by Oliver Sacks
‘You are the one living soul from whom I have constantly received sympathy’ Darwin wrote to him. From the time when [Hooker] slept with the proofs of the Voyage of the Beagle under his pillow so as to read them the moment he woke up, to the day when he helped to bear Darwin’s pall to its last resting place in the Abbey, [he] was Darwin’s closest and most frequent confidant. It was to Hooker that Darwin, in 1844, sent the first hint of his theory of natural selection and, fifteen years later, Hooker was his first convert. In 1858, when Darwin received one morning from Alfred Russel Wallace an essay setting out the identical theory of natural selection which he himself was about to publish, it was Hooker, overruling Darwin’s quixotic desire to resign his undoubted priority to Wallace, who arranged for the famous double communication of the theory to be read at the Linnaean Society. And at the centenary of Darwin’s birth in 1909, Hooker, then 92, his tall figure still full of vigour, was present at Cambridge to do homage to the friend he had helped so much.
But quite apart from his role in the history of Darwinism, Sir Joseph Hooker stands head and shoulders above his contemporaries as systematic botanist, plant geographer and explorer.
‘Few ever have known, or ever will know, plants as he knew them,’ wrote Professor Bower. His early years were spent at Glasgow, during his father’s professorship. The house, in which were accumulating the herbarium and library later to form the basis of the Kew collections, was near to the botanic garden, and he must have lived and breathed botany from morning to night. The intense love of plants acquired at Glasgow dominated his life.
80 Philip Henry Gosse, in his (anonymous) 1856 guide, Wanderings through the Conservatories at Kew, describes the cycads:
Clustered at the south-east extremity of the house, a considerable area of which they occupy, we see a group of plants having a common character, notwithstanding the various botanical appellations that we read on their labels. They bear, in their arching pinnate leaves, radiating from the summit of a columnar stem, a certain resemblance to palms, and also to the tree-ferns, but have neither the stately grace of the one, nor the delicate elegance of the other, while their excessive rigidity, and the tendency of their leaves to form spinous points, give them a repulsive aspect.
A year later, in his bizarre book Omphalos – published just two years before the Origin of Species – Gosse, who was both a brilliant naturalist and a religious fundamentalist, attempted to reconcile the existence of fossils (which seemed to testify to former ages) with his belief in a single, instantaneous act of Creation. In his theory of ‘Prochron-ism’ he suggested that the entire crust of the earth, complete with its cargo of fossil plants and animals, was created in an instant by God and had only the appearance of a past, but no real past going with it: thus there had never been any living forms corresponding to the fossils. In the same way that Adam had been created in an instant as a young man (never a child, never born, with no umbilical cord – though nonetheless with an umbilicus, an omphalos), he argued, so a cycad, full of leaf-scars, seemingly centuries old, might also be quite newly created.
Taking an imaginary tour of the earth, a single hour after the Creation, he invites the reader to look at a panorama of animals and plants:
I wish you to look at this Encephalartos. A horrid plant it is, a sort of caricature of the elegant Palms, somewhat as if a founder had essayed a cocoa-nut tree in cast iron. Out of the thick, rough, stiff stem spring a dozen of arching fronds, beset with sharp, sword-shaped leaflets, but having the rigidity of horn, of a greyish hue, all harsh and repulsive to excess. In the midst of this rigid coronal sits the fruit, like an immense pine-cone…It would be no unreasonable conjecture to suppose that this great Cycadaceous plant is seven or eight centuries old.…Nay, for this also has been created even now!
This extraordinary notion – one cannot call it a hypothesis, for it cannot in principle be either proved or disproved – had the distinction of earning the derision of paleontologists and theologians alike.
81 In his guide to Kew, Gosse includes a whimsical note on the Ci- botium there:
[It is] a singular vegetable production, of which, under the name of Scythian Lamb, many fabulous stories are told. It was said, among other things, to be part animal, part vegetable, and to have the power of devouring all the other plants in its vicinity. It is in reality nothing but the prostrate hairy stem of a fern, called Cibotium barometz, which, from its procumbent position and shaggy appearance, looks something like a crouching animal.
