The Tree (3 page)

So now to the third childish question.

HOW MANY KINDS OF TREES ARE THERE?

A simple question indeed—but of course there are complications. To begin with, as the more irritating kind of philosopher would say, “It depends what you mean by
kind.
”

In this context, “kind” most obviously means “species.” The common English oak is a species:
Quercus robur.
So is the Scotch pine:
Pinus sylvestris.
A common birch in Europe is
Betula pendula,
and so on. What’s the problem?

One problem involves how you tell the different kinds apart. Any one species is liable to be highly variable, and sometimes different species resemble each other very closely. Sometimes there is more variation
within
species than there is between species. Or then again: many creatures can be identified definitively only by their reproductive organs, which in the case of flowering plants (including most trees) means flowers. But many trees are not in flower at the time you come across them—a particular problem in the tropics, where flowering often seems to be erratic (or perhaps the tree knows when it is appropriate to flower, but the biologist does not). Some trees with similar flowers have different leaves, and both may be needed to make the identification. Willows, however, tend to produce their flowers before they produce leaves—so you never find flowers and leaves on the same tree at the same time. If you want to know what species a particular willow belongs to, you may have to make two visits.

But biologists do not define species purely in terms of what they look like. Much more fundamental, they very reasonably feel, is who mates with whom. If different individuals breed together, then it is reasonable to declare that they are of the same species.
Betula pendula
will happily breed with another
Betula pendula,
but not with
Quercus robur.
So they are different lineages of creatures, living separate lives. Easy.

Still, there are snags. Many species can and do interbreed with other species, and so form hybrids. The example that everyone knows is the mule: the issue of a male donkey and a mare. But horses and donkeys seem to be very different kinds of animals. If they can breed together, doesn’t this mean they are of the same species? No—for although the mule is a powerful animal and stubborn, as cowboys were wont to complain, it is nonetheless sexually sterile. Strong though it is, it is not, as a biologist would say, “viable.” So we can extend our definition slightly: “Two or more individuals can be considered to be of the same species if they can mate together to produce
fully viable
offspring.” “Fully viable” implies sexual potency; it also implies that the offspring should be able to compete successfully in the wild. There are some hybrids (for example, among frogs) that are sexually fertile yet generally fail in the wild, unable to compete with either of their parent species. Again, it is reasonable to rank the parent types as separate species, since the hybrids they produce between them are (relative) failures.

Still, there are problems. For example, two apparently different species that look different may fail to interbreed in the wild simply because they live in different places. Bring them together, however, and they may interbreed perfectly happily. Trees provide scores of examples, among oaks, willows, poplars, and many more. Many hybrids have arisen in gardens, where human beings bring plants from very different areas together, perhaps for the first time in many thousands of years. Among the most striking examples is the London plane,
Platanus x acerifolia
(the x indicates its hybrid status). It is tremendously successful in temperate cities everywhere. Because it sheds its outer bark (as a eucalyptus or a madrone does), it gets rid of all the soot and other pollutants that can make life so difficult for many other kinds of tree. It is the offspring of the Oriental plane from southern Europe and Turkey,
Platanus orientalis,
and the sycamore from eastern North America,
Platanus occidentalis,
and arose, so tradition has it, in the Botanic Garden of Oxford University, in the seventeenth century. An offspring of the first-ever London plane stands in a courtyard in Magdalen College, which is next to the Botanic Garden. That offspring, now several centuries old, is huge. For those who would be connoisseurs, it is well worth a diversion (assuming the porters will let you in). Of course, once novel trees appear in botanic gardens, botanists and gardeners all over the world flock to get hold of them. The London plane flourishes everywhere in the temperate world because it resists pollution so well—not least in the native countries of its parent species, in Asia and the United States.

