When British General Charles George Gordon arrived on the island of Praslin in 1881, he believed he had discovered the Garden of Eden. This was not hyperbole. For some years Gordon had been searching for the famed garden, a sort of personal side-quest between military engagements, so when he reached the lush forest of Praslin’s Vallée de Mai and encountered the island’s endemic coco de mer palm tree, he was enthralled. In a manuscript written during that visit, he described the palm as ‘a curious tree in every way, unique among its kind and among trees’, and enthusiastically explained that not only had he found the biblical Eden, but he had also found the legendary Tree of Knowledge and its infamous forbidden fruit. What he had actually found was the world’s largest seed.
The island of Praslin belongs to the Seychelles archipelago, which lies in the Indian Ocean just south of the equator and some 1500 kilometres east of the African coast. With its colourful coral reefs and turquoise lagoons, verdant tropical forests and powdery white beaches, in General Gordon’s time it would have indeed seemed like an untouched paradise. Even today, local tourism operators readily list the Seychelles’s postcard-perfect attributes – the word ‘divine’ comes up a lot. A historian might instead regale you with tales of pirates and hidden treasure. An evolutionary biologist, however, will likely tell you these islands are intriguingly weird. This is because the Seychelles are incredibly old. Many of the islands are made of igneous granite and were formed as Madagascar broke away from India nearly ninety million years ago during the late Cretaceous, making them the oldest oceanic islands in the world. Over thousands of millennia, this cluster of remote, isolated ecosystems has seen the evolution of a long list of unique species, comprising mammals, birds, insects, reptiles – including giant tortoises – as well as a handful of rare snails. There are also twelve species of endemic amphibians, which is an impressive number for remote islands because amphibians, as a rule, do not take well to salt water and avoid migrations that involve it. But if the fauna of the Seychelles are strange, wonderful and rare, then so, too, is its flora.
There are numerous endemic species of plants, one of which is the coco de mer, or coconut of the sea (Lodoicea maldivica). It is the sole species in the genus Lodoicea and only grows on Praslin and one other island in the Seychelles: the appropriately named Curieuse. The coco de mer grows surprisingly well in poor quality soil consisting mostly of weathered granite. These trees are also very long-lived, with life spans stretching hundreds of years. The tree’s hard-shelled fruit is unique and spectacularly heavy – it can weigh well over 40 kilograms. It is often called the ‘double coconut’ precisely because it resembles two bulbous, smooth-surfaced coconuts fused together. Gordon, a devout man and evidently prim even by Victorian standards, took one look at the double coconut and wrote that ‘externally the cocode-mer represents the belly and thighs, the true seat of carnal desires’. He decided, a priori, that it must be the famed forbidden fruit. To be fair, it does look a bit rude.
Due to its weight, a double coconut that somehow tumbles into the water will sink immediately. Eventually, though, the fruit and seed inside will decay, and the hard shell, now buoyant with the resulting gases, will rise to the surface. The shells can float hundreds, sometimes thousands of kilometres on ocean currents, and over the centuries they have been found along the coastlines of the African continent and Madagascar, and as far away as India, Sri Lanka and Malaysia. The story goes that when fisherfolk of ancient times witnessed the double coconut shells rising to the surface, it led them to believe they grew on mythical trees on the ocean floor. Today, the double coconut’s principal claim to fame is that it contains the world’s largest seed, which can weigh as much as 25 kilograms and reach half a metre in length. Exactly how seeds this large evolved in the Seychelles is still not entirely clear, especially given that the coco de mer grows alongside plants that produce a wide variety of seeds, including some that are no bigger than dust motes.
There are endemic orchids in the Seychelles, including the tropicbird orchid (Angraecum eburneum), which produces seeds smaller than half a millimetre long. Thousands of them clumped together appear to the naked eye as little more than a puff of pale dust. It’s interesting to imagine these tiny specks being carried on a warm breeze past enormous floating double coconuts. Both are seeds, and yet they could not be more different. Then again, maybe they could be. The tropicbird orchid does not produce the smallest seeds in the world. That title belongs to a species of jewel orchid endemic to New Caledonia, the Anoectochilus imitans. The seeds of this orchid are as small as 0.05 millimetres, roughly ten times smaller than tropicbird seeds, and they weigh around 1 milligram. With the aid of a microscope – especially a scanning electron microscope, which is quite good at imaging very small things – it becomes clear that these seeds are thin, elongated and papery. The seeds of some orchids have surfaces resembling honeycomb, while others appear scoured with fine grooves. In this, we can see that the structural diversity of seeds goes far beyond a spectrum of sizes that spans the distance between the seeds of A. imitans and the coco de mer.
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The diversity of seed shapes, textures and colours is astounding. Moreover, while some seeds go solo, others are encased in a weird and wondrous array of fleshy fruits, dry fruits and other casings that plants use to protect and disperse their seeds, from the delicious bulk of a watermelon to the painful barbs of the devil’s claw seedpod.
Some seeds, though small, are bulbous and bumpy, like those produced by rock sea-spurrey wildflowers or the succulent Crassula pellucida. There are spikey seeds, too, such as the prickly ones produced by the wild carrot Daucus carota, an almost aggressive structure for a plant otherwise known as Queen Anne’s lace. There are softer ‘hairy’ seeds which are far more pleasant to hold, thanks to fine fibres like those emerging from the cotton seed or the light tuft of the dandelion seed. The seeds of the bird of paradise appear to have a comical mop of bright orange hair. Yet other seeds have structures that closely resemble feathers, such as those produced by the cornflower or the Australian daisy (Leucochrysum molle).
