by Colin Tudge
Rimu, Totara, Kahikatea – the Most Various Conifers of All: FAMILY PODOCARPACEAE
The Podocarpaceae family currently has eighteen genera, but these are under-researched and difficult to classify not least because their cones are much reduced and hard to decipher (in most conifers it is the cones which are most informative). With more study (particularly of DNA), the Podocarpaceae might be split further. Already, however, they are the second largest family of conifers (after Pinaceae), with 185 species.
All of the genera are found in the southern hemisphere, but only half of them venture into the north. In the northern hemisphere podocarps extend from the Andes into Central America and the Venezuelan highlands, but they are most widespread in Malaysia, Indonesia, Indochina, and subtropical China north into southern Japan. It is not clear that the Podocarpaceae family as a whole originated in Gondwana (though this is the best bet) but its biggest genus, Podocarpus, almost certainly did. Fossils of this family are known from the Jurassic onwards (about 140 million years ago), all from the south.
Both in form and ecology the podocarps are impressively various. Most are trees, scattered in moist tropical or subtropical forest. Some help to form the understorey, others join the canopy, and some tower above the rest as ‘emergents’. Some grow in mossy forests in the highest tropical mountains. Some – especially in the far south – form low shrubs well above the treeline. Many grow on poor soils, including swampy peat, but others compete with angiosperms where nutrients are more abundant. Many, including Podocarpus, have broad evergreen leaves that are unlike most living conifers, and fleshy cones of various shapes and colours that again are much under-researched but are evidently dispersed by mammals and birds. Thus Podocarpus plays the angiosperms at their own game. The extraordinary island of New Caledonia as usual has the greatest oddities. There, Retrophyllum minus grows in running water – which is a most unusual way for a tree to behave. Parasitaxus usta from New Caledonia is the only parasitic conifer: it taps into the roots of another podocarp, Falcatifolium taxoides.
Podocarpus includes 107 living species, though more are likely to be found in remote tropical forests. On the other hand, says Dr Farjon, ‘revision of the genus is long overdue’, which means that it could well be divided (and Podocarpus itself will be reduced). Podocarpus is the only genus of podocarps to occur on most of the southern continents and major islands, especially Borneo with thirteen species and New Guinea with fifteen. Six are known from the Venezuelan highlands – but there is much exploration yet to be done.
Podocarps are key players in New Zealand’s ecology, and were vital to the religion and economy of the Maoris, and later prized by the Europeans. The tallest of all New Zealand’s trees is the kahikatea, Dacrycarpus dacrydioides, at nearly 60 metres. In the early nineteenth century, soon after they decided that God had obviously made New Zealand for their personal use, the British coveted the fluted, flakey-barked trunks of kahikatea as masts for their swelling fleet: warships to counter Napoleon; cutters for fast reliable trade overseas. But the timber of kahikatea is weak. It was a great disappointment. Nowadays, relationships between the Maoris and the Europeans, though far from disastrous, continue to be edgy. Recently air traffic control demanded that a lofty row of kahikateas near Rotorua airfield should be trimmed. The Maoris said no way. Rotorua is definitely Maori country. The last I heard the airfield may have to be moved.
Though it disappointed the British navy, the kahikatea is a fine tree. As in many plants, the juvenile and mature leaves are different: the former superficially yew-like in two rows, the latter fleshy and roughly cypress-like. When the female cones mature the receptacles grow red and berry-like, each with a purple seed at its tip, so the two together are like Russian dolls.
The matai is better for timber – known to the timber trade as ‘black pine’, and to botanists as Prumnopitys taxifolia. P. ferruginea is the miro, alias ‘brown pine’. The ‘mountain pine’ and the ‘yellow pine’ belong in yet another genus, Halocarpus, while the ‘silver pine’ is Lagarostrobus (and the ‘yellow silver pine’ is Lepdothamnus). But perhaps the greatest of these misnamed ‘pines’ is the rimu, aka ‘red pine’, Dacrydium cupressinuum. It is no longer cut, but existing planks and joists are rescued for furniture and household goods of all kinds, like bowls and salad servers.
