4. Try probing the pollinium with forceps or a toothpick. It will readily adhere to your probe and come out intact, same as it would if your probe were an insect. The clear sticky mass stuck to the pollinium is called the viscidium. The mucilaginous sticky stuff is viscin, a substance found in several plant groups (for example, this is the sticky pulp that allows mistletoe berries to stick to birds’ beaks and tree bark).
5. Phalaenopsis flowers have the same basic structure but the labellum, or lip, is often reduced compared with Cymbidium and Cattleya. Expose the column, which may be straight or slightly curved, by removing the petals and sepals with the forceps or tweezers.
Phalaenopsis orchid. (A) Exposed tip of the column, with anther cap (rounded pollinia visible beneath) and rostellum extending over the hollow stigma chamber. (B) Column with anther cap and pollinia removed, leaving forked rostellum (talon) behind. The talon serves to pry pollinia off a subsequent insect visitor, pollinating the flower by securing the pollinia in the hollow stigma chamber. (C) Anther cap (above) and paired pollinia (below) attached to curved stipe. The other end of the stipe bears the sticky viscidium, which adheres the pollinia to an insect pollinator. (D) Pollinia stuck to a toothpick or similar probe. Drawing by Leslie C. Costa.
6. Note its domed anther cap with the slender rostellum pointing downward. The rostellum has a sticky strip down its middle. This is the viscidium, working like adhesive tape. When a pollinator touches it, the tape sticks to the insect’s body or head. Pulling away, the insect pops the anther cap off and the pollinia, which are attached to the sticky strip by the stipe, remain stuck to the insect. The rostellum remaining behind is a forked structure (“talon”) extending in front of a hollowed chamber (the stigmatic cavity). This will help scoop the pollinia off of a later-visiting insect, and pollinate the flower by implanting it into the stigmatic chamber.
See also:
“Orchids” at the Darwin Correspondence Project: www.darwinproject.ac.uk/learning/universities/getting-know-darwins-science/orchids.
8
Plants with Volition
It all started with the sundews that he found while the family was visiting Emma’s sisters in Hartfield, Sussex, in the summer of 1860. A nice patch of sundews growing in a boggy spot caught Darwin’s eye when he was off hunting orchids one day. It might have been the pinkish hue of the dainty leaf rosettes he noticed against the green mosses, or maybe an occasional flash and sparkle as sunlight played off the clear liquid droplets the plants are named for: Drosera, from the Greek word for dew. He stopped for a closer look. What appear as reddish indistinct patches from a distance resolve into orderly sets of small spatula-shaped leaves moist with tiny dewdrops, each borne on a crimson stalk; the whole looks like a botanical pincushion with rather stubby pins bearing clear acrylic heads. A few plants may be easily overlooked but not likely a whole glistening reddish bank of them, each with a tall slender wand of a stem bearing white-petaled flowers stirring in the breeze.
