Darwin's Backyard

by James T. Costa

  Darwin was excited about his discovery, and wrote his most trusted botanist friends to share the news and pump them for information on other cases of floral dimorphism. Asa Gray wrote back with a list of candidates, and enthused that Darwin’s “discovery that the pollen of one is good for the pistil of the other, but not for the pistil of its own flower, is most important” (emphasis Gray’s).30 That was October 1861, by which time Darwin had conducted exhaustive crosses and was putting the finishing touches on a paper for the Linnean Society: “On the two forms, or dimorphic condition, in the species of Primula, and on their remarkable sexual relations” was read on November 21, 1861, and published in the Society’s journal the following year. Remarkably, this was one of some 15 papers, notes, and letters Darwin published that year, on diverse subjects reflective of his expansive curiosity and research program: fertilization of orchids and other species, horse breeding and coloration, causes of variation in flowers, mating behavior of bumblebees, the influence of brain shape on bird behavior . . . all besides revisions to the Origin (the third edition came out in April) and endless correspondence over issues raised by the Origin and other matters. No wonder progress on his domestication book project proceeded at a snail’s pace. As we shall see in the following chapter, it was in fact orchids that took center stage immediately following the Origin, but he plugged slowly away on cross-pollination and the significance of flower morphs.

  How he would have loved to have understood the light shed on heterostyly by modern genetics. His beloved primroses became the model system, largely owing to his extensive work with them. There is a fairly straightforward genetic mechanism behind pin and thrum plants. Initially the phenomenon was thought to be controlled by a single gene, called S, but more sophisticated study revealed that the S locus itself consists of at least three closely linked genes that essentially act as one: G (controlling pistil length), P (controlling pollen size), and A (controlling anther length). In general, pin morphs can be treated as ss homozygotes for the recessive s allele, while thrum plants are Ss heterozygotes. Crosses between pin (ss) and thrum (Ss) plants yield a 1:1 ratio of pin to thrum offspring, something that Darwin recognized.

  Darwin dubbed the fruitful unions “legitimate” and the less fruitful “illegitimate,” and true to form he was determined to test the generality of his Primula results. The pioneering German botanist Karl Friedrich von Gärtner (1772 –1850) had long since joined Sprengel and Kölreuter as a sort of Holy Trinity of plant pollination and hybridization in Darwin’s pantheon. In 1863 he even wrote a letter published as “Vindication of Gärtner” in the Journal of Horticulture in response to disparaging remarks made by a commentator. Darwin repeated a multitude of Gärtner’s crosses and pursued clues to interesting phenomena suggested in his work. He especially admired Gärtner’s meticulous crossing experiments and diligent follow-through with the necessary but often neglected step of counting the seeds produced by experimental crosses to quantify fertilization success. He would adopt Gärtner’s approach in his heterostyly experiments.

  “Legitimate” and “Illegitimate” crosses between long-styled “pin” and short-styled “thrum” Primula veris flowers. Redrawn from Darwin (1877), p. 27, by Leslie C. Costa.

  In the meantime, with Gray’s help Darwin confirmed heterostyly in Partidgeberry (Mitchella repens), a creeping evergreen common in eastern North America and a nineteenth-century favorite for yuletide decoration. This woodland plant bears small paired white tubular flowers in the leaf axils, giving rise in fall to a single bright red berry formed by the fusion of the two carpels. Gray also had his student Joseph Trimble Rothrock make observations and experimental crosses on the common bluet (Houstonia caerulea), confirming another case. When Darwin’s attention was drawn to common flax, genus Linum, his son William assisted by posting over 200 specimens from the Isle of Wight. This interesting case warranted yet another paper for the Linnean Society.