Some seeds appear to have entire wings. Take, for example, the four-winged seed of the bushwillow (Combretum zeyheri), which, viewed from just the right angle, looks like a butterfly. The smooth, aerodynamic foil of the sycamore maple seed (Acer pseudoplatanus) is reminiscent of the wing of a dragonfly. Often, two seeds are twinned and spin as they fall. I have plenty of childhood memories of picking up these ‘helicopter seeds’ and then throwing them in the air just to watch them spin, and I know I’m not the only one. The Hiptage benghalensis, a woody vine native to India and South-East Asia, has helicopter seeds too – with three curved wings fused together, they resemble birds in flight. Meanwhile, the papery seeds of the frangipani cause them to look more like a dense cluster of moths when they are pressed close together in their seedpod.
There are simpler seeds, like the black bean with its familiar smooth plump curves, or the rough sphere of a peppercorn seed, or the compressed curve of the poppyseed. But even a poppyseed, close up, is not quite what you might expect, with its honeycomb-like surface. There are simple hard spheres like that of the Canna indica plant. Then there are the more ornate seeds, some of which appear to be covered in ruffles or indeed entire dresses, like the poisonous larkspur (Delphinium peregrinum). There are seeds that seem at first glance like tiny cups, and the seeds of the red-eyed wattle do indeed look like red-rimmed eyes, unsettlingly so. The seeds of the dense blazing star (Liatris spicata), a North American meadow flower, has what looks like a trail of streamers.
One morning before school, my youngest daughter decides to weigh in as I flip through the pages of Rob Kesseler and Wolfgang Stuppy’s beautiful book Seeds, which is full of highly magnified images that are artworks in their own right. She peruses the strange and varied forms of the seeds and informs me that one looks like a hairy jellybean, the next one looks like a butterfly and another is like a tree branch. She keeps going: there’s a centipede, a belly button, a stingray, a set of wonky stairs, a bunch of crocodiles, a grasshopper inside a grapefruit, a bird diving into water to grab its prey, the Hungry Caterpillar, two jellyfish getting married, and ‘a person wearing pants who is having a bad day’. Indeed, there are seeds that only a child can adequately describe. So full of grooves, curves and erratic protrusions, such seeds seem like a nonsensical evolutionary hodgepodge, sort of the plant kingdom’s answer to the platypus. It’s hard to imagine they’re at all functional. And yet, that is precisely what they are. It is its function, and the handful of fundamental structures that underpin it, that makes a seed a seed.
Seeds are packages of genetic information containing all the instructions for building the next generation of the plant. Put this way, it’s tempting to think of a seed as a hardened jumble of plant cells. But for the overwhelming majority of seed plants, the next generation is a lot further along and far more organised than that.
Carol Baskin is a professor of plant ecology at the University of Kentucky and one of the world’s leading experts on seeds. She tells me of an analogy she heard many years ago that has stuck with her: ‘When I was a college student, my professor said, “A seed is a baby plant in a box with its lunch.”’ That certainly gets straight to the heart of the matter, because most seeds contain a very small immature plant, complete with a stem, a root structure and at least one leaf. In other words, a seed contains a plant embryo. That embryonic shoot is called a plumule and the nascent root is called a radicle. The embryonic leaf is called a cotyledon. Flowering plants will produce one or two cotyledons; other seed-producing plants, such as conifers, may produce a multitude. These cotyledons are the tiny leaves you see emerging from a seed when it sprouts.
The overwhelming majority of seeds also have their own food supply in the form of a nutritive tissue called endosperm, the ‘lunch’ part of the aforementioned analogy. During seed development, there’s a phase called ‘dry mass accumulation’ or ‘dry matter accumulation’. ‘It’s a period where most seeds are food-accumulating machines,’ says Christina Walters, a plant physiologist at the US Department of Agriculture’s (USDA) National Laboratory for Genetic Resources Preservation in Fort Collins, Colorado. She explains that this is the result of the parent plant redistributing its resources. Plants are always making decisions about where to send carbon, nutrients and water, such as to the roots, to new leaves or to an injured stem. During seed development, the carbon balance of the plant is manipulated to favour the reproductive units. ‘What they’re doing is packing the cells full of material,’ Walters tells me. ‘It’s not unlike parents investing in their kids.’
The endosperm surrounds the embryo and acts as a source of carbohydrates, vitamins, fats, antioxidants and proteins. The endosperm’s main job is to support the growth of the plant embryo, providing nourishment until the young plant can start making its own food. Helpfully, the endosperm can also act as extra padding. This raises another point: seeds provide protection for the plant embryo. At a bare minimum, the seed encases it in a thin covering. There may also be a tough seed coat, or even additional protection afforded by a hard or fleshy fruit that also aids in dispersal.
That’s it in a nutshell, as it were. This is what most seeds share in common: they are embryonic plants with a protective cover and, often, a food supply. It sounds simple, and yet producing a seed is one of the most intricate processes a plant will undertake in its life cycle. This begs the question, ‘Why did seeds evolve in the first place?’, because for millions and millions of years the plant kingdom got on just fine without them. Then, when seeds finally did show up, they fundamentally altered the course of life on Earth.