Darwin's Backyard
Page 21
Yes, sex weighed heavily on Darwin’s mind in more ways than one. But how did this lead him to pry into the secret lives of plants? It started with that nagging question of crossing.
Darwin, Knight, and the Law
Darwin eventually realized that Herbert’s objection was not the problem it first appeared because outcrossing was a matter of degree. Hybridization was not rampant because individual animals or plants would only produce healthy offspring if mates were of sufficiently close relationship. At the other end of the spectrum, if individuals are too closely related, then the ill effects of inbreeding are felt. Successful intercrossing, or outbreeding, is between these extremes, that golden mean where the maximum benefit is realized.
The very year of Darwin’s discovery saw the passing of the breeder who convinced him about the universality of intercrossing. Thomas Andrew Knight (1759–1838) was a renowned Herefordshire horticulturalist. In 1799 the Philosophical Transactions of the Royal Society published Knight’s “An account of some experiments on the fecundation of vegetables,” a work that Darwin seized upon in his quest for general principles of reproduction. Like Mendel later, Knight performed crossing experiments with different varieties of the common garden pea. In one such experiment Knight transferred pollen from large and luxuriant pea plants to diminutive ones, and vice versa. The results on vigor and seed size in the offspring were as unexpected as they were impressive: “I had, in this experiment, a striking instance of the stimulative effects of crossing the breeds; for the smallest variety, whose height rarely exceeded two feet, was increased to six feet.”5 Knight went on to relate mixed success doing similar experiments with wheat, apple, and grape varieties. Nonetheless, he concluded that “improved varieties . . . may be obtained by this process, and that nature intended that a sexual intercourse should take place between neighbouring plants of the same species.”6
What’s more, Knight observed, the correctness of this conclusion is apparent when one considers the many and ingenious ways devised by nature to transfer pollen. In some species pollen is so light that breezes can carry it far and wide. In others the stamens are strategically placed so as to shower visiting insects with pollen, and “the villous coat of the numerous family of bees, is not less well calculated to carry it.” So, despite the proximity of male and female parts in the same flower, nature “intended” cross-pollination and had devised a variety of contrivances to ensure that this occurs. But why? Knight thought he had the answer: sexual intercourse “certainly tends to confine within more narrow limits, those variations which accidental richness or poverty of soil usually produces.”7
In other words, Knight believed, first, that environmental conditions (the quality of the soil) induce variations, something that Darwin also believed and that was not decidedly rejected until the twentieth century with the discovery of Mendel’s work. Knight seemed to further believe that crossbreeding acts to spread variation around, a homogenizing process that prevents those environmentally induced variations from getting out of hand. Darwin agreed, but was especially interested in the stimulative effects of crossbreeding. Unlike Knight, Darwin believed that the very act of crossing engendered variations. Relying on Knight’s work, Darwin developed a working hypothesis: crossbreeding, even if accomplished only occasionally, was essential for the long-term adaptability of species. In 1844, he wrote in his species Essay:
This power of crossing will affect the races [breeds or variants] of all terrestrial animals; for all terrestrial animals require for their reproduction the union of two individuals. . . . I cannot avoid suspecting (with Mr Knight) that the reproductive action requires, at intervals, the concurrence of distinct individuals. Most breeders of plants and animals are firmly convinced that benefit is derived from an occasional cross, not with another race, but with another family of the same race; and that, on the other hand, injurious consequences follow from long-continued close interbreeding in the same family.
He called this “Knight’s Law,” and became so convinced of the critical importance of crossing that he later dedicated a chapter of over 100 manuscript pages to the subject in his Natural Selection manuscript.8 While he could observe that “the crossed offspring of two varieties, & even of two individuals in hermaphrodite plants, have their vigour & fertility increased,” he admitted not knowing why this should be so, acknowledging “how utterly ignorant we are in regard to Life & its Reproduction.”
