A Garden of Marvels Page 21
Over time, however, doubts about spermism arose. According to Leeuwenhoek, a million human spermatozoa could fit in a grain of sand. That meant that if each sperm held a tiny person, God was horrendously wasteful of creatures made in his own image. It made masturbation mass murder. Pollen, which blew about in spring in quantities great enough to fur a pond in a coat of yellow, was an even larger, if less heart-wrenching, waste of life. While Nature was obviously prodigal of youth—in early eighteenth-century London, almost half the children died before their second birthday—this level of carnage was hard to accept.
Ovism won a new lease on life in 1740 when Charles Bonnet discovered that aphids can reproduce parthenogenetically, that is, without the participation of a male. (The female effectively clones herself.) After the mid-1700s, many scientists concluded that spermatozoa are actually parasites, although why they are found only in postpubescent males was perplexing. Another possibility began to be discussed: Perhaps the fecundating power of semen lay in the liquid in which the parasites swim. The favored theory was that new creatures reside, preformed, in female ova, which are pushed into life by the liquid component of semen.
About this time, the Italian scientist Lazzaro Spallanzani supplied some actual evidence to this heretofore completely abstract debate. Spallanzani was a genial, round-faced, baldheaded man who looked a bit like actor Wally Shawn. Born in northern Italy in 1729 to a lawyer and his well-connected wife, at the age of twenty he embarked on the study of law at the University of Bologna, where his cousin Laura Bassi was the first female professor of physics and mathematics in Europe. Under the influence of the remarkable Bassi, he reoriented his studies to physics, chemistry, and natural history, and earned a doctorate in philosophy. He was also ordained and became associated with two congregations, although the Abbe Spallanzani would never spend much time on priestly duties. Instead, he taught logic, metaphysics, and Greek at the new University of Reggio Emilia, not far from his family’s home.
He also read the work of the renowned French naturalist Georges-Louis Leclerc, Comte de Buffon, and his sometime collaborator, the English Catholic priest and amateur biologist John Turberville Needham. These cross-channel collaborators researched and wrote on the subject of spontaneous generation, the theory that animals can emerge from inanimate materials. One hundred years earlier, Francesco Redi had demonstrated that the maggots that appear in putrid meat do not leap unparented into existence, but grow from eggs laid by flies. To Buffon and Needham, however, the fact that complex animals did not appear from nowhere did not exclude the possibility that microscopic life generated spontaneously. They thought it quite likely that animalcules invisible to the naked eye might pop into being out of air. In 1750, Needham reported experimental evidence that he was certain confirmed the immaculate birth of microbes. He had boiled meat broth for ten minutes—boiling was by then a well-known method of killing microorganisms—then poured the liquid into vials and corked them. A few days later, he found microorganisms bustling about in the liquid.
A skeptical Spallanzani repeated Needham’s work in the early 1760s, with some crucial adjustments to the Englishman’s experimental methods. He put the broth in a flask and boiled it for an hour (after drawing off most of the air inside the flask so the air wouldn’t expand during boiling and explode the flask). He also hermetically sealed the flask rather than simply corking it, and set up a control group that he boiled at length but only corked. The broth in his sealed flask remained sterile indefinitely while the corked broth grew cloudy with microbes. Needham was unmoved by the evidence. Such long boiling, he wrote, had killed the broth’s “Vegetative Force” and damaged the “elasticity” of the interior air, preventing any new organisms from springing into being. After a number of creative modifications to his original experiment, Spallanzani proved to his own satisfaction, although not Needham’s, that microbes do not generate spontaneously from “putridity.” Microscopic life, like all life, can only come from life.*
Spallanzani’s experiments inspired him to look into other questions related to generation, growth, and reproduction. He studied the ability of salamanders and frogs to regrow legs, and snails to grow replacement heads. (Really, they can.) He managed to isolate a single microorganism in a drop of water and watched how it either budded or fissioned to replicate. Some microorganisms, dried and seemingly dead for years, he found he could bring back to life. He studied spermatozoa of various species intensively, subjecting them to motion, chemicals, fumes, and changes in temperature in an attempt to understand just what these “spermatic worms” were. His laboratory methods were meticulous: He repeated many experiments dozens and even hundreds of times, tried to falsify his hypotheses, tested alternative explanations for results, and used control groups. A persuasive writer, he became one of the leading scientists of the day, and in 1769 he accepted a chair at the prestigious University of Pavia.
