by Colin Tudge
But nature is never satisfied. However fine its inventions may be, evolution powers on. Some time 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 multiplies 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 furthest 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 downwards, 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 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.
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 a 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 have shared a common ancestor for almost 300 million years. 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 metres tall – and their trunks may be up to 2 metres 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, seagrasses (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, and the autumn crocuses. You will find trees included in the Liliales in some traditional classifications, including those yucca and aloe: but these two genera have now been repositioned, as will become apparent. The Dioscereales are the brionies; and the Commelinales are more water-plants, including the spider-worts 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.
Joshua Trees and Dragon Trees: ORDER ASPARAGALES
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 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 fibres – sisal hemp – for ropes and fishing nets. A. americana was imported to Europe soon after Europeans became aware that the New World exists, 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 metres tall, with simple curving branches which have prickly mop-like tops: they enhance and haunt the semi-desert of the south-west 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 metres – 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 – there are several species in the beds at Kew – 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.
The‘Screw Pines’: ORDER PANDANALES
So to the Pandanales, with the single family of the Pandanaceae, colloquially known as screw pine
s. 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: up to 30 metres 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 treeline. They live mostly around the Indian Ocean and the west 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.
The Palms: ORDER ARECALES
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 south-east), the Americas and Australia, with surprisingly few species in Africa although they grow widely there. In the rainforest 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 fibres 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 metres tall, towering above the oaks and laurels all around. The wax palm, Ceroxylon, at more than 60 metres is the tallest palm of all –yet it grows high on the Andes, at 3,000 to 4,000 metres. The Chusan or windmill palm of China, Trachycarpus fortunei, can cope with snow and ice.
Most palms look as we all imagine palms should: a tall straight stem with a crown of leaves on the top. But others deviate startlingly from our imagined archetype. Some keep their stems underground, so their leaves seem to shoot straight from the soil surface. The underground stems do not grow horizontally, like the rhizome of an iris. The trunk of Attalea, for example, first grows downwards and then, as if realizing its mistake, grows upwards again, to form a U. Attalea is also the genus of the American oil palm and is a relative of the coconut – although the coconut palm itself, Cocos, grows tall and straight, though often leaning over. There are many so-called ‘stemless’ palms in the Cerrado, the dry forest of Brazil. Other palms, notably in the genus Calamus, are climbers: indeed Calamus provides rattan cane, the stuff of colonial screens and chairs that creak on the veranda. Yet there are around 370 species of Calamus and many are bona fide trees, suggesting that climbing is a late evolutionary departure, and illustrating once more the extreme versatility of nature.
In many palms the trunk is smooth; in others, patterned with the scars of earlier leaves. Many retain the ragged bases of old leaves, like the African oil palm, Elaeis; or indeed like America’s Washingtonia filifera, which Hugh Johnson in The International Book of Trees compares unkindly to ‘an Alpine haystack’, even though Californians are so fond of it. Many palms are valued for their timber – like palmyra (Borassus), much favoured in India. Palm wood, says Corner, can be ‘as hard as steel’. Yet the structure is quite different from that of dicots or conifers. The xylem and phloem run from roots to leaves in scattered strands, for there is no cambium to divide the two into neat concentric cylinders. So there can be no secondary thickening. Instead, the stem starts stubby and simply grows taller – although in bottle palms the trunk may swell at intervals, through the general proliferation of tissue and enlargement of cells. These swellings may in part serve as food stores. Bottle palms are not a discrete group: there are various kinds, in unrelated genera. Best known and most magnificent among them is the royal palm, Roystonea, whose tall, pale-grey trunks are swollen for much of their length and topped by a huge shaggy crown of dark-green pinnate leaves. Roystonea came originally from Cuba, but is grown as a street tree throughout the affluent tropics. I am told there is a most amazing avenue of royal palms in the Botanic Garden at Rio. (Memo: Always check out botanic gardens!)