The delicate beauty of these plants belied their sinister aspects. The flypaper quality of the sticky leaves of the plant had long been known, but was that accidental? Was their object to capture insects? If so, why, and how did they sense their prey as they bend and move their dew-tipped filaments to ensnare hapless insects that venture too close? After all, this is a stunning thing for a plant to do: the life of a carnivore, yet seemingly lacking a stomach let alone the speed usually associated with “sit-and-wait” predators—and what can a plant do but sit and wait, rooted to the spot? There is something disquieting in the idea of a flesh-eating plant, especially magnified to human scale in our imagination. The horror inherent in the image of a mobile human-eating plant is doubtless what inspired the ambulatory botanical monsters of John Wyndham’s Day of the Triffids. These little sundews, Drosera rotundifolia, are terrors to the insect world, but at least they don’t pursue prey. Somehow their prey come to them, so, clearly, the plants have some means of enticement. Darwin collected about a dozen plants that day, and found that around half of the fully developed leaves bore dead insects or their remains. He was intrigued; as Janet Browne remarked in her Darwin biography, “by the end of the afternoon he was caught as surely as any fly.”1
On the surface of it Darwin’s fascination with these plants and the series of experiments that ensued might seem like a pleasant distraction, coming as it did in the wake of the Origin’s publication and the sometimes bitter early round of reviews and controversies surrounding the book. Certainly Darwin was curious about the question of sense perceptions of these plants, but it was Emma who, perhaps without realizing it, put her finger on the deeper reason for his interest. Back home at Down House later that summer, she mentioned her husband’s Drosera fascination in a letter to Lady Lyell, Charles Lyell’s wife, commenting that she supposed “he hopes to end in proving it to be an animal.”2 That was just it. In Darwin’s evolutionary vision, plants and animals likely shared common ancestry in the deep past. As he put it in the Origin:
Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype . . . all living things have much in common, in their chemical composition, their germinal vesicles, their cellular structure, and their laws of growth and reproduction. . . . Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.3
The notion of common ancestry of plants and animals, let alone other forms of life such as protists and bacteria, was one unwelcome implication of Darwin’s already rather unwelcome theory. But he persevered, and had a hunch that sundews, with all of their apparent “sensibilities,” just might be seen as a linking form of sorts. If plants and animals shared common ancestry, at a fundamental level they must share some common physiology despite outward appearances. Just as there might be animals out there with plant-like features, there should be plants with animal-like features. The sundew might be just such a plant. He also thought of sundews as another kind of linking form, remarking to his American friend Asa Gray that he “began this work on Drosera in relation to gradation as throwing light on Dionaea”4—the Venus flytrap, Dionaea muscipula.
The two plants are relatives, sharing the family Droseraceae. While there are a great many sundew species, there is only one Venus flytrap species, and its native range is barely a hundred or so miles across, in boggy swales dotting a small section of the coastal Carolinas in the United States. This plant is singular in other ways: it is the only carnivorous plant in the world with a “snap trap” mechanism, modified leaves so disturbingly like animal jaws that Europeans disbelieved the earliest reports of this plant. The renowned Philadelphia naturalist William Bartram swooned over this botanical marvel in his famous Travels of 1791: “But admirable are the properties of the extraordinary Dionea muscipula! . . . Astonishing production! . . . Can we after viewing this object, hesitate a moment to confess, that vegetable beings are endued with some sensible faculties or attributes, similar to those that dignify animal nature; they are organical, living and self-moving bodies, for we see here, in this plant, motion and volition.”5
Such seemingly extreme development, such exquisite adaptation, was often cited in the natural theology tradition as evidence of special creation. As we saw in Chapter 4, examples of organic perfection in the eighteenth and nineteenth centuries—the eye and honeybee’s cells—were tackled by Darwin in the Origin. In the “difficulties” chapter, under the heading “organs of extreme perfection and complication,” his solution to the problem of the eye was to argue for a transitional series. Look to related species to see what gradations might be possible, he urged. In the case of the eye, the invertebrate world presents a rich diversity of eye anatomies. “We can commence a series,”6 he argued in the Origin, from mere spots of light-sensitive pigment to fairly complex structures and a great many variants in between. The many and varied eye structures of invertebrates showed
that a continuous gradation of forms is possible, just what would be expected on his theory. Far from isolated islands of perfection, such forms were connected to all others by stepping stones, and this was his interest in the flytraps.
Dietary Supplements
There are many kinds of carnivorous plant, belonging to several plant families. They thrive in marginal, low-nutrient habitats like bogs and seeps by supplementing their photosynthetic diet with fresh meat: trapping and digesting small animals. The Botanical Society of America notes five ways that carnivorous plants trap their prey:
• Pitfall traps. The leaves of pitcher plants (family Sarraceniaceae) form tubular structures that fill with rainwater. The plant then secretes digestive enzymes into these leaves, which dissolve insects and other small animals for food.
• Snap traps. Venus flytrap (D. muscipula) and the related waterwheel plant (Droseraceae; Aldrovanda vesiculosa) have hinged leaves that snap shut at the touch of trigger hairs on the leave’s inner surface.