His work on sperm led him to the central question of whether semen is essential to fecundation or not. If it is, what exactly does it do? Spallanzani turned to aquatic green frogs for his experimental subjects. No one had properly observed amphibian sex, and everyone assumed, because male frogs clasped females, that fertilization took place internally as it does in mammals. Spallanzani was the first to recognize that this is not so, that the male frog’s milt, ejected into the water during frog amours while the female emits strings of eggs, is the equivalent of semen. But what exactly did the milt do and how did it do it?
Spallanzani focused on the proposition than an aura seminalis fertilizes eggs. Could the aura work through the air? He tied strings of frog eggs above a glass dish of frog semen, but the eggs did not turn into tadpoles. Could an aura radiate through water? In one of history’s most charming experiments, he outfitted dozens of his male frogs in tight-fitting, waxed, taffeta trousers (I imagine pink) and set them swimming with female frogs ready to release eggs. Uninhibited by their formal attire, the male frogs responded as male frogs ought, which Spallanzani knew by examining their trousers afterward. None of the nearby eggs became tadpoles. His data should have led him to conclude that direct contact between sperm and eggs was essential to conception. Instead, he concluded that tadpoles were already complete in the eggs and the role of semen was to inspire them to grow. How could this be?
For starters, he was a committed ovist. After having spent years discrediting the myth of spontaneous generation, he found it impossible to believe that a “shapeless body, whether liquid or solid” could become an organized being. There had to be a creature in either the egg or the sperm, and his microscopic inspection of sperm revealed no tiny creatures.
A quirk of amphibian reproduction further led him astray. It happens that when a frog or other amphibian egg is pricked and its surface breached, the egg can respond as if a sperm has passed through its membrane. It begins to divide and soon a tadpole, and eventually a frog—a clone of its mother—develops. Spallanzani, in describing his lab procedures, noted that he used a needle or a pencil to manipulate frog eggs. From time to time, he must have pierced an egg because he described the development of virgin eggs into tadpoles. Although these accidents didn’t happen all the time, he couldn’t help but be impressed when they did. Frog eggs, he wrote, “are not eggs, as Naturalists suppose, but real tadpoles. . . . The egg is nothing but the tadpole wrapped up and concentrated.” Moreover, “There is no essential difference between impregnated and un-impregnated eggs.” He then verified his discovery with similar experiments on several species of toad and newts. “The aspersion [sprinkling] of the seed of the male,” he concluded, “is a condition necessary to the animation and evolution of the fetus,” but not to its original formation.
If frog eggs were really slumbering frog embryos, were plant ova likewise quiescent plant embryos? He ran experiments on hemp, pumpkin, spinach, dog’s mercury, and a dozen other species. Like Camerer, he strove to destroy any source of pollen and to prevent any accidental fecundation. Many times he was successful in isolating his subjects,
and no viable seeds resulted. But sometimes his female flowers—even if cloistered indoors—did produce fertile seeds. Again, quirks of nature were responsible. The tendency of some species, like hemp and spinach, to produce the occasional hermaphrodite individuals resulted in seed set. He didn’t know that the female flowers of his pumpkins could be pollinated by other varieties of Cucurbita, such as zucchini. In some plant species, distantly related pollen does not fertilize an ovum but can stimulate it to divide parthenogenetically, as the needle provoked the frog eggs. And he had no idea how far pollen of his experimental subjects could travel: hundreds of yards easily and much greater distances if transported on hair or clothes. Spallanzani conducted many of his experiments outdoors, so when he confidently reported that there were no hemp yards nearby to threaten his virgin hemp, who knows what wild and subtle Romeo had breached his convent’s walls? In any case, he became convinced—and thanks to his reputation, many others did, too—that pollen, like semen, played only a minor role in creating offspring.