Then there is the extremely important phenomenon of polyploidy. Genes, as everyone knows these days, are aligned along chromosomes. Every kind of organism has its characteristic number and arrangement of chromosomes. Eggs and sperm (or the appropriate cells in ovules and pollen) contain only one set of chromosomes, and are said to be “haploid.” When they fuse in the act of fertilization, the resulting embryo has two sets of chromosomes and then is said to be “diploid.” Most organisms (at least of the most familiar kinds) are diploid: for example, human beings have forty-six chromosomes—twenty-three acquired via the egg of the mother and twenty-three from the sperm of the father. Chimpanzees have forty-eight chromosomes, twenty-four from each parent.

Sometimes, however, apparently spontaneously, the chromosome number will double. (The chromosomes divide in the normal way they do in preparation for cell division, but then the cell fails to divide.) Thus the diploid cell becomes tetraploid, with four sets of chromosomes. This does not apparently happen much in animals (or not, at least, in mammals), but it is extremely common in plants. The newly formed tetraploid organism can breed successfully with other tetraploids of its own kind, but it cannot usually breed successfully with either of its parents. So it forms an instant new species. Many plants in nature turn out to be tetraploid, and many more tetraploids have been produced in cultivation. The common potatoes grown in Europe are tetraploid derivatives of diploid potatoes that grow wild (and are cultivated) in the Andes. Many trees, wild and cultivated, are tetraploid. Sometimes the chromosomes of the tetraploid plant double again to produce octoploids. These octoploids form new, discrete species—generally unable to interbreed with the tetraploid parents who gave rise to them. “Polyploid” is the general term that describes any organism with more than two sets of chromosomes. Sometimes the complications become too much even for the plants and they end up with an odd number of chromosomes (some having been lost among all the cell divisions and matings). Plants with anomalous numbers of chromosomes are said to be “aneuploid.” Aneuploidy in animals generally leads to various degrees of disorder; aneuploid animals usually die, and if they live they tend to be compromised at least to some extent. But many plants put up with aneuploidy. Sugarcane is aneuploid; but that doesn’t stop it from being an extremely vigorous, major crop.

There is one further complication. As we have noted, diploid organisms that are of different species sometimes mate to produce fully viable offspring (as the eastern and the western species of
Platanus
evidently did). But usually such crosses fail, and often this is because the chromosomes of the two parents are incompatible. The two different sets of chromosomes might be able to support body cells that work well enough (as in the mule). But even if cells with two different kinds of chromosomes succeed this far, they will not necessarily produce sound gametes (eggs and sperm or ovules and pollen), because this requires close cooperation between the chromosomes.

Yet if a hybrid organism doubles its chromosomes, it often
can
produce viable gametes. So we find diploid parents of different species mating to produce diploid hybrid offspring that are sterile; but the hybrids then double their chromosomes and become tetraploid—and the hybrid tetraploids are fertile. This happens a lot among plants, and has produced many, many new plant species, both in the wild and in cultivation. Indeed, the complications seem endless. For instance, a tetraploid plant might mate with a closely related diploid plant to produce a triploid offspring—two sets of chromosomes from the tetraploid parent, and one set from the diploid parent. Triploids are sterile—they cannot produce gametes at all—but they may still form viable plants. Thus the cultivated banana is triploid. Because it is sterile, its fruits contain no seeds (as wild banana fruits do). So the domestic banana has to be reproduced vegetatively, by planting cuttings. In other cases, though, triploid hybrids double their chromosomes to become hexaploid (with six sets of chromosomes). The most famous and important hexaploid organism of all is bread wheat (as opposed to pasta wheat, which is tetraploid).

If you have been brought up with animals and are innocent of botany, you may find all this fantastical. But among trees, hundreds of examples of polyploids are now known; the more that botanists look, the more polyploids they find. Some of the polyploids simply represent a doubling (or redoubling) of chromosomes within one species. Others are polyploid hybrids. For good measure, breeders have produced many hundreds of polyploids by artificial means. (Some chemicals induce polyploidy almost to order.)

