A Garden of Marvels Page 23

  Amborella is in a confused state when it comes to sex, as if transitioning from gymnosperm to angiosperm anatomy. It has separate male and female organs on the same plant like many conifers. The male organs have stamens only, but they don’t look like modern stamens, that is, with filaments topped with two pollen-bearing sacs. Instead, the two pollen sacs are carried on the edge of flat and broad petals that look very much like the scales in male conifer cones. But the amborella also has organs that look like hermaphrodites, with both carpels and stamens. These stamens, however, are sterile, making them staminodes.

  How did these other organs (which function as females) manage to acquire both carpels and nonfunctional stamens? The debate is far from over, but it seems the first and critical step was a “whole genome duplication” in the common ancestor of all flowering plants. A gymnosperm (like a human) is diploid, which means it has one copy of its genome in the nucleus of all of its cells. (The term diploid indicates that the genome is a set of paired chromosomes, one from each of its two parents.) After the whole genome duplication, the mutated plant and its tetraploid descendants had two copies of its genome in the nucleus of its cells. These plants became instant winners in the survival game. They now had spare genes for experimenting, spare genes that might mutate freely without endangering the viability of the individual. If a mutation in a spare gene conferred an advantage, the mutation could be passed on. If the spare gene mutated in an unproductive way, it would be lost.

  Today, most flowering species have more than one copy of its genome, and thousands have a dozen or more copies. Plants with multiple copies are called polyploids, and tend to be larger, have more complex flowers, and bear bigger fruits than their genomically less endowed relatives. Humans have shaped plant evolution by selecting polyploid plants for cultivation. Modern garden strawberries, for example, are octoploids and some have berries so large they need to be cut in quarters to be eaten, while their wild relatives are diploid or at most tetraploid and have berries the size of a pea. Plant breeders often propagate garden flowers with doubled genomes to produce double the usual number of petals. In the era just before the amborella emerged, however, a doubled genome simply meant a new plasticity for basic forms and functions. That plasticity was critical to evolving leaves into colorful petals and into sepals, the leaves that protect unopened buds. Gene redundancy also was a factor in developing male and female organs in the same flower.

  Gender in gymnosperms of the era must have already been nuanced and mutable. Only four divisions of gymnosperms are extant, but some of their members have interesting sexual variations. Some gymnosperms change genders as they mature. Male gingkoes occasionally metamorphose into females, much to the distress of city residents. (Gingkoes are popular urban street trees—they’re so tough some survived the nuclear blast zone in Hiroshima—but female seeds are terribly messy and their odor has been compared to vomit or rancid butter.) The Mediterranean cypress starts off female but later becomes male. Many pines have their male cones on the lower branches and female cones on the upper, but several varieties are sexually upside down. So, when the gymnosperm genome first doubled and was chock-full of extra and as yet unassigned genes, gender would have been particularly malleable. Amborella, with its organs that are unisexual in function but hermaphrodite in appearance, is not a shocking experimental result. And amborella is known to go transgender, too.

  One of the most important innovations of amborella and later angiosperms was to enclose its seeds in an ovary. No gymnosperm seed has an ovary: Their fertilized ovules are “naked,” covered only in a single, thin protective layer. An angiosperm’s ovary protects its embryo from desiccation and physical harm. Some species’ ovaries enlarge significantly to become an edible fruit. If you’re a plant and you want to have your seeds spread widely, fruits are the way to go. Animals transport them inside their digestive tracts, and then deposit the embryo far from the parent, along with a nice pile of nitrogen-rich manure. Botanically speaking, grains and nuts are also fruits. It’s just that their ovaries are dry and tough.

  So where did the ovary come from? Gene duplication again. The ovary is the bottommost part of the carpel, and the carpel, genetic analysis shows, is a repurposed leaf, which by folding and fusing its edges together came to form a second, impermeable covering over the seed. In fact, the amborella carpel has a visible seam where the leaf has not quite fully fused. An edible fruit is not the only transportation method that angiosperms developed for their seeds. Thanks to those duplicated genomes, external seed structures such as burrs and hooks, plumes, and sticky liquids have evolved to send seeds far down the road. Wait a few million years, and angiosperm seeds will be hailing cabs.