• Suction traps. The aquatic bladderworts (Lentibulariaceae; Utricularia spp.) have small globular bladder-shaped structures on their leaves, hollow with a hinged door lined with trigger hairs. Minute aquatic animals brushing against the hairs trigger the trapdoor to spring inward, sucking the prey into the hollow bladder where they are digested.
• Lobster-pot traps. The leaves of corkscrew plants (Lentibulariaceae; Genlisea spp.) are labyrinthine tubular channels lined with hairs and enzyme-secreting glands. It is easy to get in, but all but impossible to get out again.
• Flypaper traps. The leaves of sundews (Droseraceae; Drosera spp.), Portuguese sundew (Drosophyllaceae; one species: Drosophyllum filiformis), and butterworts (Lentibulariaceae; Pinguicula spp.) bristle with stalked glands that exude a sticky substance. Once prey is stuck, digestive enzymes are secreted to break down the body.
Not all of these groups have the power of movement to catch prey, and it was the combination of movement and digestive power that most intrigued Darwin. His sundew studies proceeded along two paths. The most immediate, and experimental, was in pursuit of mechanistic questions: how did the dew-tipped “tentacles” move and how did they sense prey? Did the plant produce digestive enzymes? What sorts of matter could it digest? The other path was more comparative: how did the movement and digestion by this plant compare with related plants? This path resonated with his argument about the evolution of eyes in the Origin: with enough versions perhaps gradations in the prey-catching and -digesting mechanism could be traced, gradations along a general evolutionary pathway.
An evolutionary link between sundews and Venus flytraps was not farfetched: in Darwin’s day it was recognized that the two are in the same taxonomic family. Just as the flytrap’s lobes close to form a stomach-like digestive chamber, the leaves of some sundew species inflect to form a similar cavity, sealed by the dewy stalked glands. A plant with a stomach: the very idea of it fires the imagination. A sundew, Darwin would write in Insectivorous Plants, “with the edges of its leaves curled inwards, so as to form a temporary stomach, with the glands of the closely inflected tentacles pouring forth their acid secretion, which dissolves animal matter, afterwards to be absorbed, may be said to feed like an animal.” Asa Gray agreed that the sundew’s mode of leaf movement is akin to that of the flytrap. But are there sundews out there with leaves that fully close upon their prey? Darwin found what he was looking for in John Lindley’s 1846 book The Vegetable Kingdom. The Asian species Drosera lunata did just that, prompting him to swing into crowd-sourcing mode, posting a query to the readers of the Gardeners’ Chronicle. “In Lindley’s Vegetable Kingdom (p. 433) it is stated that the leaves of Drosera lunata ‘close upon flies and other insects that happen to alight upon them.’ Can you refer me to any published account of the movement of the viscid hairs or leaves of this Indian Drosera?”7 Lindley himself responded to Darwin’s request, giving his source for the observation and even noting that he was so struck by its similarity to the Venus flytrap that he initially gave it the same specific name, Drosera muscipula, until he realized that the name “lunata” had already been given. It is now known that this species is one of many sundews with somewhat bowl-shaped leaves that can close upon insects, most dramatically in the Australian Drosera huegelii, a ground-hugging species with dangling bell-shaped leaves fringed with glandular hairs. How Darwin would have marveled at this species, had he known of it.
Now, this is not to say that the snap trap of the Venus flytrap is strictly homologous to the disk concavity of sundews, but they have some important physiological features in common that reflect their close relationship. For example, the structure of the stalked glandular hairs of Drosera and the trigger hairs of Dionaea and Aldrovanda are very similar and likely have a common origin. And then there’s the genetic evidence: DNA sequence data confirm that the two are indeed sister groups; in fact, all Drosera sundews (about 150 species) form a single evolutionary group (clade) which is sister to the single-species lineage of Dionaea, the flytrap, with Aldrovanda falling intermediate between the two.