Willows, genus
Salix,
provide many fine examples of polyploid trees. There seem to be around four hundred species—although there must be many more that are still unknown, including an entire phalanx in western China, yet to be properly studied. Some willow species have a haploid number of 19 chromosomes, so that the diploids have 38 (2 × 19). But another group of willows has a haploid number of 11 (diploid 22), and the third group has 12 (diploid 24). There doesn’t seem to be much hybridization between willows with different haploid numbers, but there is a great deal of hybridization between different species with the same haploid number, and this has produced a whole array of polyploids, some with as many as 224 chromosomes. Most of those polyploid hybrids are fertile, and some willows have been bred artificially from combinations of up to fourteen different species. For good measure, many of the hybrids are all of one sex and reproduce by suckers, so that all the members of such “species” in fact form a clone (of which more later). Thus, the hybrid known as
Salix x calodendron
is all female. Many willows, too, both wild and in cultivation, are aneuploid. All in all, identification of the multifarious willows—the diploid types and all their polyploid hybrids—is a nightmare (even when they are not tucked away on some remote Chinese hillside).

Acacias show a similar picture. Acacias are those lovely, lonely, sprawling trees of tropical grasslands worldwide that provide such essential shade and fodder for giraffes, camels, gazelles, and the domestic cattle and goats of nomadic pastoralists.
Acacia
is a huge and messy genus, with 1,300 species—which should probably be further subdivided, perhaps into five or more different genera. Be that as it may, the basic haploid number of the whole group is 13, so the default diploid number is 26, but there are polyploids with up to 208 chromosomes—sixteen times the haploid number. In some of these, it’s clear that the ancestor simply doubled (and then sometimes redoubled) its chromosomes. Other acacias arose as polyploid hybrids.

In birches, the haploid number is 14, so the diploid number is 28—but some species have up to 112 chromosomes, which means they are octoploid: and there are some aneuploid hybrids in cultivation. In northern Europe, the silver birch,
Betula pendula,
and the downy birch,
B. pubescens,
can look very similar, and some botanists have suggested that they are the same species. But the silver birch is a diploid, with 28 chromosomes, and the downy birch is a tetraploid, with 56. The downy birch presumably arose from the silver birch, but now, following polyploidy, it is very clearly a separate species. Alders, too, show much the same kind of thing. Clearly the variety depends in part on past hybridization of what had been separate species.

How many more species of trees will turn out to be hybrids or polyploids or fertile polyploid hybrids? Another century or so of serious study will throw a great deal more light. Science takes time.

There is one final set of complications. If different populations of trees become isolated one from another, eventually they may evolve into separate species. But in the shorter term, the separated populations may remain similar enough to breed easily together—that is, they are still the same species—and yet become genetically distinct to some extent, and may look different. Then biologists say that the two populations are different “races” or “varieties” of the same species; and if the variety is really distinct, they may call it a “subspecies.” In Great Britain, varieties of plants that arise through informal selection on traditional farms are called “landraces.” Domestic varieties of plants that have been produced through formal breeding programs are called “cultivars,” for cultivated variety (and domestic races of animals are called “breeds”).

Sometimes, both in the wild and in domestication, “variety” simply means a subset of the species. Among domestic crops, the different varieties of runner beans are of this kind: subsets of the runner bean species as a whole, but breeding sexually (by seed) and genetically still diverse. Many plants, however, also reproduce vegetatively as well, by means of bulbs or tubers—or, as with many trees, by suckers from the stem or roots. A tree produced vegetatively in the wild may remain attached to its parent so that parent and offspring together form an entire copse (as in English elms or groves of giant redwoods). Indeed, the parent tree and the offspring that grow from its suckers may cover many acres, as with the aspens of Canada. Growers and foresters often reproduce their favored trees by cuttings, which of course they separate from the parent. Whether they are separated from the parent or remain attached, all the offspring that are produced vegetatively are genetically identical with one another, and with their parent (who, of course, is a single parent). All the offspring are then said to be “clones” of one another and of the parent; and the whole genetically identical group is collectively called a clone.