Tuesday, January 17, 2012

Vivipary: a seed within a seed *

Take a good look at the photo below, and tell me: what's wrong with this picture?

If you pointed out the little green shoots, give yourself a pat on the back, because I'm pretty sure that corn isn't supposed to do that. The kernels usually wait until they're planted before they sprout. What we have here is a mutant plant, showing off a phenomenon called vivipary.

Want the explanation? Fruit is not, generally speaking, a particularly good place for a seed to sprout. Take apple trees, for instance. Apple seeds start out buried deep inside a sweet, fleshy ball of fruit: ideally, they shouldn't germinate until the apple has been eaten and its core discarded to rot. If they wait, they need only break through the relatively thin, brittle walls of the core, rather than having to expend lots of energy pushing out through the whole apple. Most importantly, if they wait til the apple is eaten, the animal that ate it has probably carried them a good ways away from the parent tree: they'll be able to grow without having to compete with Mom for sunlight and water.

Since nobody really benefits if a seed jumps the gun, plants have developed ways of keeping seeds quiet until it's time. In its fruits, the parent plant produces a hormone called abscisic acid, which suppresses germination and keeps the seed dormant. As long as the seed is inside the fruit, it gets a steady dose of the stuff and it stays dormant. Once the fruit is off the parent plant, though, and the seed broken out of the fruit, the abscisic acid stops flowing and the seed can sprout.

The trouble is that this system only works when both the parent plant can produce abscisic acid and the new seed is sensitive to it. Sometimes, one end or the other fails, and that's when vivipary happens. Corn sprouting on the cob! Pumpkin vines splitting your jack-o-lantern in two! Dogs and cats sleeping together! It's chaos!

Well, perhaps it's not as bad as that. The phenomenon is called vivipary -- "live-bearing", in the sense of an animal giving birth to live young -- and it's not common in the plant world, because it frankly doesn't work very well for the plants. Viviparous mutant strains are very useful in studying how abscisic acid works, but the naturally-occuring ones tend to die out fairly quickly. A few species, such as garlic and mangroves, have embraced vivipary as a way of getting around some reproductive limitation; mostly, though, it's not the norm.

Should you ever shuck an ear of corn and find a seedling inside, then, you let me know. You'll have a rare oddball on your hands, and I'll want to see it for myself!

Image source:
Hughes, Wayne. Corn4. 15 September 2005. Niches. Retrieved 16 January 2012 from <http://sparkleberrysprings.com/v-web/b2/?p=300>. Used with permission.

    * All Inception jokes should be directed in person to Leonardo DiCaprio.


    1. What's the difference between vivipary and adventitious growth?

      1. For adventitious growth, you have to have a change in function: plant cells in one organ turning into another organ. A common example is when tissue in the stem suddenly starts to grow roots. It's perfectly normal for plants to do that, but it does represent a change in function: tissue that was "destined" to be one sort of organ is turned into another sort.

        With vivipary, there's no change in function. The seed was always going to sprout and become a new plant; that's what embryos do. Vivipary is just when the embryo jumps the gun and sprouts before it's really time.

        Am I making sense?

    2. I found this article after finding several apple seeds sprouting in my apple today. I eat a lot of apples and have never noticed this before, so I googled it and found my way here. I took a couple photos of it and would be happy to send them to you if you wanted them. Thanks for the great explanation!

      1. Oh, neat! Yes, I'd love to see your photos. If you could send them to photosynthetic.430 AT gmail DOT com, that'd be wonderful. Thanks!