To the Maoris, however, the greatest of all trees was the totara, Podocarpus totara (with the smaller upland version, P. cunninghamii). P. totara grows on both islands up to 40 metres, with trunks 2 metres across. The Maoris revered it, not simply for its general magnificence but for the red of its timber, the colour of royalty. They also hollowed out the trunks to make from a single piece of timber canoes to be paddled by a hundred men.
The Celery Pines: FAMILY PHYLLOCLADACEAE
The single genus Phyllocladus, with just four species, is commonly included within the Podocarpaceae. These are the ‘celery pines’, which grow in Malaysia, Indonesia, New Zealand and Tasmania, usually as large canopy trees, though stunted up towards the treeline in mountain cloud forests. They are called celery pines because their leaves are celery-like, though fleshy; not what you would associate with a conifer. Botanically speaking, however, these are not leaves at all. They are ‘phyllodes’: flattened green stems that do the job of leaves. The true leaves have gone missing. But such vegetative features are not usually considered of huge taxonomic significance. True evolutionary relationships are more reliably revealed by sexual characters, which with conifers means cones; and these, and the fruit-like bodies that develop from them after fertilization, are very like those of podocarps. But Phyllocladus was first placed in its own family in the 1960s; and although many taxonomists do not favour this separation (including Judd), Aljos Farjon feels that the phyllodes are such a striking feature that they justify separation; he also points out that the celery pines have a different number of chromosomes from the Podocarpaceae; and their mechanism for pollination is also distinct. The discussions will doubtless continue. More data are clearly needed, not least from DNA.
A One-off from Southern Japan: FAMILY SCIADOPITYACEAE
Here is another very small family – indeed with just one species: Sciadopitys verticella. It’s an evergreen conifer endemic to southern Japan, where it grows on steep slopes and ridges, sometimes in clumps and sometimes mixed with broadleaved trees. It competes because it can tolerate such poor soils.
The Sciadopityaceae are an ancient group – fossils that possibly belong to this family are known from the upper Triassic, around 200 million years ago, before the dinosaurs got fully into their stride. Traditionally it has been included in the Taxodiaceae (which means it would now be in the Cupressaceae). But its needle-like leaves are not leaves at all – again, they turn out to be phyllodes. Also, studies of structure and of DNA support the notion that the remarkable Sciadopitys indeed belongs in a family of its own.
The Yews: FAMILY TAXACEAE
In the family Taxaceae are five genera, with twenty-three species between them. Mostly these are in the northern hemisphere, but there are a few in the south including – inevitably it seems – in New Caledonia. The only widespread genus is Taxus, the yew, with ten species found throughout North America and down into Honduras, and in Eurasia down into the southern hemisphere in Malaysia–Indonesia. The British are used to common yew (Taxus baccata) lowering gloomily in dank churchyards. Yet it lives in the tropics too, albeit confined to mountainsides. Toreya is less widespread, but also occurs in Asia as well as North America. Trees of the Taxaceae are slow growing and long-lived – hence the somewhat speculative though not to say outlandish claim in the preface, that Pontius Pilate might have dozed beneath one that is still growing in Scotland. Taxaceae seem doleful by nature: adapted to the shady understorey in coniferous or mixed forests. The fruit-like arils, of various colours, are dispersed by birds. Like the podocarps, the group as a whole has not been studied enough. In the past, because of the peculiar and much-reduced female cones, yews were thought to form a group on thei
r own, separate from the conifers. But closer study suggests that their cones have evolved from more complex seed cones and this, plus evidence from DNA, confirms yews as bona fide conifers.
The wood of yew is valuable in many ways. In particular, it provided the medieval English with their longbows.
It would be good to devote this entire book to conifers. They are so various and wondrous and in many ways, in many contexts, they have turned the course of human history. Yet they are at least matched and in some ways for outstripped by the flowering trees that will occupy the next five chapters.
6
Trees With Flowers: Magnolias and Other Primitives
Beautiful but simple: is magnolia the most ancient flowering tree?