Still, Darwin was getting at the very reason behind the phenomena of sex and sexes. This was the backdrop to many of his experiments and other investigations through the 1840s and beyond. Darwin’s case for the near-universality of crossing was so compelling that “Knight’s Law” was called the “Knight-Darwin Law” by naturalists in later years, and he became fascinated by the stunning creativity of nature when it came to methods of achieving cross-pollination. In the garden, greenhouse, and meadows of Down House a bevy of experiments and other investigations kept Darwin and his family as busy as the bees he came to appreciate as the plants’ main pollination agents—researches culminating (after literally thousands of experiments and related investigations) in his remarkable treatises of the late 1870s: The Effects of Cross and Self Fertilisation in the Vegetable Kingdom, published in 1876, and The Different Forms of Flowers on Plants of the Same Species published the following year. At first glance these books may seem to treat unrelated botanical topics, but to Darwin the significance of crossing was of a piece with the phenomenon of separate sexes in flowering plants. He tried to think like a plant, considering reproduction from their point of view, and came to appreciate the central importance of those six-legged go-betweens whose abundance and ubiquity make them most excellent pollen couriers. His investigations proceeded along two partly overlapping paths: he first began a program of documenting insect-mediated cross-pollination, and next documented the beneficial effects of cross-pollination.
Not-So-Secret Agents
Darwin built his case bit by evidentiary bit in characteristic fashion: close observation, meticulous experimentation, extensive inquiries, and, eventually, what we might call crowd-sourcing. One of his early acquaintances and correspondents was Robert Brown (1773–1858), a giant of British botany (and of Brownian motion fame) who, in spring 1841, recommended that Darwin consult Christian Konrad Sprengel’s 1793 book Das entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen (translated as Nature’s Secret in the Structure and Fertilization of Flowers Unveiled). In this book Sprengel (1750–1816) argued for something that we take for granted today, namely, that the object of showy flowers is in most cases to attract insects for the purpose of pollination. He meticulously documented a bevy of plant-pollinator relationships but didn’t bother with the “why” of these relationships, something that Darwin found disappointing: “Has no notion of advantage of intermarriage,” he scribbled in the margin; “Seems to think fact of insects being required at all, does not deserve any explanation.”9 Darwin dismissed as hopelessly naive Sprengel’s suggestion that stamens and pistils were found together for the convenience of visiting insects. For all of his criticism, however, he was enthralled with Sprengel’s book. He systematically followed up on the botanist’s observations and added a great many of his own; there may have been “some little nonsense,” but the book was nonetheless “full of truth,” he declared to Asa Gray.10 Years later his son Frank said that Sprengel’s book “not only encouraged [his father] in kindred speculation, but guided him in his work . . . It may be doubted whether Robert Brown ever planted a more fruitful seed than in putting such a book into such hands.”11
Initially, his insect- and flower-watching was done on the side, a relaxing distraction from other pursuits in the early 1840s. At that time Darwin was in the middle of the “Zoology of the Voyage of the Beagle” project, which was ultimately published in five taxonomic sections between 1838 and 1843. He effectively acted as editor-in-chief for these volumes, even as he labored over geological projects stemming from the Beagle voyage. As
we saw in Chapter 1, Darwin had become a rising star in the London geology scene, elected a Fellow of the Geological Society immediately upon his return to England in 1836 and then Secretary just two years later. He gave up the secretaryship in 1841 though—it was all becoming too much, not least owing to the arrival of a new addition to the family: a daughter, Anne Elizabeth, born March of that year. Charles and Emma were exhausted, and he was already showing signs of the mysterious illness that would plague him for the rest of his life. So in late spring 1841 they fled Macaw Cottage for a much-needed respite with family at Maer Hall in Staffordshire, where Emma grew up. They arrived at Maer in late May, 2-year-old William (“Doddy” as they called him then) and infant Annie in tow, to the delight of Emma’s parents (and Charles’s uncle and aunt) Jos and Bessie Wedgwood.
