Promiscuous Plants and the Pollinators they Tantalize

Plants live to have sex, just like us!

bat-pollinatorPollination, the transfer of pollen from male to female reproductive parts of a flower, can be carried out by insects, by animals like birds and bats, or just by a gentle breeze in the air. Pollination by insects is essential for the production of fruit by many common crops. Some crops that require insect pollination include those in the rose family (apples, pears, stone fruits, almonds), cucurbits (melons, cucumbers, pumpkins, squash), onions, and carrots. The most common insect pollinators in agricultural systems are bees. Flies, wasps, beetles, moths, and butterflies can also serve as pollinators. Commercially, honeybees are by far the most common managed pollinator. And all of this is arranged and managed simply because plants want to have sex!

All plants do, or at least all plants try to have sex. They exist to reproduce. To many this is a startling fact to learn. Now you will have to forgive me for the explicit or harsh nature of what follows but the truth must be spoken here! I must tell you that plants produce beautiful, elegant, fragrant flowers for one reason and one reason only, and it is not so that we can enjoy and appreciate them. No, plants produce flowers to have sex. And the reason they produce flowers to have sex is so that they can reproduce. As I said, it is why they exist. Not to blanket the landscape in lovely colors and fragrances. Nope, plants exist for the purpose of making babies. These plant babies (for a clearer understanding let us call these babies seeds) are nurtured in a womb that is known to us as a fruit. Every plant that produces a flower, if that flower is fertilized, produces a fruit of some type, be it a berry, a capsule, a husk, a pod, an apple, a pear, a plum, a cherry, a peach etc.

Every plant you encounter, all of the trees you see, all of the shrubs you walk past, all of the lawns (and their weeds) you work so hard to maintain, even all of the plants in your garden that feed you – every one of them is at some point in its life going to engage in this passionate pastime. It is an invigorating way to see the world around you, isn’t it? And some of the plants around us have sex in ways that most people have never imagined…

hummingbird-pollinatorTo better understand this lets take a moment to discuss flowers. Flowers are incredible – beautiful to people, attractive to pollinators like bees and birds, and full of surprises too! Did you know that most flowers on planet Earth are hermaphrodites? It is true – while some flowers are formed with only male or female parts, most are hermaphrodites – being neither strictly male nor female, but rather being both at once. Male flowers have male reproductive parts and produce pollen, which contains the sperm. Female flowers have ovaries that contain ovules (eggs). Once the eggs are fertilized they are kept safe and sound in the fruit of the plant.

This can give us a whole new appreciation for the “fruits and vegetables” we consume. There really is no such thing as a vegetable. The term vegetable was made up by the produce industry to mean “edible plant.” And botanically speaking, plants produce fruits to protect and often distribute their seeds.

The most common type of flower is what biologists call a “perfect” or “complete” flower, which has both male and female parts within a single flower. Most garden vegetables such as beans, peppers, and tomatoes have perfect flowers. In all cases, the male pollen must reach the female eggs in order for fruit to be produced. This transfer of pollen is called pollination and it can play out in several different ways.

bee-pollinatorWhen it comes to their sex lives and preferred partners, plants are quite open-minded. Many plants engage in third-party sex. Many have sex that intimately involves insects or other creatures — and that’s where the bees come in. But some prefer other partners such as beetles, flies, wasps, ants, butterflies, moths, and occasionally, even birds, bats, and more. A few somewhat desperate plants, particularly those that have been hanging around people for too long, now have sex primarily with themselves. Cringe!

People used to understand this whole plant kingdom/pollinator courtship thing far better than we do today. The term “the birds and the bees” is an English-language idiomatic expression and euphemism that refers to wooing and sexual intercourse. We have this expression because we largely used to understand plant sex! The “birds and the bees talk” is generally the event in most children’s lives in which the parents explain what sexual relationships are. According to tradition, the birds and the bees is a metaphorical story sometimes told to children in an attempt to explain the mechanics and good consequences of sexual intercourse through reference to easily observed natural events. For instance, bees carry and deposit pollen into flowers, a visible and easy-to-explain parallel to male fertilization of a female’s eggs in people. Another example, birds lay eggs, a similarly visible and easy-to-explain parallel to female ovulation. These metaphors are very easy for children to understand, but the truth is a bit more complicated.

Take, for example, the fascinating orchid, which often lures pollinators with the promise of uninhibited sex with strangers! In one case, male bees with an ‘urge’ to add to the gene pool are victimized by orchids that pose as irresistibly scented exotic females, new research suggests. Sexually deceptive orchids rely for their own reproduction on the male insect’s drive to copulate. If a male mistakes the flower for a female insect, his seemingly fruitless mating attempts are actually effective to pollinate the orchid. Some orchid flowers are so exciting to males that they even pass up sex with real female bees to have sex with the flower instead! Although the visual misrepresentation is impressively brilliant, what really drives the male bees nuts is the female sex pheromone cocktail that the orchid imitates.