  About 80 percent of angiosperms are pollinated by insects; most of the remainder use wind to transport pollen. I assumed wind pollination evolved first since that seems both the simplest method, as well as the way gymnosperms get the job done. Not so. In the Carboniferous era, insects were already crawling into gymnosperms’ male cones in search of pollen, which is a highly nutritious food. These ancient thieves didn’t waste time and energy visiting pollenless female cones. The innovation of the amborella and later angiosperms was to put the thieves to work as go-betweens, connecting male stamens and female carpels. Although the staminodes in the amborella’s female flowers are sterile, they act as decoys, attracting hungry insects who, having visited the real male flowers, inadvertently drop off pollen on the nearby stigmas.

  The insects that crawled into the first amborellas 140 million years ago were searching only for pollen and had no expectation of finding nectar. Nectar and nectaries, which are specialized glands found at the base of flowers, had yet to evolve. Darwin might have considered nectaries to be another abominable mystery, since they also seem to have come out of nowhere. But nectar is made of concentrated sugars manufactured in leaves and distributed throughout a plant via phloem. Phloem (and not, to my surprise, xylem) also bring water to flowers. It is thought that phloem at the base of the flower, under the increased pressure of water that accompanies flower development, sometimes leaked a little sap. You might think leaky phloem at the base of flowers would be a disadvantage. After all, what is draining away is hard-earned energy that the plant could use for growth and repairs. But, like the leaky phloem at root tips that succor mycorrhizae, leaky phloem in flowers had an offsetting advantage: Insects favored flowers that provided not only a pollen dinner but a sweet postprandial drink. Over time, as plants whose flowers always kept a well-stocked bar prospered, mutations in nearby structures evolved into nectaries. Ever more attractive petals and scents evolved, too, to ensure that the location of the restaurant would be no mystery, abominable or otherwise.

  twenty-five

  Cheap Sex

  Of Darwin’s voyage on the Beagle and his publication in 1859 of On the Origin of Species, the foundation of evolutionary biology, much has been written. Less well-known is the fact that after he published the Origin at the age of fifty, he directed much of his intellectual effort in his remaining years to the study of plants, writing five major books and some seventy articles on botanical subjects. Had he never developed his understanding of evolution, he would still have been renowned as a major figure in the history of botany.

  Darwin’s botanophilia was born at his childhood home, the Mount, in bucolic Shropshire. The Mount had extensive gardens where he and his five siblings played and an orchard of scientifically bred apple trees. His mother, who died when Charles was eight, nurtured tropical species in a heated greenhouse. In a portrait painted in 1815, a six-year-old, rosy-cheeked Charles—the beetle-browed scientist with the Mosaic beard far in the future—kneels with his arms hugging a big red pot of yellow flowers. Making collections of natural objects was a fad of the period, and Charles became an avid collector of beetles. He also claimed to have had an early curiosity about the way plants work, and would later recount how he told another little boy that he “could produce variously coloured polyanthuses and primroses by wateri
ng them with coloured fluids.”

  Charles started his university career in medicine at the University of Edinburgh, but the gore and mayhem of the operating room horrified him, and he switched to Cambridge to prepare for the ministry. (This goal he later judged ironic, “considering how fiercely I have been attacked by the orthodox.”) He was hardly a devoted student during his three years at the university, admitting that he “got on slowly” in math, had forgotten his prep school Latin and Greek, and his attendance at most lectures was “nominal.” The only course he attended regularly, and he took it at least twice, was that of John Stevens Henslow, clergyman and professor of botany. “I became well acquainted with Henslow,” he wrote, “and during the latter half of my time at Cambridge took long walks with him on most days; so that I was called by some of the dons, ‘the man who walks with Henslow.’ ” Under Henslow’s influence, his former passion for beetling cooled and he took up “herborizing” with avidity. He began to imagine his future self as minister of a country parish, a position that appealed primarily because it would leave him plenty of time to pursue botany and his other naturalist interests. First, though, having read and reread the German explorer Alexander von Humboldt’s 3,754-page account of his travels through the wilds of South America in the early years of the century, he wanted to go on a voyage of discovery. He schemed to get to the Canary Islands. When that trip fell through, Henslow helped him get hired as a naturalist and companion to the captain of the HMS Beagle, who was charged to collect navigational information about the world’s coastlines. The ship set sail in 1831.