Charles and Emma both had extremely fond memories of Maer, with its grand Georgian country house set on a low hill, and extensive park and gardens (originally laid out by renowned eighteenth-century landscape designer Lancelot “Capability” Brown). A long, curved, rhododendron-lined drive led to the house, against a knoll overlooking a placid lake ringed by a sandy path the Darwin’s loved taking walks on (and would later inspire Darwin’s thinking path, the sandwalk, at Down House). There were also acres and acres of surrounding woods and farmland. The Wedgwoods and Darwins were very close—recall that Charles’s mother was a Wedgwood, and it was his uncle (and future father-in-law) Jos who had convinced Charles’s father to give him a fair hearing on the benefits of voyaging round the world on HMS Beagle, an idea that his father initially dismissed as “a wild scheme.” Emma was the youngest of eight children, and Charles and his siblings visited often growing up; together the kids had the run of the place. Emma with her sisters and mother took great pleasure in planting the flower beds, with exuberant, layered clouds of color and texture: reds, yellows, purples, whites, the spreading masses punctuated by gracefully branched beauties and showy spikes. Many years later Charles and Emma’s daughter Henrietta (Etty) commented that “Maer Hall . . . was so deeply beloved by the whole group that their children even have inherited a kind of sacred feeling about it. . . . My father used to say that our mother only cared for flowers which had grown at Maer.”12
And what a palette those flowers made: yellow daylilies, deep purple geraniums, pink azaleas, blue monk’s hood, larkspurs, lupines, and campanulas, white fraxinellas, scarlet poppies, multicolored speedwells, heartsease, and many more. On a sunny day the beds were alive with the hum of innumerable busy bees—which meant that Maer’s beautiful gardens offered more than peace and quiet for Darwin’s frazzled nerves; they were also an outdoor laboratory for testing his latest ideas. Bees and flowers were on his mind, and maybe more: Sprengel’s volume was a veritable Kama Sutra of the plant-pollinator world, and Darwin studied the manual assiduously. Every flowering plant had a story, a particular relationship to reveal, and Darwin probed and peered from petal to pistil like a paparazzo to learn the secrets of each. He spent many an hour observing floral structures and the movements of bees as they went from flower to flower, how they probed for nectar, and noting which ones they stopped at and which they passed over, how long their visits lasted, and timing how many blossoms could be visited in a set interval of time. Here is a typical notebook entry:13
Maer. June 15 /41/. Watched plants of Fraxinella, with seven flower stalks for ten minutes, it was visited by 13 Bees—& each examined very many flowers. = 22d—/during several succeeding days many most numerous bees visited this same bunch & on this day in five minutes eleven Humbles came & each visited many flowers—
Saw Bees frequent these flowers till late in evening—On rough calc. 280 flowers—allowing each Bee visits 10 flowers in minute each flower will be visited in 28 minutes—say then each flower is visited 30 times a day is considerably under mark, & this has now gone on 14 days. (except some wet ones/ & wd go on longer—
His stream-of-consciousness notebooks convey the excitement of discovery. Fraxinella (Dictamnus albus), a perennial from the Mediterranean region, is also called “burning bush” or “gas plant” for its highly flammable aromatic oils. Its flamboyant perfumed flowers have white to pink backswept petals and long, protruding stamens—a favorite of bees. Darwin watched a plant with seven flower-laden spikes visited by 13 bees in 10 minutes, but they worked so fast it was difficult to tell just how many individual flowers they visited. Turning that around and estimating how many flowers were visited by a bee in a minute and then extrapolating to the number of available flowers, he figured that each flower was visited about 30 times per day, as a conservative estimate. At that rate transfer of pollen by insect emissaries was virtually guaranteed.
Later that summer, back in London, Darwin weighed in on a discussion in the pages of the Gardeners’ Chronicle, touched off by a letter signed “Ruricola.” The broad-bean crop had been doing poorly that year, and Ruricola laid this at the multiplicitous feet of bumblebees. He spied these bees chewing through the base of broad-bean flowers to extract the nectar, and concluded that this rendered the flowers incapable of fruiting. His advice was both remedy and punishment for the iniquitous bees: “it would . . . be a very desirable object with the bean-grower to avoid such a loss, and no better means could be adopted than to destroy the Humble-bees’ nests at the end of summer, and employ children to catch and kill the females in the bean-fields as soon as the first blossoms have expanded.”14 In the course of his bee-watching that summer Darwin had also observed flower-perforating behavior (now called “nectar robbery”), but he had a different take on how it related to failure of the beans.
