, by people who know about them; how entire economies and cultures can flourish under our noses without us noticing; and how easily and often those ways of life are swept aside—for what developer would care about the wild jujube trees? The lac insect also feeds on the peepul, the rain tree, and the mango.

Both the Rosaceae and the Rhamnaceae families are at the edge of the Rosales order, however. The remaining families all seem to group roughly together in one great clade. And what families they are.

First come the Ulmaceae, the family of the elms
(Ulmus)
and the favored park tree
Zelkova.
There are 6 genera, and about 40 species, all trees, mostly in the temperate north. Elms until recent times were so common in England they largely defined the lowland landscape: they dominate John Constable’s Suffolk landscapes in the east, and in the west were known as the “Wiltshire weed.” They commonly grew in hedges and formed fine trees whose timber was often used in great slabs—for example, to make the buttock-molded seats of rural wooden armchairs and the sides of wheelbarrows. In the 1970s, Britain’s elms were struck down by Dutch elm disease, caused by the fungus
Ophiostoma novo-ulmi
and carried and introduced beneath the bark by bark beetles of the genus
Scolytus.
Within a few years, despite the best efforts of foresters and biologists, mature elms had all but disappeared. The original types hang on as hedgerow bushes—but as soon as they reach a critical height, within the beetle’s flying zone, they are attacked again and die off. New resistant strains are being developed, but England’s lowlands will never be the same (although, of course, the transformations wrought by urbanization and agribusiness are far more dramatic). Dutch elm disease occurs on mainland Europe and in America as well, where the fungal pathogen is carried both by
Scolytus
and
Hylurgopinus.

There are also the Celtidaceae, which include the hackberry or sugarberry
(Celtis),
whose colorful fruits are for the birds that disperse them, and are rarely eaten by humans. But hackberry is used for timber and grown as an ornamental.

Then comes a huge and supremely important family: the Moraceae. Its 53 genera (1,500 species) include shrubs, climbers, and herbs—but also some intriguing and supremely important trees that grow throughout the tropics as key players in rain forests. The jackfruit or breadfruit
(Artocarpus)
has a pale grayish-green warty-skinned fruit that is really a fused mass of fruits (an infructescence) and may be huge: as big as a sack of coal and up to ninety pounds in weight.
Brosimum
is the breadnut. There are some fine timber trees, too: the iroko
(Chlorophora excelsa)
from central and western Africa is often used as a substitute for teak. The snakewood
(Piratinera guianensis)
from tropical America is extremely heavy (much heavier than water when dried), with a black-brown tortoiseshell pattern favored for everything fancy, from the backs of brushes and umbrella handles to violin bows—and for native bows for shooting arrows. More temperate is the mulberry
(Morus):
the white kind grown to raise silk moths (nine thousand pounds of leaves for one silk blouse), the black kind favored for its glorious edible blackberry-like fruits, popular from the seventeenth century and now featured, gnarled, in many an old walled garden.