A Most Sagacious Animal
The Darwin family returned home from Hartfield in early September 1860, sundews in tow, but 17-year-old Etty’s continued ill health prompted them to return to seaside Sussex by the end of the month. They stayed 7 weeks at a house on Marine Parade, Eastbourne, where Darwin continued his sundew studies—as much to take his mind off of his daughter’s dangerous illness, just a decade after her elder sister’s death, as to satisfy his curiosity. He wrote in mid-November to his neighbor John Lubbock: “[Etty] was desperately ill at Eastbourne & we gave up all hope; but she has rallied considerably. God knows what the ultimate result will be. I never passed a more miserable time than at Eastbourne.”8
Late summer of 1862 found the family back at the seaside, in Bournemouth where they rented a little holiday place called Cliff Cottage. Twelve-year-old Lenny had contracted scarlet fever at boarding school the previous June and was sent home; making matters worse his mother soon came down with it too, en route to the coast. By September both were recovering well, but Darwin was having a difficult time with his enforced holiday between fretting over the family and itching to get back to work: “This is a nice, but most barren country & I can find nothing to look at,” he wrote Hooker. “Even the brooks & ponds produce nothing—The country is like Patagonia.—My wife is almost well, thank God, & Leonard is wonderfully improved.”9 He took rambles with the children, and was tickled when Horace, his 11-year-old, showed a good grasp of natural selection. Relating the story to Gray, Darwin recounted how “Horace said to me, ‘there are a terrible number of adders here; but if everyone killed as many as they could, they would sting [bite] less.’—I answered ‘of course they would be fewer.’ Horace [said] ‘Of course, but I did not mean that; what I meant was, that the more timid adders, which run away & do not sting would be saved, & after a time none of the adders would sting’’—Natural selection!!”10 Darwin was amused and proud in equal measures, and described Horace’s scenario as “natural selection of cowards” to Lubbock.
About this time he realized he could easily procure his beloved Drosera in that “most barren country”—just the thing to calm his nerves. He picked up on his digestion experiments, first making a fly-sized bundle of his own hair and watching over the next couple of days how the sundew’s filaments first covered the bundle as if to consume it but then, as if the plant realized its error, uncurled and rejected the hair. What other substance to try? One can imagine him looking around, wondering what else was at hand . . . how about a bit of toenail? On the morning of Tuesday, September 16th, he wrote: “Put a bit of old nail of my toe on another leaf of same plant.”11 Once again the filaments closed, only to reject the offering. That must have been a curious sight—the esteemed naturalist sitting in a seaside cottage feeding bits of his hair and toenails to sundews transplanted in soup dishes.
Darwin had commented two years earlier, during his initial explorations, that sundews are �
��first-rate chemists”—the plants seemed to be able to distinguish between organic and inorganic substances. Now he had found out that, like animals generally, the sundews can’t digest tough organic material and could distinguish between edible and inedible organic matter. It makes sense to us now that the plants would be well adapted to distinguish edible objects from inedible and so avoid wasting time and energy on substances that aren’t digestible or nutritional. But we know this with the benefit of hindsight. No one thought of these little plants as “eating” to gain nutrition—a most unbotanical thing for any self-respecting photosynthetic plant to do—and as no one had thought of the question to ask, naturally no one had thought of experiments to test such a question until Darwin did. And these were only the beginning; over the next few years he launched himself into experiment after experiment with sundews, and fired off letter after letter to friends and associates asking for information on other sundew species, suggesting experiments, and reporting on his own experimental successes and failures.
Experiment builds upon observation, and so Darwin began with a close look: “The whole upper surface is covered with gland-bearing filaments, or tentacles, as I shall call them, from their manner of acting,” he later wrote in Insectivorous Plants.12 Note his word choice: “tentacles” are evocative of marine invertebrates—animals, not plants. Consciously or not, that was how Darwin viewed his little sundews, as his playful comment about them in 1863 to Gray indicates: “a wonderful plant, or rather a most sagacious animal.”13 Darwin set about observing his sundews closely, describing in minute detail the production of the viscid droplets that give the group their name, and documenting the structure and movement of the tentacles. He watched in fascination as they bent in unison in response to food, and set about testing just what would tempt them most.
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