  In September 1835, near the end of the five-year voyage, Darwin arrived in the Galapagos Islands, off Ecuador in the Pacific Ocean. Henslow had taught him that oceanic islands were often home to unique collections of plants, an interesting phenomenon that the reverend attributed to the work of the divine Creator. Indeed, as Darwin traveled around the Galapagos, he saw vivid demonstrations of what we know as “island endemism.” He assiduously collected blooming specimens, per Henslow’s instructions, carefully labeling them according to where he’d found them. Island endemism, he noticed, didn’t appear to be limited to plants; the mockingbirds also looked different from island to island. Although he was unsure whether these were varieties of a single species or separate species, he was already mulling over the significance of their differences.

  Darwin returned to England in October 1836 and handed over his collections to specialists for identification. Within months, ornithologist John Gould told him that his Galapagos birds that he had thought were blackbirds, grosbeaks, finches, and wrens were all different species of finch. The mockingbirds were not varieties, but separate species. “In October 1838,” Darwin wrote in his autobiography, “that is, fifteen months after I had begun my systematic enquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species.” Two years later, he was well on his way to an understanding of how natural selection works on variations to create, modify, and extinguish species.

  Something, however, troubled him about evolution and plants. The operation of natural selection presupposed a supply of individuals with variations. Those variations, he knew, resulted from the crossing of male and female individuals who looked different from each other and produced offspring that looked different from their parents. But if this was true, the common understanding of the way sex worked in flowering plants must be wrong. Everyone assumed, and Henslow had taught him, that a hermaphroditic flower fertilizes itself. Why else would its carpels and stamens be mere millimeters apart inside their petals? But, Darwin realized, if individual flowers fertilized themselves, then the seed would yield offspring identical to the parent. (He was unaware that random genetic mutations also create variation.) There would be no variation among individuals, and no individual would be any more or less fit than any other. There would therefore be no natural selection, and evolution of species would not occur. Clearly, this had not happened; anyone could see that flowering plants are rampantly speciated. So, Darwin had to reconsider the nature of sex in flowering plants. During the summers of 1838 and 1839, he later wrote, he “was led to study the cross-fertilisation of flowers by the aid of insects. . . . My interest in it was greatly enhanced by having procured and read in November 1841, through the advice of Robert Brown, a copy of C. K. Sprengel’s wonderful book, Das entdeckte Geheimnis der Natur.”

  This inspiring book (whose full title in English is translated as The Secret of Nature in the Form and Fertilization of Flowers Discovered) had been published nearly fifty years earlier, in 1793. Its author, Christian Konrad Sprengel, was forty-three years old at the time, living alone in attic rooms in Berlin and surviving on a small pension. A year before, he had been the director of a school. After his doctor advised him to take up outdoor activities for his poor health, he had become fascinated with flowering plants in general and obsessed by the puzzle of pollination.

  It was understood at the time, thanks to Kölreuter, that insects helped fertilize flowers, but Kölreuter believed that their visits were haphazard. Insect pollination, he thought, was an accidental occurrence and certainly not essential for the propagation of the species. It might be more important for dioecious species whose male and female flowers were separated, but for hermaphrodites, he assumed it was a secondary strategy. What Sprengel discovered was that there was nothing accidental, nothing casual, about insect pollination. In fact, “nature had arranged the whole structure of the flower for this method of fertilization.” For one, he realized that “those flowers whose corolla is differently colored in one place than it is elsewhere always have spots, figures, lines, or dots of particular color where the entrance to the nectary is located.” These “nectar guides” are a method of leading the insect to its reward. Fragrance is another guide. Night bloomers, he wrote, have “a large and light-colored corolla so that they catch the eyes of insects in the darkness of the night. If their corolla is inconspicuous, then this shortcoming is substituted by a strong scent.”

