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A Natural History of Ferns
In Shakespeare's Henry IV, Falstaff, Prince Hal and Poins scheme to rob a rich merchant on his way to London in the dark hours of the early morning. Because they need help with the heist, one of Falstaff's henchmen tries to persuade another thief to join them. He says to the thief, "We steal as in a castle, cock-sure; we have the receipt of fern-seed, we walk invisible," to which the thief replies, "Nay, by my faith, I think you are more beholding to the night than to fern-seed for your walking invisible" (act 2, scene 1, lines 95-98).
What do the thieves mean by fern seed? Anyone who has taken a botany course knows that ferns do not have seeds; instead they disperse by tiny dust-like spores. Did people in Shakespeare's day believe that ferns had seeds? And what is this about walking invisible?
In 1597 when Henry IV was written and first performed, the belief that ferns had seeds was common. To be sure, no one had ever seen a fern seed, but how could ferns — or any plant, for that matter — reproduce without such propagules. Therefore, the reasoning was that ferns must have seeds. "The views of those who believe all plants have seeds are founded on very reasonable conjectures," wrote Joseph Pitton de Tournefort, a celebrated French botanist, in 1694.
But sometimes the conjectures went too far. The early herbalists, for example, claimed that fern seed had to be invisible because no one had ever seen it. Furthermore, they asserted that it conferred invisibility to the bearer, that if you held the fern seed, you walked invisible. They also specified that the seed could only be collected at midnight on St. John's Eve (Midsummer's Night Eve, June 23), the exact moment it fell from the plant, during the shortest night of the year. You could catch it by stacking twelve pewter plates beneath a fern leaf; the seed would fall through the first eleven plates and be stopped by the twelfth. If you came up empty-handed, it was because goblins and fairies, roaming freely that one night of the year, had snatched the seed as it fell, much as Puck, Oberon, and the other fairies caused mischief, some of it botanical, in Shakespeare's A Midsummer Night's Dream.
Of course, not everyone believed all this about invisibility, but they did believe that ferns had seeds. The only problem was, what was the fern seed? Many early botanists suspected it was the dust liberated from the dark spots or lines (the sori) on the underside of the fern leaf. Other botanists thought that this dust was not seed but instead equivalent to pollen that impregnated a female organ somewhere on the plant.
The first person to investigate fern dust scientifically was Marcello Malpighi, the Italian anatomist. In the late 1600s he focused his microscope on the curious dark spots or lines on the undersides of fern leaves. He observed that the spots or lines resolved into hundreds of tiny "globes" or "orbs" (the sporangia), each encircled by a thick, segmented band, the annulus. Inside the orbs sat the dust, which appeared as round or bean-shaped bodies. He noted that the dust was hurled out of the orb by the catapult-like action of the annulus. Nearly half a century later, Malpighi's observations were confirmed and elaborated by Nehemiah Grew, an English microscopist. But the observations of neither man solved whether the dust was equivalent to pollen or seed.
Even the great Swedish botanist Carl Linnaeus was puzzled about the nature of fern dust. In a letter written in 1737 to Swiss botanist Albrecht von Haller, he said, "this powder seen under a microscope, exactly agrees with the dust of the anthers in other plants." But one month later he said, "[I know] nothing about the imperfect tribes of plants [mosses and ferns] and must confess my ignorance whether what I see is seed, or dust of the anthers." In 1751, however, he changed his mind and asserted that the dust was the true fern seed. Despite his flip-flopping, Linnaeus was sure about one thing: ferns have seeds.
Uncertainty reigned until 1794 when John Lindsay, a British surgeon, proved that ferns reproduced from their dust. He discovered this while stationed in Jamaica, where he noticed hundreds of young ferns arising on freshly exposed soil after rains. With a microscope, he examined the soil in the hope of finding a fern seed but was unsuccessful. Undaunted, he decided to sow some of the dust. (For some unknown reason, fern gametophytes abound on freshly exposed soil, generally less than 3 years old since exposure. Of course, the gametophytes from which the young ferns grow are easier to find on exposed soil because they are not obscured by vegetation, but there is something peculiar about freshly exposed soil that promotes their growth. Even if the soil is kept free of vegetation, particularly mosses and grasses, fern gametophytes will become fewer over time. To find fern gametophytes in the field, look for freshly exposed soil along trails, under tipped-up tree bases, along roadsides, and on landslides.)
Lindsay gathered the dust from several weedy ferns and sprinkled it over soil in a flowerpot. He placed the pot in a window of his room, watered it daily, and every day or two examined a small portion of the soil with his microscope. Lindsay (1794) describes what happened:
I could always readily distinguish the dust or seeds from the mould, but observed no alteration till about the 12th day after sowing, when many of the small seeds, represented at 6 in the annexed plate, had put on a greenish colour, and some were pushing out their little germ, like a small protuberance, the rudiment of the new fern, as at 8. This little protuberance gradually enlarged, and successively put on the appearances at 9, 10, and 11. They had acquired small roots, and the remains of the little seeds were still discernible where the roots of the infant plant commenced. Although the young ferns were now very conspicuous by the microscope, the naked eye could see nothing but a green appearance on the surface of the mould, as if it were covered with some very small moss: this was the numberless young plants from the quantity of the seed sown. In some weeks this moss began to appear to the naked eye like small scales, as at 13, which gradually enlarged, as at 14: they were generally of a roundish figure, somewhat bilobate, but sometimes more irregular; they were of a membranous substance, like some of the small lichens or liverworts, for which they might readily be mistaken, and of a dark green colour. At last there arises from this membrane a small leaf, different from it in colour and appearance, as at 15, and shortly after another still more different, as at 16. Now each succeeding leaf grows larger than the last, till they attain the full size, and are complete in all the parts and discriminating characters of their respective species.
Clearly, Lindsay thought he had seen a full-sized fern develop from a mote of fern dust. He felt certain that the dust is the true fern seed.
A busy medical practice kept Lindsay from making further observations, until one day he received a letter from Sir Joseph Banks, president of the Royal Society of London and scientific advisor to the Royal Botanic Gardens, Kew. Banks asked Lindsay to collect Jamaican plants, especially ferns, and send them to England for cultivation. Lindsay wrote back that given the risk of transporting green ferns over such a great distance, he would send some of their seeds instead. Banks must have been flabbergasted that Lindsay claimed knowledge of the true fern seed. He wrote back that if Lindsay could furnish the means of making ferns grow from seed, he would be given the credit of having made a valuable discovery, one that Banks would communicate to the Linnean Society.
Lindsay sent Banks the seeds along with instructions for their sowing. The result was pteridological history. Thanks to Lindsay's information, gardeners in England learned to propagate ferns from spores, and they passed this knowledge to colleagues in other countries. Ferns began to enrich greenhouses, gardens, and parks around the world. Furthermore, the horticulturists at Kew began raising ferns sent from far corners of the British Empire. They amassed the world's largest and richest collection of living ferns — a distinction held to this day (the Kew collection is important scientifically as well as horticulturally). James Edward Smith, a pteridologist and one of England's leading botanists, commemorated Lindsay for his discovery by naming a genus of tropical ferns after him: Lindsaea.
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