  Mad over Lythrum

  In late 1861 he made another startling discovery: a species with trimorphic flowers. It was hiding in plain view: colorful stands of purple loosestrife (Lythrum salicaria) and its relative thyme-leaved loosestrife (L. thymifolia) are a familiar sight in England along rivers, streams, and other wet places, their tall waving spikes of conspicuous purple flowers giving vivid splashes of color amid the lush green banks. (Purple loosestrife is now a familiar sight along American waterways too, where it is considered a noxious weed.) He came across the account of multiple flower morphs in loosestrifes while making his way through the encyclopedia of botanical geography by Henri Lecoq, director of the natural history museum and botanical garden at Clermont-Ferrand, in central France. The nine volumes were tough going, but well worth the effort to find a prize like this. He had to investigate at once: he thought this might be the most complex plant reproductive system yet known, and what better evidence that heterostyly has nothing to do with evolution of dioecy (separate sexes) than a species with not two but three distinct flower morphs!? Frustratingly, winter was bad timing for this revelation. By late March, Hooker supplied him with plants. Darwin’s orchid book was now with the printer (it came out on May 15, 1862), so he could happily turn his attention to the secret life of Lythrum.

  In no time he confirmed that there are indeed three distinct floral morphs: short-, medium-, and long-styled. There are three types of corresponding stamens too, and for each pistil length two of the three stamen types are found in the same flower: the long- and mid-styled flowers both bear long and short stamens, while the short-styled morph bears long and medium stamens. Thus, as Darwin later described it:

  In the three forms, taken together, there are thirty-six stamens or males, and these can be divided into three sets of a dozen each, differing from each other in length, curvature, and colour of the filaments, in the size of the anthers, and especially in the colour and diameter of the pollen-grains. Each of the three forms bears half-a-dozen of one kind of stamens and half-a-dozen of another kind, but not all three kinds. The three kinds correspond in length with the three pistils: the correspondence is always between half the stamens borne by two forms with the pistil of a third form.31

  Whew! But if you think that’s complicated, just imagine keeping track of all the possible crosses between stamens and pistils of different length. There are 18 crosses based on stamen and pistil length, but he also thought he detected differences in the pollen of some of the morphs, giving many additional permutations. Darwin was also determined to repeat each cross 20 times to be absolutely sure of the results. Before long he was “stark staring mad” over Lythrum, as he put it to Gray: “it is grand case of Trimorphism with 3 different pollens & 3 stigmas; I have castrated & fertilised above 90 flowers, trying all 18 distinct crosses which are possible within limits of this one species!”32

  That June, in the middle of these experiments, he and Emma grew increasingly concerned about their son Lenny, 12 at the time. Darwin wrote Willy, in Southampton, about his brother’s symptoms: “his kidneys hardly act & his urine is tinged with blood—His liver is much disordered & he vomits. Poor dear little man, he is so patient.”33 He commented on his plans for purple loosestrife, but acknowledged that he didn’t have much heart for botany just then. A few days later Lenny was diagnosed with scarlet fever, which could be deadly. In fact their youngest child, 18-month old Charles, had succumbed to the disease almost exactly 4 years earlier. As July wore on Lenny seemed to be out of the woods but was very weak and recovering slowly. Gray kindly sent US postage stamps for Lenny’s collection to cheer him up; Darwin wrote Gray with gratitude: “He actually raised himself on one elbow to look at them. It was the first animation he showed. He said only ‘You must thank Prof. Gray awfully’—In evening after long silence, there came out the oracluar [sic] sentence ‘He is awfully kind’.”34

  Trimorphic flowers of purple loosestrife, Lythrum salicaria, based on a sketch Darwin made for William Darwin in 1862 to illustrate “legitimate” and “illegitimate” crosses. Note (top to bot
tom) the long, medium, and short pistil lengths, and different size classes of stamens. From Darwin (1864), p. 171.