That August he wrote to the Gardeners’ Chronicle:15 “Perhaps some of your readers may like to hear a few more particulars about the humble-bees which bore holes in flowers, and thus extract the nectar,” he began. He proceeded to give a litany of examples he had observed: snapdragons and penstemons, salvias and stachys. The rhododendrons he observed at Maer were instructive: while he did witness nectar robbery with these flowers he concluded that most bees nonetheless obtained nectar from them in the usual way, dipping into the corolla, as evidenced by the fact that the specimens were sterile hybrids that could produce no pollen of their own, yet their pistils were covered with many fine grains. He put his finger on the real reason the perforated broad-bean flowers failed to develop fruit. The bees were responsible, not in the way Ruricola imagined, “but by their not extracting the nectar in the manner nature intended them,” and therefore failing to transfer pollen. He continued, half-jokingly:
If all those flowers, even hermaphrodite ones, which are attractive to insects, almost necessarily require their intervention . . . to remove the pollen from the anthers to the stigma, what unworthy members of society are these humble-bees, thus to cheat, by boring a hole into the flower instead of brushing over the stamens and pistils . . . ! Although I can believe that such wicked bees may be injurious to the seedsman, one would lament to see these industrious, happy-looking creatures punished with the severity proposed by your correspondent.
On the contrary, Darwin pointed out with tongue firmly in cheek, florists should praise the bees for their “ingenious method” of obtaining the nectar, since in that way they save wear and tear on the delicate petals, which become ragged when the bees constantly scramble in and out of the flowers: “The little orifice which the bees make, in order to avoid clambering in at the mouth, is hardly visible; whereas all the flowers in some beds of the Mimulus, at the Zoological Gardens, are sadly defaced.” (That comment revealed what Darwin was up to when visiting the zoo: rather than viewing the animals, he was in the flower beds making observations.)
Rube Goldberg Flowers
It may seem odd to a modern reader that Ruricola didn’t see that the reason for the failure of the beans to develop was want of pollination. But even as late as the mid-nineteenth century the role of insects in pollination was far from clear, or generally accepted. The prevailing assumption for centuries was that plants self-fertilize, and the side
-by-side placement of stamens and pistils in flowers of many types reinforced this view and was even cited as evidence of the wisdom of the Creator. Darwin sought to not only establish that insects, especially bees, were the agents of pollination, but even more importantly that the pollen they transferred was by and large from other individuals of the same species.
Darwin became more and more intrigued with the quirks and oddities of pollination itself. He noted at Maer in 1841 how in certain flowers the stamens and pistil curved upward so as to place their tips precisely in the path of bumblebees heading for the nectaries (what he called the “fairway” or “gangway”). Rhododendron was a good example, and Fraxinella with stamens that actually moved into position in the gangway as they ripened. Starting out straight as pins, that changed when on a hot day the “anthers all burst . . . & I found their positions all changed & their tips now stood in the direct gangway to the nectary, & were brushed by every Bee which entered.”16 Flowers like those of rhododendron and fraxinella were the simple cases, and no one in Darwin’s day would have batted an eye at the seemingly providential placement of the stamens and pistil. That was true of other flower “contrivances” too, but before long Darwin went down the botanical rabbit hole into a world of cross-pollination mechanisms that few in his day could have even conceived existed.
There were curious spring-action stamens to fling pollen, lever-action structures where pressure on a petal here results in a stamen poking out over there, flowers where stamens and pistils ripen at different times, and elaborate structures and chambers that force hapless insects down one-way pathways in the flowers or even imprison them for a time like floral houses of horror. Darwin shed light on odd flower morphs, discovering in some cases as many as three coexisting in the same species, and the fact that many of our favorite botanical beauties supplement their showy blossoms with small hidden flowers that never open, which they seem to use as insurance policies against poor insect pollination years. Rube Goldberg, the master of intricately complicated devices (incidentally born the year after Darwin’s death), would have appreciated the odd contrivances for pollination that Darwin elucidated.