82 One cannot look at the cycads in Kew or in the Hortus without a sense of their frailty too, the extinction which constantly threatens species which are special and rare. This came home to me especially in the Kirstenbosch Gardens in Capetown, where more than fifty species of the African cycad Encephalartos grow. Some of these are common, some are rare. One is unique, because it comes from a single (male) plant, E. woodii, discovered by Dr. Medley Wood in 1895. Though cuttings of the original have been cultivated (propagated asexually and thus clones of the original), no other trees of this species, male or female, have ever been found – and unless an unknown female exists somewhere, E. woodii will never pollinate or mate; it will be the last of its kind on earth.
Seeing the magnificent solitary specimen at Kirstenbosch, unla-belled and surrounded by an iron fence to discourage poachers, reminded me of the story of Ishi, the last of his tribe. I was seeing here a cycad Ishi, and it made me think of how, hundreds of millions of years ago, the numbers of tree lycopods, tree horsetails, seed ferns, once so great, must have diminished to a critical extent until there were only a hundred, only a dozen, only a single one left – and finally, one day, none at all; only the sad, compressed memory held in the coal.
(Another unique cycad, a female Ishi, Cycas multipinnata, has recently been found in a temple garden in China; no other specimens are known to exist. It is portrayed, with others, in a set of postage stamps issued in May 1996, commemorating cycad species native to China.)
83 In the northern part of Guam, there is a tropical dry forest, dominated by cycads; in Rota the cycad forest is wetter, ‘mesic,’ though not true rain forest such as one sees on Pohnpei. The last few years have seen the destruction of Rota’s unique forests on a fearful scale, most especially with the building of Japanese golf courses. We encountered one such development as we were walking through the jungle – huge bulldozers tearing up the earth, mowing down an area of several hundred acres. There are now three golf courses on the island, and more are planned. Such clear-cutting of virgin forest causes an avalanche of acidic soil into the reef below, killing the coral which sustains the whole reef environment. And it may break up the jungle into areas too small to sustain themselves, so that within a few decades there will be a collapse of the entire ecosystem, flora and fauna alike.
84Chamberlain, in The Living Cycads, described how he estimated the age of a Dioon edule, which reaches maturity (in the wild) around the age of fifty, and then puts out a new crown of leaves every other year on average. By counting the number of leaf scales on the stem, and dividing by the number of leaves produced each year, he arrived at the age of the tree. He described one beautiful specimen which, by this criterion, was 970 years old, even though less than five feet in height. Indeed, Chamberlain wondered whether some cycads might approach the sequoias in age.
85The cones of cycads vary in character and shape and size: the vast cones of Lepidozamia peroffskyana and Encephalartos transvenosus may weigh more than a hundred pounds, and the cones of the smallest Za-mias no more than thirty milligrams. But all of them exhibit, in the arrangement of their cone scales, intricate geometric patterns similar to the corkscrew spirals or helices we see in pinecones, the leaf arrangement of cylindrical stems, or the whorling florets of sunflowers. The study of these patterns, this phyllotaxis, has intrigued botanists and mathematicians for centuries, not only because the spirals themselves are logarithmic, but because there are numbers of accessory helices (or parastichies) running in the opposite direction and
these two sets of helices occur in a fixed ratio to one another. Thus in cycad cones, as in pinecones, we almost always see spirals in five and eight rows, and if we express as fractions the number of parastichies, we find a series of 2⁄1, 3⁄2, 5⁄3, 8⁄5, 13⁄8, 21⁄13, 34⁄21, and so on. This series, named after the thirteenth-century mathematician Fibonacci, corresponds to a continued fraction which converges to 1.618, the numerical equivalent of the Golden Section.