There is more to flowers than meets the eye. Of course they may be beautiful. They are also, beautiful or not, superb essays in engineering, wonderfully efficient first at achieving fertilization, and then at producing and dispersing their seeds. The archetypal flower is supported by a circle of sepals, usually green, that make the calyx; inside that is a circle of petals, which between them form the corolla; and in the middle are the sexual parts – the male stamens, with the anthers at the tips, containing pollen; and the female carpels. Each carpel has an ovary with its ovule (or ovules) inside, a projecting style, and is tipped by the stigma which receives the pollen. Immediately we see a key difference between angiosperms and all other seed plants. In the angiosperm the ovule is completely enclosed within the ovary, and the male gamete (reduced to a nucleus) is carried to it via a pollen tube that must burrow through the full length of the tissue of the style. In conifers and all other seed plants, the ovule is not completely enclosed. The pollen tubes do not have to burrow through living tissue. In cycads and ginkgoes, motile sperms do the fertilizing.
Then there is another key difference – much less obvious. Uniquely, angiosperms practise ‘double fertilization’. As noted earlier, the pollen contains the male gamete – and other cells too. So too does the ovule – a true egg cell, and also subsidiary cells. In all but the most primitive angiosperms, the male gamete fuses with the egg to form a new embryo, as in all other organisms that practise sex. But a second cell in the pollen fuses with two of the cells in the ovule to form a combined cell with three sets of chromosomes; and this peculiar triploid cell then multiplies to form a food store, rich in carbohydrates, protein and often fat, that surrounds the embryo. Double fertilization is a very neat trick – and unique to angiosperms.
The unique ability of angiosperms to take so many different forms – mighty trees and creepers, tiny floating duckweeds, and everything in between – seems at first sight to have nothing to do with the innovations of flowers and seeds and fertilization. But it happens anyway. Perhaps the more sophisticated devices of sexual reproduction simply allow new ways of living.
Flowers are immensely variable. Some are huge and showy, others cryptic to the point of invisibility. Some have all the standard parts – sepals, petals, stamens and carpels – but others have abandoned one or several of the basic components, and some incorporate various bracts (modified leaves) and other structures. In some the petals are green like sepals, and in others the sepals are coloured like petals, and in some the two kinds of structure are more or less indistinguishable. Most flowers are hermaphrodite (like those of magnolias) while others are single sex; and some ‘monoecious’ angiosperms keep both sexes of flowers on the same plant (like oaks) while other ‘dioecious’ types have only one sex per plant (like holly). The sexual organs themselves take many different forms (as is also true of animals) and are key features for identification.
Many flowering plants are pollinated by various kinds of animals – not just insects, but also bats, birds (such as hummingbirds) and sometimes mammals, from fruit bats to giraffes (or so it is said). Best known among the insect pollinators are bees, butterflies and moths, but flies and beetles are important too. Beetles were probably the first insect pollinators of all – and probably developed the trick in association with cycads. The brash new angiosperms lured at least some of them away.
But many flowering plants, including most temperate trees, are pollinated by wind. Some – probably more than is yet appreciated – make some use both of animals and of wind. In a few, like the sea-grasses, which flower under water, and perhaps some plants of the mangroves, the pollen is conveyed by water. In general, the animal-pollinated flowers are showy and the wind-pollinated ones much less so. But flies, for example, hunt largely by scent and pollinate many very inconspicuous flowers (like those of the garden shrub Fatsia, a close relative of ivy), while wind-pollinated flowers often take the form of catkins, which can be very spectacular indeed.