If only Spallanzani had met Josef Gottlieb Kölreuter. Kölreuter was born in 1733 in a small town in the Black Forest region in the southwest of modern Germany, the son of an apothecary. At fifteen, he matriculated at the nearby University of Tübingen, where he studied medicine and botany and published a review of all the experiments to that date on plant sexuality. Degree in hand, he went to St. Petersburg, Russia, in 1759 to work as a natural historian at the Imperial Academy of Sciences. That year, the Academy offered a prize for the best essay providing new evidence on the question of whether plants reproduce sexually or not. Linnaeus, already an ardent supporter of plant sexuality, submitted an essay that asserted that hybrids proved that plants are sexual. (A hybrid is the result of the mating of a male and female of two distinct species.) As proof, he offered up dozens of plants he said had the leaves and epidermis of the male parent and the fruit and bark of the female parent. His essay won him fifty gold ducats.
The problem, or rather the greatest problem, with Linnaeus’s essay was that his hybrids were not hybrids. To Linnaeus, any plant that looked like an intermediate between two species was a hybrid, and with those criteria he found hybrids everywhere. (Had he been looking for mammalian hybrids, he might have concluded that the thirty-pound spotted ocelot is a hybrid between a hundred-pound leopard and a domestic cat.) Kölreuter was appalled at Linnaeus’s supposed evidence, what he called those “premature births of an over-excited imagination,” and launched the first scientific study of hybridization.
He started with tobacco plants, which are hermaphrodites, using the pollen of one species to pollinate the castrated flowers of another. Not all crosses produced progeny, but the successful ones consistently produced offspring that exhibited physical characteristics of both parents. Over the course of six years, Kölreuter made more than five hundred different crosses among 138 different species, repeating the crosses thousands of times, using control groups, and minutely describing the characteristics of the offspring. Between 1761 and 1766, he published four reports on his experiments, reports that should have made it clear that the male parent contributes substantially and in a consistent way to the appearance of its offspring. But Kölreuter’s brilliant work went either unread or unappreciated, as Camerer’s had seventy years earlier and as Gregor Mendel’s would be a hundred years later.
Instead, the old debate about the mechanism of reproduction burbled along. In the first decades of the 1800s, German scientists were just coming into their own, and developed a new theory. Grounded in the new chemistry of Lavoisier, their model for reproduction was a chemical reaction. In plants, according to the German physician and botanist Karl Friedrich von Gärtner, “the liquid in the pollen reaches the ovules after being combined with the liquid secreted on the stigma, so as to give birth there to the embryo.” Reproduction in plants was vegetal chemistry.
While the true nature of fertilization remained as obscure as ever, the improved microscopes did uncover an extraordinary secret about pollen. In 1822, Giovanni Battista Amici, a leading Italian microscopist, was closely observing the sticky stigma of a Portulaca flower onto which some grains of pollen had fallen and adhered. “Suddenly,” he wrote, a grain “exploded and sent out a type of transparent gut” down into the stigma. What he saw for the first time was the pollen tube, which is a cell in a pollen grain that after landing on a compatible stigma, elongates, and burrows its way down the style into the ovary and then into an ovule inside via a pore on the ovule’s surface. (Amici couldn’t see this, but the tube, which is filled with liquid, becomes the conduit for two sperm cells to swim down into the ovule. One fuses with the ovum to create the embryo; the other unites with two “polar nuclei” to become the endosperm, the tissue that nourishes the embryo.) Here at last was the answer to the impenetrable mystery of how pollen grains pass through the all-too-solid style to fecundate seeds: They tunnel their way in.
Ironically, Amici’s discovery led to a revival, or rather a reinvention, of the old spermism. Matthias Schleiden was a young professor of botany at the University of Jena who, in 1838, cofounded modern cell theory, which states that all living things are made of cells and new cells are created by the division of old cells. Marrying his idea with Amici’s discovery of pollen tubes, Schleiden suggested that the pollen tube delivers a single-celled embryo to the ovary, where it then divides and grows. “The anther,” wrote Heinrich Wydler, professor of botany at the University of Berne in 1839, “far from being the male organ, is on the contrary the female organ: It is the ovary. The grain of pollen is the germ of the new plant, the pollen tube becomes the embryo.”