Many palms have vicious spikes. Those on the trunk may be like bodkins, up to a foot in length. Some that grow from near the base originate bizarrely from the tips of adventitious roots, rising directly from the trunk. I have grabbed the leaves of palms to keep my balance along Amazonian forest paths and come close to shredding my hands on barbs as sharp as fishhooks, ranked along the midrib. Not nice. Yet on islands in the Indian Ocean that lack insouciant ramblers and rapacious herbivores the palms are spikeless – just as birds on islands that traditionally had no humans or cats tend to be tame (and were wont to perch on the rim of sailors’ cooking pots). Armaments, and the emotions of fear and aggression, require energy. With no threat, an air of relaxed innocence can prevail. There is an obvious moral in here.
The roots of palms are peculiar. Firstly, they are adventitious, growing directly from the trunk; generally from the base, of course, but also from higher up, occasionally forming significant stilt roots as in the screw pines, so that the whole tree can look like a rocket about to take off (and sometimes remaining dwarf but with hard and sharp tips to the roots, which form some of the most murderous spines). Big dicot trees have extremely thick roots – but they become thick in the same way as dicot trunks do, by secondary thickening. Palm roots and screw pines have no secondary thickening, but have instead the thickest primary roots seen in nature. They can also be, as Corner says, ‘exceedingly numerous’ – so that 8,000 roots a centimetre thick may emerge from the base of a coconut tree or an African oil palm, and sometimes up to 13,000. The root system begins normally enough in the seed – in the dicot manner, from the radicle (the primary root) of the embryo. But that first effort soon dies, and the adventitious roots take over. Because there is no secondary thickening, the roots remain cylindrical, though they taper off towards the end and finish with a stout root cap. Seldom are there root hairs of the kind that dicots generally rely upon: water is absorbed from a short region just behind the root cap. Palm roots (like screw-pine roots) are very rich in xylem. They are highly efficient conduits.
Palm roots are lignified through the middle and have huge tensile strength. The roots of the coconut spread up to eight metres in any direction, branching more or less at right-angles two to four times as they advance. Their spread far exceeds that of the crown, and they provide prodigious anchorage. Coconuts grow on windblown islands by the edge of the sea. The waves dig out the sandy soil around them until the trunks collapse to the horizontal, held out like the bowsprit of a ship to a length of 20 metres, with a huge mop of leaves and a couple of score of coconuts at the end. The strain is fierce. Yet the trees remain stable. As Corner says, nothing less than a hurricane can dislodge them. Perhaps it’s the roots of palms above all that
have made them so successful. They can hang on through storms where dicots and conifers are toppled. Perhaps palms established themselves first as swamp and riverside trees and then spread to the surrounding forests.
The apical bud at the top of the palm, from which the crown arises, is the biggest bud in all of nature. The buds are harvested from many species as ‘palm heart’ or ‘palm cabbage’, common on Brazilian menus. Few palms can grow a replacement bud, so harvesting kills the tree: a huge loss if they are taken carelessly from the wild. But some species, like the Brazilian Euterpe, can be coppiced like a European chestnut or a hazel. When a stem is cut, more stems grow to replace it. Euterpe also yields a fine fruit with the common name of acai (pronounced ‘ass – ay – ee’). Its deep-purple juice is a delightful drink, makes excellent ice cream and is one of the most valuable of the ‘non-timber forest products’ from Amazonia (though of course behind rubber, cocoa and brazil nuts). For small farmers, a copse of Euterpe, constantly harvested but then regenerating, can be a fine source of income. For good measure, it has lovely, fernlike leaves.
In general the leaves of palms are either pinnate (feather-like) as in the coconut or palmate (hand-shaped), as in the European fan palm. For some reason the palmate types include most of the hardiest species. The flowers may be of one sex or both. If they are unisexual, the tree itself may be of one sex only (dioecious), as in the American oil palm; or carry both sexes (monoecious). The inflorescences may be huge –up to 250,000 separate flowers. In some species, like the date palm (Phoenix), the inflorescence grows from between the leaves and hangs out from the side of the tree, typically below the leaves. In others the inflorescence erupts from the top like a slow firework. Sometimes this is the tree’s last fling, for some apical flowerers die after reproduction. The flowers are mostly pollinated by insects – commonly beetles, bees or flies. They often have nectar to reward the pollinators.