  Darwin worked away at his Lythrum studies, and wondered if the proportion of the three morphs varied geographically. Hearing that his Wedgwood nieces, Katherine, Margaret, and Lucy, were on holiday in north Wales, he wrote to enlist their help. Margaret replied in early August: “Dear Uncle Charles, of 256 specimens of Lythrum gathered this morning from different plants, we find 94 with long pistil, 95– middle length pistil, 69—shortest pistil.”35 These were all found in one field, but they would check other locales as well. “My dear Angels!” came Uncle Charles’s swift reply; “I can call you nothing else.—I never dreamed of your taking so much trouble; the enumeration will be invaluable.”36

  Willy helped too. His father asked him to check out at least 100 plants in the field, tallying up the three morphs and marking flowers with long styles with three strings, medium-styled ones with two strings, and a single string for short styles. “I much want a few pods, (wrapped up separately when ripe) to count seeds in each, to know natural product, & to compare shape of pods.” The idea was to see if some crosses were more productive than others. He had instructions for Georgy as well, a “rare hand at watching insects,” his father said. “Beg him to observe carefully what part of body of bee the knobbed stigma of long & mid-styled rubs against.”37 His sister-in-law Sarah Elizabeth Wedgwood managed to procure specimens of the rare Lythrum hyssop­ifolia from several countries, making for a valuable comparison with specimens Oliver sent from Kew. Darwin was in crowd-sourcing mode, and everyone was eagerly lending a hand.

  Charles and Emma decided by early August that with Lenny well enough to travel they would head to Bournemouth, near the coast, stopping en route to see Willy in Southampton. In the meantime they sent Etty, Bessy, and little Horace off with their former nurse, Brodie, now a dear family friend down from Scotland for a visit. They planned to meet up in Southampton. Charles’s sister Susan had gone ahead separately with Franky and Georgy, arriving in Southampton on the 2nd of August. Darwin wasted no time enlisting the two of them as field assistants: Willy reported in a letter home later that week that he had “packed off the boys” to observe and collect Lythrum flowers, instructing them to take care to collect from different plants. Charles and Emma traveled with Lenny to Southampton the following week, but en route the boy suffered a relapse of scarlet fever and Emma ended up contracting it too. Charles sent all the other kids on to Bournemouth, tending to Lenny and Emma in Southampton for another 2 weeks before the family was together at last.

  Enforced holidays were not endured gladly by Darwin, but he always made the best of them. He went on countryside rambles, and despite complaining to Hooker that Bournemouth was “most barren country,” some things caught his eye. Earlier in this chapter I related how honeybees visiting red clover caught his attention. He also came across his old friend the sundew and performed a few experiments (see Chapter 8), inspected local flowers, and pondered Lythrum.

  The Darwins returned from Bournemouth at the end of September 1862. Everyone was well at last, and Darwin was keen to get back to work. He immediately returned to Variation, “dull, steady work”—but his heart lay with loosestrife. “My geese are always at first Swans,” he wrote to Gray, “but I cannot help going on marveling at Lythrum.”38 That summer he had managed to complete 94 crosses before Southampton and Bournemouth and the results were spectacular, but now it was getting too late in the season to do much more. He picked up where he left off the following summer, and by August he was able to write to Gray that he had “worked Lythrum like a Trojan,” having just completed a heroic 134 crosses. “No slight labour,” he pointed out with understatement, “but the case seems to me worth any labour, for I declare I think it about the oddest case of reproduction ever noticed—a triple marriage between three hermaphrodite.”39 And there was more labor to come; in the end his paper was not read at the Linnean Society until June 1864. It was a masterful work of 30-odd pages, with a multitude of tables summarizing the results of hundreds of experimental crosses and meticulous seed counts, and an overview of flower morphs in a host of other species with trimorphic flowers. All told, he had amassed incontrovertible evidence that the evolutionary imperative of intercrossing is the point behind heterostyly. That is, trimorphic flowers exist to maximize the likelihood of mating with a genetically distinct individual.

  Darwin’s 1864 paper on the three forms of L. salicaria was the tip of the iceberg when it came to this botanical hobbyhorse—over the ensuing decade he produced another dozen papers and notes bearing on pollination, cross-fertilization, flower structure, and further studies of heterostyly amid the many other subjects he wrote on, even while managing to finally complete the two-volume Variation of Animals and Plants Under Domestication (1868); two volumes on human evolution (Descent of Man and Expression of the Emotions in Man and Animals, published in 1871 and 1872, respectively); another three Origin editions (they culminated in the sixth edition of 1872); and his volume on insectivorous plants (1875; see Chapter 8). So perhaps he can be forgiven for taking until 1876–1877 to come out, at long last, with the magnum opus duo on cross- and self-fertilization and heterostyly.

