Life evolved in the oceans and stayed there - nothing bigger than a bacterium - for a couple of billion years. Slowly, life developed complex metabolic pathways, cellular anatomy, and organelles such as nuclei that would eventually allow production of large specialized cells. Ages later, cells banded together into multicellular forms and, by half a billion years ago, produced individuals with billions of cells and separate lives. Sex could no longer occur simply by tickling a nearby cell.
The key to gene exchange between multicellular individuals is haploid sex cells, each with just half the adult DNA. They form gametes, sex cells that can fuse with an opposite type. Ours are called eggs and sperm. In the sea, haploid gametes produced by adults simply float away. Nearly all are quickly eaten, so it’s all a numbers game. When male and female gametes meet by chance, a tiny embryo begins to grow.
Deep in the Carboniferous Era, 300 million years ago, vast swamps, bogs, and soupy bays teemed with life. Tall spreading tree-ferns, newt-like amphibians the size of crocodiles, and dragonflies bigger than gulls released gametes into the water as their ocean ancestors had done. Some gametes fused and grew, creating babies. These days fish, frogs, and some water plants still do it that way.
Plants began to colonize in a far harsher environment about 270 million years ago with multiple challenges to reproduction. Dry land offered huge opportunities for those who could survive its rigors—but, how were they going to get those haploid cells to meet up?
How Land Plants Get It Done
Evolution is infinitely creative, given enough time. The simplest solution for plants was to let the wind carry away lightweight haploid cells—aka pollen—that just might find a female sex cell clinging to a twig or bract somewhere. The gymnosperms (“naked seed” in Greek), including primitive gingkoes and cycads, plus pines, firs, and their relatives, were the first to use the wind to carry pollen on a massive scale.
Many flowering plants still make use of air transport. Grasses produce tiny dangling flowers that wait for a warm, dry day to release vast numbers of pollen grains, as hay fever sufferers will confirm. But wind is chancy. And it’s not efficient in damp climates or sheltered locales.
Most of the flowering plants we see address the risks of chance with a more surefire solution. They invite an animal partner, most often an insect, to help fertilize the embryo. In fact, the whole reproductive purpose of lush and lovely flowers is to attract these willing workers in the plant sex game.
The pollinator visits the beckoning flower seeking nectar, inadvertently touching a male stamen covered with sticky pollen, then carries it to the next flower. With luck, a grain sticks to a female stigma atop a tall pistil in a flower of the same species. There, it sends down a tube through which new genes reach the ovule below. Around the growing seed, the ovary may swell to become a succulent fruit or berry, inviting another animal to enjoy a meal, carry away indigestible seeds, and defecate them in a new spot. Of course, manure helps young plants thrive.
Today, we delight in thousands of varieties of flowery sex organs, shining in the sun or shade. And we realize flowers exist almost entirely to attract bees, butterflies, wasps, flies, beetles, moths, bats, monkeys, lemurs, possums, rodents, hummingbirds, and others. Pollinators respond to aroma, color, patterns, and sweet nectar. To discourage nectar robbers such as ants, flowers create traps, clever protective structures, and diversionary tactics. It’s all about getting genes together, and those tantalizing blossoms use some unlikely tactics.
Charles Darwin was fascinated with the co-evolution of plants and their insect pollinators. In some cases, a single species of plant and a single species of insect are totally dependent on each other for successful reproduction. Three species of moth are the only pollinators of yuccas. And yucca moths can only lay eggs in a ripe yucca flower, where larvae hatch and feed entirely on that flower. Fig species each have their own species of wasp for pollination, while the wasps lay eggs only in their special figs.
Dioeceous and Monoeceous Sex in Plants
About six per cent of more than 250,000 flowering plant species are fully heterosexual or dioeceous (Di = two, Oikos = house, in Greek). Staminate (male) and carpellate (female) flowers are found on different plants. Having separate sexes prevents self-fertilization, assuring cross-pollination. Seven per cent of flowering plant (angiosperm) genera contain some dioecious species.
Familiar examples of dioeceous plants include hops, date palm, yew, hemp, bay laurel, willow, African teak, holly, gingko, and common nettle. Obviously, they are all over the evolutionary map and in many unrelated families. Some dioeceous plants change sex over time, usually making male flowers earlier in the season and female flowers later.
Many flowering plants are monoeceous (“one house”) or bisexual. They use either self-fertilizatoin (allogamy) or cross-pollination (xenogamy). About 7 percent of higher plant species are partially cross-fertilizing and partially self-fertilizing.
In monoeceous plants, male stamens with pollen and female pistils are borne on the same plant, though some have separate male and female flowers. In a so-called “perfect” flower, each bloom has both stamens and pistil.
Some familiar monoeceous angiosperms are birch, hazelnut, oak, pine, spruce, corn, and squashes. Monoeceous also describes ancient plants, including many mosses and algae, that lack flowers but have both male and female reproductive organs to produce spores. Monoeceous species of all sorts can be wind pollinated. The European ash has tiny flowers that are either bisexual or male and that depend on wind. Others lack apparent flowers but have highly effective pollination schemes. In corn (maize), the tassel atop the forming ear bears pollen. Inconspicuous female flowers form rows along the tiny cob and silks grow out from each ovary, ready to receive pollen and direct it to the ovary. Once fertilized, the rows become swelling kernels.
In self-pollinating species with “perfect” or complete flowers, male stamens may be aligned so that pollen can simply fall onto the female carpel that’s ready to convey its genes to the ovule, where seed develops.
One way monoecious flowering plants discourage self-fertilization is through “self-incompatibility.” In these flowers, if anthers and pistils mature at the same time, structure and placement of flowers can make it unlikely any pollen will fertilize the same plant. More common are flowers with an early season male (staminate) phase, followed by a female phase. In still other plant populations, early season whole flowers are nearly all male, while more female flowers develop later in the growing season.
And some plants undergo a sex change. In jack-in-the-pulpit (Arisaema triphyllum), juvenile plants are asexual. Small plants produce all or mostly male flowers. As they grow larger over the years, individuals have a mix of both male and female flowers. The large ones produce mostly female flowers.
Only about eight per cent of higher plant species reproduce exclusively by non-sexual means, without gametes. The next generation can sprout from runners, from stems in contact with soil, or from bulbs or bulb-like corms. And some produce viable seeds without any fertilization at all.
In case you were wondering, a so-called complete or perfect flower has both anthers that make pollen with pistils ready to receive it. A perfect flower displays a calyx of outer sepals, usually green, and a corolla of inner petals. The sepals and petals together form the perianth. Closer to the center, stamens produce pollen grains, each containing a microscopic haploid sex cell. The male parts of a flower collectively form the androecium. Finally, in the middle, are the carpels. At maturity, each holds one or more ovules containing a tiny female gametophyte. The female parts of a flower form the gynoecium.