Apple of My Eye

Posted: March 29, 2011 in evolution, genetics, just for fun, plants

Next time you bite into a sweet, juicy piece of fruit, don’t let your enjoyment be diminished by the realization that you are both eating an ovary and being manipulated into playing an active role in plant sex. Ignorance might seem like bliss, but really; that plump, fleshy goodness is just as satisfying nonetheless. Kind of. Mostly. But many plants NEED animals to help them with both copulation and then also distribution of offspring, because plants can’t just get up and walk to the singles bar, nor can their children move away. So these chlorophyll-laden Casanovas devised tactics to entice unsuspecting motile creatures to do their dirty work for them. In exchange for a little sugar high, plants romance animals into carrying off feats of reproduction and migration without so much as ruffling a leaf.

Angiosperms, or flowering plants, began to dominate the planet (taking over from the Gymnosperms, which bear “naked” seeds) as insects also began to radiate into incredible abundance, diversity, and ubiquity. Coincidence, you might ask? Not so fast – with flying pollinators unabashedly doing the deed for immobile plants, these clever vegetables developed all sorts of sneaky tricks to entice the growing insect population to carry their sperm for impregnation of ova far and wide. This co-evolution gives us both the diverse beauty of the plant kingdom and the specific flower (sight, taste, smell) preferences of the animal kingdom.

I think now is a good time to discuss the difference between a fruit and a vegetable… So: All a plant’s somatic, or non-reproductive cells, make up vegetative structures. Why do we (perhaps unkindly) refer to comatose patients as existing in a “vegetative” state? Well, they’re not moving, they’re not reproducing; in fact, they are rather plant-like! On the other hand, certain structures may become specialized for reproduction – in plants, cells dedicate themselves to one or the other: normal body growth and development, OR reproduction. Flowers and their resulting fruit are such specialized tissues. Thus, any plant part NOT involved in reproduction (i.e. vegetative) is called a vegetable, while those parts directly involved in sex are called fruit. You can distinguish this by determining whether a piece of produce has seeds, which are plant embryos. Sans seeds, you have some vegetative structure from the plant. Squash? Fruit. Peppers? Fruit. Nuts? Fruit. Avocado? Fruit. Olives? Fruit. You see where I’m going with this. Seeds = Fruit. Beans and peas ARE the seeds! (The pod is the fruit.) This defies the conventional wisdom of amateur chefs around the globe. Consistently, we call items traditionally seasoned with savory flavors “vegetables” while reserving “fruit” for items incorporated in sweet dishes. You may want to take a moment to reflect and recoup after that reality-altering realization.

When it’s time for an angiosperm to have sex, the plant diverts energy and nutrients into creating differentiated reproductive structures. Leaf and other tissues specialize into distinct flower components by altering gene expression and up-regulating exactly those genes necessary for each cell’s particular role. A flower often has female parts – collectively called the carpel, and male parts – comprising the stamen. On the anthers (tips) of the stamen, pollen grains are hollow structures housing millions of individual male gametes (sperm!). Insects pick up these grains and transport them to the carpel of a different plant. Flowers, then, share a bit of nectar in exchange for fertilization!You know that bulbous structure at the base of flowers? Within that ovary lies the ova, or eggs, of the flower. When a bee brings foreign sperm into contact with a viable egg, the receptive flower begins to produce ethylene gas, which triggers the decline of any flower parts not essential in seed and fruit formation (this is the same compound that causes ripening of fruit). Simultaneously, the receptacle and/or ovary wall tissue grows, swelling in size to form a protective (and often tasty!) structure in which to house the developing offspring. Eventually, the structure begins to look like a fruit as we know it; the seeds are the embryos (or house the embryos).

When trees establish themselves in the ground, they compete with one another for resources; access to sunlight, and surface area to absorb water and nutrients from the soil. Mature trees then will often be found spaced apart, at whatever distance that particular ecosystem has the nutrients to support them. So to prevent the parent tree from competing for valuable resources with offspring, trees would like to be able to distance themselves from their seedlings…..And so: The fruit was born. When you or I or some other creature comes along and spies a shiny treat dangling tantalizingly from the tree, we promptly grab it and go. In doing so, we are feeding right in to the plant’s plot – As we wolf down the sweet snack we inadvertently consume the seeds (or, for picky humans, toss them far from the parent plant). Fruit, then, share a bit of nutrition in exchange for seed dispersal (where do you think the term “spreading his seeds” comes from?!).

Intact seeds pass through our digestive system without great harm to the protected embryo. It’s no coincidence that fruits contain fiber and other compounds that aid in excretion; in fact, many fruits contain natural laxatives that induce peristalsis (smooth muscle contraction in your digestive tract) and literally force expulsion of the seedlings. This is pure plant genius. Not only have you safely carried (dispersed) the angiosperm offspring to a new location, you have planted it happily within a cushy pile of fresh, hydrated fertilizer. What more could a germinating baby plant ask for??

Incidentally, it isn’t only plants who have evolved these sneaky tactics – fungi do their fair share of animal manipulation as well. After all, what is the stereotypical place to find ‘shrooms? All’s fair in love and war. And love is war.

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Comments
  1. Cody Kyrk says:

    Ah, I remember the trouble I’ve gotten in elementary school for correcting my teachers constantly on simple subjects; one even said that Texas was the largest state in America… My third grade teacher loved me the most after I’ve corrected her multiple times that a tomato was NOT a vegetable, nor was squash (though due to their taste and how they’re grown, they’re labeled in the vegetable category.) She “threatened” to skip me to fifth grade to watch me fail if I didn’t stop; of course I got detention after replying, “what papers do I need to sign?”

    I have a question though: A pomegranate, my favorite fruit, has a cluster of seeds, one in each of its many juicy “crystals” of flesh beneath the skin of the fruit. So exactly why are there so few pomegranate plants grown, despite all of these seeds that able to grow? Are they more competitive, are they difficult to grow due to picky conditions/pests or animals? Are the plants take a long time to fully grow the fruit, or do farmers just not grow them due to bad profit/unpopularity?

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