These patterns probably represent no more and no less than an optimum way of packing leaves or scales together while avoiding their superimposition (and not, as Goethe and others thought, some mystical archetype or ideal), but they are a delight to the eye and a stimulus to the mind. Phyllotaxis fascinated the Reverend J.S. Henslow (professor of botany at Cambridge, and Darwin’s teacher), who discussed and illustrated it in his Principles of Descriptive and Physiological Botany, and it is pondered at length in an eccentric (and very favorite) book, D’Arcy Thompson’s On Growth and Form. It is said that Napier’s discovery of logarithms at the start of the seventeenth century was stimulated by a contemplation of the growth of horsetails, and the great botanist Nehemiah Grew, later in the century, observed that ‘from the contemplation of Plants, men might first be invited to Mathematical Enquiry.’
This sense of the mathematical determination (or constraints) of nature, especially of organic form and growth, divested of idealism or idiosyncrasy, is very strong now, especially with the development of chaos and complexity theory in the last few decades. Now that fractals are, so to speak, part of our consciousness, we see them everywhere – in mountains, in landscapes, in snowflakes, in migraines, but above all in the vegetable world – just as Napier, four centuries ago, saw logarithms in his garden, and Fibonacci, seven centuries ago, found the Golden Section all about him.
86 The forms of plants exercised Goethe endlessly – we owe the very word ‘morphology’ to him. He had no sense of evolution, but rather of a sort of logical or morphological calculus whereby all higher plants might be derived from a simple primordial type, a hypothetical ancestral plant he called an Ur-pflanze. (This idea came to him, he recorded, while he was gazing at a palm in the Orto at Padua, and ‘Goethe’s palm,’ as it is now called, still grows there in a house of its own.) His hypothetical Ur-pflanze had leaves, which could metamorphose into petals and sepals, stamens, and anthers, all the complex parts of flowers. Had Goethe concerned himself with flowerless plants, I could not help feeling, he might have seized on Psilotum as his Ur-pflanze.
Alexander von Humboldt was a close friend of Goethe’s, and adopted his theory of metamorphosis in his own Physiognomy of Plants (indeed, he widens Goethe’s notion and hints at a cosmic, universal organizing power acting not only on plants but on the forms of rocks and minerals and on the forms of mountains and other natural features as well). The physiognomy of the vegetable kingdom, he argues, ‘is principally determined by sixteen forms of plants.’ One of these – a leafless branching form – to his mind, binds together plants as diverse as the Casuarinas (flowering plants), Ephedra (a primitive gymnosperm) and Equisetum (a horsetail). Humboldt was a superb practical botanist, and very well appreciated the botanical differences between these, but he was looking, as Goethe was, for a principle orthogonal to biology, to all particular sciences – a general principle of morphogenesis or morphological constraints.
The arborization of plants originates not in accordance with some primordial archetype, but as the simplest geometric way of maximizing the ratio of surface area to volume and thus the area available for photosynthesis. Similar economic considerations may apply to many biological forms, such as the branching dendrites of nerve cells or the arborizations of the respiratory ‘tree.’ Thus an ‘Ur’ – plant like Psilotum, lacking leaves or other complications, is an exemplar, a diagram of one of nature’s most basic structures.
(In more recent times, a specific analog of Goethe’s theory, which traces how all higher plants might be derived morphologically from primitive psilophytes, has been proposed by W. Zimmerman, in his theory of telomes. And a general analog to Goethe’s morphology may be found in some of the current theories of self-organization, complexity, and universal morphogenesis.)
87 Such a feeling of transport to the distant past struck Safford when he saw the cycad forests of Guam: their ‘cylindrical, scarred trunks, and stiff, pinnated, glossy leaves,’ he wrote, suggested ‘ideal pictures of the forests of the Carboniferous age.’
A very similar feeling is described by John Mickel, writing of horsetails:
To wander among them is a kind of science-fiction experience. I well remember the first time I encountered a stand of the giant horsetail in Mexico. I had the feeling that I had found my way backward into a Carboniferous forest, and half expected dinosaurs to appear among the horsetails.
Even a walk in the streets of New York can evoke the Paleozoic: one of the commonest trees here (apparently well able to resist pollution) is the maidenhair tree, Ginkgo biloba, a unique survivor little changed from the ginkgophytes of the Permian. But the ginkgo exists now only in cultivation; it is no longer found in the wild.
