The Fascinating Sex Life of Plants

We were taught this in school: bees collect pollen from one flower and carry it to another, ensuring pollination. Once fertilized, the flower will produce seeds, and the seeds will produce new plants. Such is the sexual life of plants… in general. But there is much more to it than that!

May the Wind Blow Favorably

Not all flowers are pollinated by bees and other insects. About 20% of flowers are pollinated by the wind, particularly grasses and conifers, but also several hardwoods, such as oaks, birches, and maples. To ensure that pollen reaches the right flower, these plants must release large amounts of pollen into the air in the hope that at least one grain of pollen will land on a receptive flower. This may seem like a gamble, but in fact, most anemophilous flowers (as flowers pollinated by the wind are called) will be fertilized.

Wind-pollinated flowers do not need to be attractive, as they do not have to attract pollinators and are therefore usually dull in color—green or brown, sometimes yellow when they carry pollen—and without petals.

This enormous production of wind-borne pollen is the source of seasonal allergies in humans. Ragweed (Ambrosia artemisiifolia), with its inconspicuous green flowers but troublesome pollen, is an example of a typical wind-pollinated plant.

A Drop In the Ocean

Curiously, some plants are pollinated by water. These are aquatic plants, of course, and the pollen is carried by currents. This method seems to be relatively ineffective, as most aquatic plants reproduce more by suckering than by seeds.

Everything to Please

Flowers pollinated by insects (or mammals or birds) are rarely allergenic. On the contrary, they do not waste their pollen by distributing it through the wind, but wait for a pollinator… and all means are good for attracting one. Some give off a sweet scent, while others adorn themselves with colorful petals or fill themselves with sweet nectar… and many use all three strategies. Competition is often fierce, as there are more flowers than pollinating insects, so the tricks to attract the right host are multiplying.

Daisies and their relatives in the Asteraceae family have two types of flowers. The outer flowers, called ray flowers, are long and colorful, but often sterile: they serve to attract insects initially and then, when the insects approach the flower, as a landing pad. Once an insect lands on it, the Asteraceae present the insect with a host of tiny fertile flowers forming a central disc. By leaning in to reach the nectar at the bottom of each flower, the insect accidentally collects pollen. This combination of a “landing strip” and “tightly packed flowers” appeals to somewhat lazy insects, such as butterflies and flies, which like to land and stay on a flower for a long time.

Detailed Instructions

Often, insects that land on flowers receive very specific instructions: contrasting colored veins serve as arrows, pointing the way to nectar. In irises, for example, veins guide insects to a nectar tube where pollen sticks to their bodies as they pass by. They drink their share of nectar, then fly on to another flower. When it lands, it encounters a stigma shaped like a carpet. The stigma is sticky and the pollen sticks to it, ensuring the fertilization of the flower… and the insect repeats its passage through the tube to ensure future fertilization.

Often, these veins are clearly visible in the throat of the flower, but sometimes they appear to be absent. Very often, they are indeed present, but we cannot see them because they are ultraviolet in color, a hue that the human eye cannot perceive, but which is very visible to insects, especially bees.

Nighttime Scents and White Petals

Some flowers bloom at night, such as sweet tobacco (Nicotiana sylvestris) and moonflower (Ipomoea alba). Almost inevitably, these flowers are pale in color—pale pink, pale yellow, and, above all, white—and also strongly scented, but only at night. Why? First, their penetrating scent is carried by the wind, attracting moths from up to 1 km away. But when the scent is diffused, the insect may arrive close to the flower without being able to find it. However, pale colors reflect the light emitted by the moon and are therefore clearly visible at night, allowing moths to find them without fail.

Des généralistes et des mutualistes 

Most flowers are somewhat promiscuous: they accept almost any pollinator indiscriminately. So they need to produce a lot of nectar (the favorite reward of most pollinators) in order to attract many visitors and also a lot of pollen, because only some will carry the pollen to another flower of the same species, thus ensuring the fertilization of the flower. The daisies mentioned above belong to this group of generalists.

Mutualistes

Others have developed specific strategies to secure the services of a pollinator, so specific that sometimes the flower can only be pollinated by a single species of insect. The advantage of such an exclusive relationship is that the flower can produce fewer flowers and less nectar, a clear energy saving, knowing that its host will not waste it on a flower of another species. The disadvantage is that if the insect becomes rare or disappears, the plant will have no means of reproducing. It is a symbiotic or mutualistic relationship: the flower benefits from the insect to be pollinated and in turn feeds the insect with its nectar, pollen, or other food.

The pretty cardinal flower (Lobelia cardinalis) found in our moist woodlands is a good example. It produces long red tubular flowers, but few insects have a proboscis long enough to reach the nectar. Worse still, insects cannot see the color red. As a result, the cardinal flower is pollinated only by an animal that can see red well and has a beak of just the right length: the ruby-throated hummingbird.

The dark blue flower of bottle’ gentians (Gentiana andrewsii), native to northeastern North America, remains tightly closed: no insect can penetrate it. Except one: the bumblebee. It lands on the flower and forces its way in, leaving with its back full of pollen.

Other Examples

Yuccas (Yucca spp.) can only be pollinated by the yucca moth, the only species capable of penetrating their complex flowers. The insect even lays its eggs in the flower’s ovary, and the larvae feed on the yucca seeds… but not all of them. There are always enough left to ensure the yucca’s reproduction. This is therefore a symbiotic relationship: everyone benefits. Some yuccas are grown in Canada, but since they are not native to the region, the yucca moth is not present. As a result, they bloom in vain and never produce seeds.

Datura (Datura spp.) is only fragrant at night and attracts moths as pollinators. Its nectar contains a narcotic that appeals to moths: once they have tasted it, they want more. By frequenting datura flowers, they can become so addicted to their nectar that they refuse to pollinate any other flowers.

When the Flower Is a Decoy

Several flowers have found an interesting technique: they promise the moon, but give nothing in return. This way, they will be pollinated without having to offer nectar, which is expensive to produce.

Mimicry

Often, they have learned to resemble another flower rich in nectar: this is called mimicry. The insect, believing it has found a good deal, enters the flower, but comes out empty-handed… except for the pollen it has accidentally collected. The Epidendrum radicans orchid perfectly mimics the yellow and orange coloring of lantana flowers (Lantana camara), which are rich in nectar. Insects willingly visit and pollinate the orchid… which gives them absolutely nothing to eat.

By Smell

Other orchids, such as the bee orchid (Ophrys apifera), go even further: they perfectly mimic the female of a specific insect, not only in shape, but also in color and even smell. The male insect, finding the flower to its liking, tries to copulate with it, but leaves frustrated… with a load of pollen. It is not as intelligent as it needs to be, because if it sees another flower of the same species, it will try to copulate again… and thus transport the pollen necessary for the fertilization of the flower.

The skunk cabbage (Symplocarpus foetidus) found in our forests also belongs to this opportunistic group. It produces a huge, unappetizing-looking brown inflorescence that smells like carrion. To complete the imitation, the inflorescence gives off heat, further mimicking rotting meat. Bees are repelled, as are butterflies, but carrion flies visit it greedily and even lay their eggs within the inflorescence. Unfortunately, the larvae that emerge will find no rotten flesh to eat and will die. A sad end for the fly, but a happy outcome for the skunk cabbage, as its seeds are now fertilized!

Such is the way of the plant world: when it comes to sex, any means are acceptable. Fortunately, humans are more scrupulous!

---

Larry Hodgson has published thousands of articles and 65 books during his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Le Soleil.