Walks on the Wild Side, Fall-Winter, 2008-9

Source: http://www.ou.edu/cas/botany-micro/ben/ben120.html
In Search of Fern Seed (Part 1 or 3)

From: "Robbin C. Moran" (biobrm@aau.dk) originally published in the Fiddlehead Forum 22: 37-40. 1995

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 don't have seeds; instead, they disperse by tiny dustlike spores. Did people in Shakespeare's day believe that ferns had seeds? And what's this about walking invisible?

In 1597 when Henry IV was written and performed, the belief that ferns had seeds was common and widespread. To be sure, no one had ever seen a fern seed, but they couldn't imagine how ferns (or any plant, for that matter) could reproduce without such propagules. Therefore they reasoned 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 the fern seed had to be invisible because no one had ever seen it. Furthermore, they asserted that it conferred invisibility to the bearer; 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. You could catch it by stacking 12 pewter plates beneath a fern leaf; the seed would fall through the first 11 plates and be stopped by the 12th. If you came up empty-handed, it was because goblins and fairies, which were allowed to roam freely that one night of the year, had snatched the seed as it fell.

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 scientifically investigate fern dust was Marcello Malpighi, the famous Italian anatomist. In the late 1600s, he focused his microscope on the curious, dark spots or lines on the undersides of fern leaves. These 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 catapultlike action of the annulus. Nearly half a century later, Malpighi's observations were confirmed and elaborated by Nemiah 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, the Father of Botany, was puzzled about the nature of fern dust. In a letter written in 1737 to fellow Swedish botanist Albrecht von Haller, he said that "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, Linneaus was sure about one thing: ferns had seeds. :... continuing in BEN 121:

In Search of Fern Seed (Part 2 or 3)

From: "Robbin C. Moran" (biobrm@aau.dk) originally published in the Fiddlehead Forum 22: 37-40. 1995

Uncertainty reigned until 1794 when John Lindsay, a British surgeon, showed 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 searched the soil in the hope of finding a fern seed, but was unsuccessful. Undaunted, he decided to sow some of the dust-which he suspected as the true fern seed-and keep it in his room for observation.

“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 had put on a greenish colour, and some were pushing out their little germ, like a small protuberance, the rudiment of the new fern. This little protuberance gradually enlarged. 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 which gradually enlarged: 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, and shortly after another still more different. 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 therefore felt certain that the dust was 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 Botanical 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 he (Banks) would communicate to the Linnean Society of London.

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 most species-rich 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.

Yet Lindsay's observations raised more questions. Were the "membranes" or "scales" he observed equivalent to the seed leaf or cotyledon of flowering plants? If the dust was equivalent to the seed, where were the pollen-producing anthers? (The pollen, of course, was necessary to "stimulate" the development of the seed.) How and when did pollination take place? :.... conclusion in BEN 122:

In Search of Fern Seed (Part 3 or 3)

From: "Robbin C. Moran" (biobrm@aau.dk) originally published in the Fiddlehead Forum 22: 37-40. 1995

We smile at these questions today, knowing that they are completely misguided, but they were valid questions to botanists in the 1700s and early 1800s. It wasn't until 1844 that Karl von Naegeli, a German botanist, steered questions about the fern seed in the right direction. By focusing his microscope on the undersurfaces of the prothalli (the membranes or scales that Lindsay originally reported), von Naegeli saw globose papillae containing dark, spiral filaments. He noticed that the papillae, when wet, burst at the tip and released the spiral filaments, which then began to wiggle and swim away. He knew that similar papillae and filaments had been found in mosses and liverworts, where they were called antheridia, in allusion to the "male" anther of the flower. Thus, von Naegeli adopted the name antheridia for the papillae he saw on fern prothalli. But where did the spiral filaments swim to?

This question was answered in 1848 by Michael Jerome Leszczyc-Suminski, a Polish count with a botanical bent. He found that the spiral filaments swam to another kind of papilla also located on the undersurface of the prothalli. This type of papilla, which we now call an archegonium, was flask-shaped with a long neck and a single, large cell at the base. When the sperm swam to the archegonium, they wiggled downwards between the neck cells and penetrated the large basal cell. After penetration, this cell (now known to be an egg cell) developed into an embryonic fern with roots, stem, and leaves. This baby plant eventually grew into a mature fern with spore-bearing leaves.

What developed from Leszczyc-Suminski's observations was the picture of fern reproduction still taught today. In a series of quick nutshells this is it: The spores (fern dust) are produced on the undersides of the leaves in sporangia. They are liberated from the sporangia, land on a suitable substrate, and germinate. They grow into prothalli that bear the sex organs-archegonia and antheridia-which produce egg and sperm, respectively (the prothalli of some ferns produce only one kind of sex organ). The sperm are released from the antheridia when water is present and swim to the archegonia and fertilize the egg. The resulting cell, the zygote, develops into an embryo with stem, roots, and leaves. This embryo grows by widening its stem and producing larger and larger leaves until a spore-bearing leaf eventually appears. At this point the process is complete.

This sequence of events is known as the fern life cycle-the bugbear of many Introductory Botany students. It has two distinct phases, or generations. The first is called the gametophyte generation because it produces the gametes or sex cells. The second is called the sporophyte generation because it produces the spores. The gametophyte consists of the prothallus, and the sporophyte consists of the "normal" fern plant we typically think of-the one with roots, stems, and leaves. Each generation develops from a single cell: the gametophyte from a spore, the sporophyte from a zygote.

One point must be made about these two generations, a point often dimly understood: The gametophyte is the sexual generation because it produces the sex cells, egg and sperm. In contrast, the sporophyte is the asexual generation because it produces asexual spores; it does not produce sex cells. Remember this the next time you spot a leafy fern luxuriating in the wild. What you are looking at is an asexual being, one that does not and cannot engage in sex. This point is difficult to grasp because we tend to equate, erroneously, our own bodies with that of the fern sporophyte. But unlike plants, humans and other animals produce their gametes directly by meiosis; we have no intervening gametophytic (sexual) stage that produces gametes by mitosis.

But to return to the fern-seed. Botanists today realize that spores and seeds are completely different structurally. A spore consists of a single cell and contains no preformed embryonic parts. In contrast, a seed (typically) consists of hundreds or thousands of cells and contains stored food (the endosperm) and an embryo. Moreover, spores and seeds differ in what they give rise to. A fern spore gives rise to the prothallus of the gametophyte generation; a seed, to the baby plant of the new sporophyte generation.

These differences between spores and seeds seem so great that most of us are astonished when we learn that early botanists once seriously considered spores were seeds. But our astonishment is only proof that botany has progressed. Nowadays, it is the belief in the fern seed that walks invisible.

Selected References and Notes:

* The history of ideas about sexual reproduction in plants and animals, especially how it reflects prevailing social attitudes about sex, is treated by John Farley, Gametes & Spores, Ideas about Sexual Reproduction, 1750-1914 (Baltimore: The Johns Hopkins University Press, 1982)

* John Lindsay described his observations on fern reproduction in "Account of the Germination and Raising of Ferns from the Seed," Transactions of the Linnean Society 2: 93-100 (1794).

* The life of Leszczyc-Suminski is documented by Cezary W. Domanski in "M.J. Leszczyc-Suminski (1820-1898), an Unknown Botanist-Discoverer," Fiddlehead Forum 20: 11-15 (1993).

Author's address: Dr. Robbin C. Moran, Dept. of Systematical Botany, University of Aarhus, Denmark

Return to Fern Friends