Recently, Nicolas Vereecken and Florian Schiestl at the University of Zürich in Switzerland have discovered a new trick in the orchids’ repertoire of sexual lures. When they studied geographic variation in pheromone mixes between 15 different populations of the bee Colletes cunicularius and the orchid that mimics it, Ophrys exaltata, they were surprised to find that the flowers consistently smelled slightly different than the female bees in any given population. Why was this more effective at attracting the male bee? “This was not at all what we expected. If the orchids thrive on imitating female bees, the match should be as perfect as possible,” said Schiestl, “Unless, of course, the males like their girls just a little bit different.” Since the bees enjoy a quiet life and do not travel much, Vereecken and Schiestl speculated that inbreeding may be a common problem. Mating with a stranger may be a welcome opportunity to stir up the gene pool for a bit more variety. Somehow the ‘fancy woman’ orchid has figured this out and set her devious plan in motion. In essence, they learned that male bees have the hots for exotic perfume. The study shows that given the choice between a dummy infused with the pheromone cocktail produced by the ‘girl next door bee,’ and another one with the bouquet of a female from another population, the males visited the scent that was new to them 50% more often.

But the scent of orchid blooms, with even greater differences in the pheromone mix, was even more popular. In choice tests with bees and wasps, orchids attracted males up to five times as often as the pheromones of a local female bee. Manipulating the natural perfume blend of the local female bees to mimic that of the orchids nearly doubled the male bees’ attraction to those females. In other species, such as mice, rare females are popular when inbreeding is common. “When out-breeding is desirable, it makes sense for males to look for females with exotic scent,” says chemical ecologist Manccfred Ayasse at the University of Ulm in Germany. “But in this case, what they find instead is the orchid.”

thynnid-waspAnother orchid mimics the scent and appearance of a female wasp so well that it attracts male wasps like crazy! Like a lady of the evening standing on the corner calling, “Hey big bee, why don’t you come up and see me sometime!” They draw the male wasps into their fold. Then just as the wasp lands ready for some sexy fun, the orchid quickly steals any pollen the wasp is carrying and then smacks it in the back and head, depositing its own pollen. As the wasp responds, “Oh! Sorry miss, I thought you were someone else!” and turns to leave, the orchid must just smile and wave goodbye, happy in the knowledge that its trap has worked again.

Since plants can’t run off to look for a mate and reproduce, many, like the orchids above, have evolved elaborate mechanisms of pollination — often cheating or bribing animal pollinators into doing the work for them. The focal point of these efforts is almost always the flower. Flowers attract pollinators by tempting them with the promise of a reward (usually nectar) and then use the opportunity to distribute and gather pollen (essentially plant sperm) via the pollinator. It is not just bees and wasps, other different pollinators look for different things in a flower. Hummingbirds, for example, like bright red tubular flowers. Bees like flowers with a landing platform and a UV nectar guide (floral markings which reflect light that bees — but not humans — can see and which guide the bees to the nectar source). Flies like dark flowers that smell like rotting meat. These sets of characteristics that attract different pollinators are called ‘pollination syndromes’ and somehow over the millennium the members of the plant kingdom have figured out how to use those of the animal kingdom to their benefit. They may be smarter than any animal! They have even used me! Not long ago while gazing longingly at the ready to pick peaches on my peach trees I could almost hear the tree calling to me, “Here animal, animal, animal. Come eat this juicy peach!” That is just what I did too, I took a peach and as I walked along on my merry way I enjoyed the wonderful fleshy fruit. After a bit I had nothing left but the pit and guess what I did with it? Yep I did just what the plant wanted me to do – I carelessly tossed the peach pit away. Only then did I realize I had been used to distribute the seed of the cleaver peach tree ‘mother.’ Oh well, at least I got to enjoy the tasty fruit!

peachesIf you think about this for very long, it may very well modify the way you look at your garden. You would not, for example, be the least bit surprised to discover that oaks and lawn grasses have flowers, just as your roses do. And when a strange cone or globe arises in the middle of your sago palm, unfolding to become what is in essence a very primitive version of a flower, you’d recognize immediately that even plants like pines and cycads bloom and have sex. Knowing something about plant sex, you might help your female holly find a suitable male mate, at least if you want her to have bouncing, round, red babies. If you were smart about the many ways plants have sex, you’d realize that your blueberry bush, which is neither male nor female, but in fact both at once, doesn’t need a companion of the opposite sex, but it must mate with a distant cousin to produce a good crop of berries.

Fully appreciating the vagaries of plant sex will also help you to understand why you only need one tomato plant to tango, and why all southern figs are virgins. Perhaps, if you study the basics of plant sex, you won’t worry every spring that the blossoms on your squash plants aren’t producing fruit. Instead, you’ll understand that squash produce separate male and female flowers. And, who’d have guessed it, only female flowers bear fruit. Those first, seemingly unproductive, flowers are often male.

You’ll also immediately recognize that the round hard capsule hanging from your camellia branch is not some kind of disease or plant goiter, but rather the fruit the camellia flower was designed to deliver. Open that capsule up, and you’ll find a seed, or two, or three. Plant that seed carefully, and with a little luck, eventually you will see a seedling camellia.

But before you plant it, it’s good to remember that this seed is the product of sex between two very different camellia parents. And just as human children are never precise copies of their parents, it is highly unlikely that the seedling, when it develops and eventually flowers, will look exactly like the plants that produced it. That’s just the simple facts of life.

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