Darwin, in the Origin, introduced the term ‘living fossil’ to describe primitive organisms which could be seen as relics from the past – members of groups once widespread but now greatly reduced and occurring only in very isolated and restricted environments (where ‘competition…will have been less severe than elsewhere’). Ginkgos, for example, were very widespread once – they were a dominant species of the Pacific Northwest before the great Spokane Flood fifteen million years ago – but are now restricted to a single species, found only in cultivation and in a small area of China. The most spectacular discovery of such a ‘living fossil’ in this century was that of a fish, the coelacanth Latimeria, in 1938; a more recent one, which shook the botanical world, was the discovery in 1994 of a gymnosperm long thought to be extinct, the Wollemi pine, in Australia. (I still hope, in some irrational, romantic part of myself, that a giant club moss or horsetail will turn up one day.)
But whereas ‘Latimeria chalumnae is a single species, only managing to survive in the special conditions off the Comoros, cycads (though no longer the dominant flora, as in the Mesozoic) still number more than two hundred species and still thrive in a wide variety of eco-climes, so they cannot really be called ‘living fossils.’
88 The unexpected adaptation of crabs to coconut eating fascinated Darwin, who describes them in the Beagle:
I have before alluded to a crab which lives on the cocoa-nuts: it is very common on all parts of the dry land, and grows to a monstrous size: it is closely allied or identical with the Birgos latro.
The front pair of legs terminate in very strong and heavy pincers, and the last pair are fitted with others weaker and much narrower. It would at first be thought quite impossible for a crab to open a strong cocoa-nut covered with the husk; but Mr. Liesk assures me that he has repeatedly seen this effected. The crab begins by tearing the husk, fibre by fibre, and always from that end under which the three eye-holes are situated; when this is completed, the crab commences hammering with its heavy claws on one of the eye-holes till an opening is made. Then turning round its body, by the aid of its posterior and narrow pair of pincers, it extracts the white albuminous substance. I think this is as curious a case of instinct as ever I heard of, and likewise of adaptation in structure between two objects apparently so remote from each other in the scheme of nature, as a crab and a cocoa-nut tree…It has been stated by some authors that the Birgos crawls up the cocoa-nut trees for the purpose of stealing the nuts: I very much doubt the possibility of this; but with the Pandanus the task would be very much easier. I was told by Mr. Liesk that on these islands the Birgos lives only on the nuts which have fallen to the ground.
(In fact, coconut crabs do climb tall palm trees, and cut off the coconuts with their massive claws.)
89 It used to be held that cycads were wind-pollinated, like ferns and conifers, though early authors (includin
g Chamberlain) had occasionally been struck by the presence of certain insects in or near the male cones at the time of pollination.
In 1980, Knut Norstog and Dennis Stevenson, working at the Fairchild Tropical Garden in Miami, were struck by the failure of many introduced cycads there to produce fertile seeds, even though healthy male and female plants had been planted just a yard or two apart, whereas the native Zamia was quite fertile. They found that snout weevils would feed as larvae on the male Zamia cones, emerging as adults by boring through the microsporophylls, covered with pollen. Could this be the way in which the female cones were pollinated?
Stevenson and Norstog, along with other researchers (Karl Niklas, Priscilla Fawcett, and Andrew Vovides), have confirmed this hypothesis in great detail. They have observed that weevils feed and mate on the outside of the male cone and then enter it, continuing to feed not on the pollen, but on the bases of the microsporophylls. Their eggs are laid, and larvae hatched, inside the microsporophylls, and the adult weevils finally chew their way out through the tips of the sporophylls. Some of these weevils go to the female cones, which exude a special warmth and aroma when they are ready for pollination, but the weevils cannot feed here, since the female cones are toxic to the insects. Crawling into the female cone through narrow cracks, the weevils are divested of their pollen and, finding no reason to stay longer, they return to the male cones.
The cycad thus depends on the weevil for pollination, and the weevil on the cycad cones for warmth and shelter – neither can survive without the other. This intimate relationship of insects and cycads, this coevolution, is the most primitive pollination system known and probably goes right back to the Paleozoic, long before the evolution of flowering plants, with their insect-attracting scents and colors.