What is most strange about both the ovist and spermist theories is that they flew in the face of what everyone commonly observed: Offspring tend to have features of both their parents. Ordinary folk knew that by whatever mechanism, little Johnny’s chiseled looks came from his mother and those long legs came from his dad. Farmers selectively bred animals to produce thicker-coated plow horses, better milk cows, fatter pigs, and more prolific hens. Pigeon fanciers crossbred their birds to create a vast array of sizes, plumage forms, and colors. In 1865, Mendel pointed out (to a completely indifferent world) that if the egg cell of a plant “fulfilled the role of a nurse only, then the result of artificial fertilization could be no other than that the developed hybrid should exactly resemble the pollen parent.” But the academics’ theories trumped everyday wisdom. “It is the opinion of most physiologists,” Charles Darwin wrote in On the Origin of Species in 1859, “that there is no difference between a bud and an ovule.” In other words, a new plant is in the egg. Pollen simply—somehow—awakens it. In the mid-nineteenth century, nearly two hundred years after Nehemiah Grew first floated the idea of plant sex, botanists were not much closer to an understanding of how it works.
twenty-three
Black Petunias
The only flowering plants my mother ever grew were petunias, which she put in large pots by the front door. They delighted me as a child, with their simple, open trumpets in uncomplicated colors: a pure white, a pink the color of Hostess Sno Balls, a crayon red, and a bright purple the shade of my favorite party dress. It was my job to pinch off the spent blooms, which my mother said encouraged more flowering. The leaves were unpleasantly sticky, but the petals were even softer than velvet, and infinitely lighter in weight. They seemed friendly flowers and eager to please: All summer they blithely spilled flower after flower as if inspired by their own naïve delight in the season.
The petunias I am looking at today in the display gardens of the Ball Horticultural Company headquarters outside West Chicago are not my mother’s petunias. No such petals ever unfolded in her pots by the front door. These petals are black, completely and utterly black, without the slightest pentimento of deep purple that you will find in other purportedly black flowers. The only color here, in the deepest depths of its trumpet, is a single dot of brilliant yellow. It is as if all the sun-sent photons that ever had the misfortune to approach this black ho
le of a bloom had been sucked down its dark throat and were distilled in its depths.
It is a day in late August, and I am in these spectacular gardens waiting to meet Dr. Jianping Ren, the breeder of the black petunia. I had found her petunias at the entrance to the garden, commingled in a large pot with pure white and lipstick red varieties. I can’t imagine who would want a garden full of black petunias other than a funeral home operator, but in this mixture the effect is striking, in a Madame X kind of way. Black petunias can be sexy.
Dr. Ren finds me, and together we walk back through the garden and across the lobby. She has agreed to show me her breeding greenhouses, and we’ll have to drive about twenty minutes to get there. Ping, as she insists I call her, has thick, black hair cut in a pixie style, a brilliant and ready smile, an energetic stride, and an English that is fluent if just a little eccentric. She is charmingly candid. She is also one of the country’s foremost plant breeders and is responsible for Ball’s petunia program, one of the company’s biggest-selling product groups.
Ping came to the United States after finishing her undergraduate work in 1998 in China. After earning a Ph.D. from Cornell, she started at Ball in 2001, breeding the traditional petunias I knew from my childhood. She joined the long line of breeders who have been manipulating and improving petunias for almost two hundred years. In 1834, a British nurseryman named Atkins created the garden petunia (Petunia hybrida) when he crossed two species that had recently been shipped from South America. One, Petunia axillaris, has an upright habit and white flowers that have a long, narrow tube, something like the trumpets that medieval heralds played. Axillaris emits a scent that attracts its primary pollinator, the hawk moth, which has a long proboscis capable of reaching the nectar produced deep in the tube. The other species is Petunia integrifolia, which has a ground-hugging habit, violet-purple flowers with short, wide tubes, and no scent. Its pollinators are primarily bees, which are drawn by color and can maneuver their big bodies comfortably into the wide tube. (The tube also serves as a pied-à-terre for trysting insects.) In the wild, the two species do not produce hybrids even when they grow in the same area. Hawk moths are active only at dusk and, relying on fragrance, rarely stumble into the scentless integrifolia. Most bees cannot fit into the narrow axillaris tube.