  The ins and outs of pollination had not been Kölreuter’s primary interest, and his observations had not been rigorous. He had examined only a handful of species, but Sprengel studied nearly five hundred, observing many of them continuously for days in their natural habitats to catch “nature in action.” Pollination can occur at dusk or at night or only once for a few seconds in the space of several days. (Orchids can remain in bloom for months because their highly particular pollinators take so long to find them.) Gnats and tiny flies can be stealthy pollinators: You need to practically put your eye to the blossom, or you may miss the event. But watch long enough, Sprengel knew, and a pollinator almost always shows up. Kölreuter had also been misled by plants that developed seeds even without an insect visit. He hadn’t thought to test the seeds’ fertility, but Sprengel did, and found them sterile. Insects are essential to angiosperm reproduction.*

  For all his trouble, Sprengel was rewarded with a revolutionary discovery. In nature, a hermaphroditic flower “cannot be fertilized by its own pollen but only by the pollen of another flower.” Anatomy ruled against self-fertilization, for one. Despite their proximity in a flower, anthers and stigmas do not routinely touch each other. Often they have mismatched heights that reduce the likelihood of contact. Timing is another issue: A flower’s anthers ripen and release their pollen, and only then do its stigmas become receptive, or vice versa. This nonsimultaneity, or dichogamy, prevents self-fertilization.

  Sprengel also put to rest—or would have put it to rest if anyone had read his book—some far-fetched theories about nectar. Some botanical authorities had asserted that the purpose of nectar is to feed the developing seeds in the ovary, and claimed that insects harm the
flower by stealing it. Others thought nectar was a danger for flowers, and that bees removed it in order to protect their source of pollen. If nectar wasn’t collected, it was thought to accumulate, thicken, and destroy the development of the fruit. Sprengel knew otherwise. “The nectar is for the flowers what a spring is for a clock. If one takes the nectar from the flowers, one renders all their remaining parts useless; one thereby destroys their final purpose, namely, the production of the fruit.”

  Botanists of Sprengel’s generation simply couldn’t absorb his unsettling ideas, no matter how much data he provided. This obscure amateur was proposing that the Creator puts male and female organs in the same flower—a perfectly sensible arrangement for beings that can’t get up and move to find their mates—and then prevents their union? Absurd. To top it off, He then concocts a labyrinthine system in which insects on the hunt for nectar inadvertently deliver some distant flower’s pollen? The theory seemed ridiculous. Sprengel didn’t help his cause because he couldn’t propose any purpose for this convoluted method of congress. Besides, this business of cross-fertilization reminded people of hybridization, and everyone knew that hybrids, such as donkeys, were often infertile. One expert called his theory of flowers “an amusing fairy tale.” His book was never translated into English (and still isn’t in full), and he never wrote the second volume he intended. “Poor old Sprengel,” as Darwin called him, died in 1816, his reputation so obscure that to this day no one knows where he is buried.

  Darwin was perfectly primed to appreciate Sprengel’s work; the German amateur confirmed what he already suspected. Sprengel’s data, along with the information Darwin collected on his voyage, the tables on genus/species ratios he derived from the scientific literature, conversations with animal breeders, and examples gleaned from his reading went into refining his theories and into the Origin, which he started drafting in 1851. Eight years later, when the book was published, he had moved far beyond Sprengel. Sprengel had recognized that “Nature abhors perpetual self-fertilization,” but didn’t know why. Darwin did. Cross-fertilization results in more vigorous and greater numbers of offspring than selfing. The more vigorous offspring of crosses pass down the inherited traits, including flowers whose carpels and stamens mature at different times, that contribute to their superior survival rates.