  The Forms of Flowers volume, dedicated to Asa Gray, is more than a botanical classic. Most of the conclusions Darwin drew from his observations and experiments are still valid well over a century later. That’s impressive, but more to the point of the present book, botanist Herbert G. Baker of UC–Berkeley noted that Darwin’s volume is especially valuable “in illustrating the principle that carefully thought-out but simple experiments can substantiate or falsify quite important ideas”—a welcome message for would-be Darwin-inspired experimentisers who might, as Baker put it, “be discouraged by the seeming necessity for high technology in all branches of contemporary science.”40 Baker rightly pointed out that plenty can be done with the simplest of equipment and, above all, a good eye. Darwin’s sentiments exactly. The incomparable Henry David Thoreau, who was fascinated with Darwin’s ideas and whose untimely death in 1862 meant he never knew of Darwin’s exciting investigations into the secret lives of plants, captured the spirit of this philosophy in one lovely journal entry: “Nature will bear the closest inspection. She invites us to lay our eye level with her smallest leaf, and take an insect view of its plain.”41

  Experimentising: Darwinian Encounters of the Floral Kind

  Darwin was convinced that the benefits of outcrossing in plants are many—the number, vigor, and fertility of offspring produced by outcrossing are superior as compared to self-fertilization—and he marveled at the means plants use to entice, reward, or dupe insects into visiting their flowers and carry their pollen. Here is a Darwin-inspired selection of easily observed pollination “contrivances.” Like Darwin, you can learn to see once-familiar plants with new eyes.

  I. Pollination Machinations

  A. Materials

  • This investigation is seasonal in regard to the availability of flowers. With enough lead time you might grow your own, or try to obtain the following plants in flower from a local nursery: kidney bean (Phaseolus vulgaris), sage or salvia (Salvia), mountain laurel (Kalmia latifolia), barberry (Berberis), foxglove (Digitalis)

  • Dissecting probe, toothpick, or stiff camel-hair brush

  • Forceps or tweezers

  • Craft knife or razor blade

  • Magnifying lens and/or dissecting microscope

  B. Procedure

  1. Kidney bean (Phaseolus vulgaris)

  Darwin described the lever-action mechanism of kidney bean flowers: the style is curved in a left-hand loop (“like a French horn,” he thought) within the modified petal called the keel. The stamens are housed within this looped tube as well. He realized that the two lower petals protruding at the bottom present a landing platform for bees. When they land their weight depresses these petals, popping the stigma (tip of the pistil) out of the tube. You can imitate the action of a bee like Darwin did.
Carefully tug on the lower petals (you may have to hold the flower steady), and observe how the stigma tip emerges from the coiled tube. Note the brush of fine hairs just below the stigma. Tug and release the lower petals a few times: note the brush moves backward and forward, as it would each time a bee lands on the flower. The bees indirectly help the flower self-pollinate, as the brush sweeps shed pollen out of the tube and gradually pushes it into contact with the stigma.

  2. Sage or salvia (Salvia spp.)

  Stamens can move as levers too, as in Salvia species—salvia, sage, and related garden plants in the mint family (Lamiaceae). Each functional stamen is joined near its base with a modified sterile stamen. Shorter and stouter than the fertile elongated member of the duo, the sterile stamen takes the form of a basal spur or process. These are situated right in the gangway, so bumblebees trying to get at the nectaries behind must push these spurs and in the process tip the overarching fertile anthers down onto the back of the bee, dabbing it with pollen—a simple but effective lever. Using the dissecting probe, toothpick, or camel-hair brush, carefully push at the base of the sterile stamen spur and note how the attached fertile stamen dips down by lever action.