In the plants that are considered to be most primitive, all the petals are much the same, all the sepals are much the same, and so on; and all the parts of the plant are separate. So the flower consists simply of repeated modules, arranged in spirals like the scales of a cone (although flowers are not, just to emphasize, botanically related to cones). The whole is a kind of Kiddicraft flower. Magnolias and waterlilies are of this primitive type. Such flowers are radially symmetrical: symmetrical whichever way you look at them. In other flowers the different parts may be fused and extended in all kinds of shapes, and are commonly designed to attract very particular kinds of insects, and then to ensure that the visitors are well furnished with pollen. In such types (and many wind-pollinated types) the flowers are bilaterally symmetrical, like a face, or indeed are asymmetrical. Orchids are perhaps the most famous floral elaborators; but many other families have elaborate, specialist flowers too, including the pea family Fabaceae (Leguminosae), which includes many of the world’s most significant trees. But primitive flowers can also trap insects in very clever ways. ‘Primitive’, after all, is not a pejorative term. It merely means ‘close to the ancestral state’. Daisies, incidentally, may look primitive (radially symmetrical, repeated parts). But in truth their flowers are complex inflorescences (collections of flowers), each of which is highly modified. So the daisy family, the Asteraceae (formerly called the Compositae) is commonly considered to be the most ‘advanced’ of all. Perhaps unsurprisingly, though in truth for different reasons, the Orchidaceae and the Asteraceae both include a spectacular number of species – far more than any other family. Of the two, however, only the Asteraceae has trees – though not many; and little to write home about. The family with the most trees (as well as many of the grandest), is the Fabaceae.
The fruits of flowering plants, containing the seeds, are also immensely variable. Some are big, bright and fleshy and are intended to be spread by animals. Some (notably orchids) are tiny and wind-blown. Some wind-blown types are bigger but are fitted with wings. This is the case in many trees – such as the familiar sycamore and ashes, but perhaps most spectacularly in the dipterocarps, the great forest trees of South-East Asia, which South-East Asians at least consider to be the most important tropical trees of all. Other airborne seeds are fitted with cottony extensions that serve as parachutes. We all know dandelions and groundsel. In Yorkshire I have driven through blizzards of migrating thistle seeds that seemed to go on for miles. Cotton, too, of course (a relative of the hollyhock) produces its fibre as a way of spreading its seeds. Among trees, perhaps the most spectacular and significant parachute seeds are those of the various kapoks – the cotton used to stuff mattresses and parkas. Some fruits, notably the coconut and the giant coco de mer, are expressly designed to float.
So various are flowering plants, it has often been suggested that they cannot be a single group: not a true clade, with just one common ancestor. But they clearly are. All but the most primitive practise double fertilization, and that is so weird, and so complicated, that it surely could not have evolved more than once. So flowering plants, for all their marvellous variety, are a single invention, all derived from one common ancestor, which arose about 145 million years ago. Truly they are one of nature’s greatest inventions.
But it is not clear who the c
ommon ancestor was, or what it looked like. There are two ways to go about finding out. One is to look at the fossil record, to try to see what the very first flowering plants looked like. The other approach is to look at living plants, decide which are the most primitive, and then assume that the very first flowering plants must have looked roughly like the still-existing primitives. (The latter exercise is helped by applying cladistic principles to the DNA, and inferring which plant’s DNA is the most basic, with the most shared primitive features. But the details need not delay us.)
Both approaches are necessary, but both are riddled with traps. The fossil record is notoriously patchy (or ‘spotty’, as the palaeontologists tend to say). We are most unlikely to find the first organisms in a new lineage because, obviously, new lineages begin with just a few individuals – and rare organisms have scant chance of being fossilized, and then recovered. More generally, if we fail to find what we are looking for, that does not mean that our quarry did not exist. But at least if we do find something, that shows that it did exist. In reality, some of the earliest known angiosperm fossils are waterlilies, which seems to fit very nicely with expectation, since waterlilies have a simple kind of flower. But waterlilies are herbs. The problem with this is that the timber of flowering or broad leaf trees closely resembles the timber of conifers. That structure is very complex, and is not likely to have evolved more than once. This and other evidence suggests that conifers and angiosperms shared a common ancestor. If flowering plants had first arisen as herbs, then they would have to have reinvented timber that was very like that of conifers, and this seems most unlikely. So it seems that the very first flowering plants must have been trees – and thus for all their primitive flowers, waterlilies are highly evolved specialists, devoid of wood. They could not have been the first. The first angiosperm must have been a tree. Magnolias are reasonable candidates, but the oldest-known magnolias are not quite old enough, and it is hard to imagine that the first ever angiosperm took quite such spectacular form.
