The Science of Gardening Part 2: Germination
In part two of this gardening series, Ask Science talks about germination.
Germination has always seemed a little magical to me. You have this tiny rock-like thing that can sit dormant for years, then you stick it in the ground, give it a little water, and in a few days you have a plant–just like magic.
But of course, it isn’t actually magic. So let’s take a look at some of the really amazing things that happens to a seed during germination.
Parts of a Seed
Plant seeds have a lot of cool stuff packed into a tiny package. First, you have the seed coat, which is the outer covering of the seed. Not only does the seed coat protect the delicate internal parts of the seed, but it also serves as a sort of sensor system to let the seed know when it is time to germinate.
Inside the seed coat we have the plant embryo. This consists of the seed leaves, or cotyledon, which we learned about last week; the radicle, which will form the the root of the plant; and the hypocotyl, which will form the stem.
You might wonder why scientists couldn’t have just named these things the “baby root” and “baby stem.” Is it because they just like to make things difficult for biology students? Actually it’s because science has a tradition of naming things in Latin and Greek. (Occasionally, there is some French thrown in there, too, because French used to be the language of science.) This might seem annoying, but on the bright side, if you study a little Latin and Greek, you’re almost sure to see a dramatic boost of your science vocab scores on any test!
The final part of the interior of the seed is the endosperm, which provides the food for the plant embryo when it first starts to grow.
Take off Your Coat and Stay a While
In order for germination to occur, certain conditions have to be met, and those conditions vary from plant to plant.
Some seeds will only germinate when water is absorbed through their seed coat. Others will germinate once the seed coat has cracked due to extreme heat, like in a forest fire. Still others won’t germinate until they have experienced sufficient sunlight.
All of these mechanisms serve to help the seed have the best possible chance of survival. For example, the seed of a tree won’t have much of a chance for survival if it germinates under the darkness of a thick forest canopy. So it stays dormant until it is exposed to sufficient sunlight.
After the seed coat comes off or breaks open. the radicle, or baby root, emerges and starts moving down through the soil. One of the cool things about this is that most plants have a special ability called “gravitropism,” which means “gravity turning.” This means that even if the seed is planted upside down, the root will sense this and turn around while they grow, so that they are pointing downward. (This is sometimes also called “geotropism,” which means “earth turning.”)
Next, the stem and leaves emerge from the seed coat and start moving upwards. Again, this process is controlled by gravity, and so even in a seed planted upside down, the stem will figure out which way is up.
Another interesting aspect of germination is that the way the stem grows before it emerges from the ground is different than how it grows after it emerges from the ground. Before emerging from the ground, it proceeds straight upwards–away from gravity–in a long, vertical climb. Once it emerges, however, it begins to thicken, turn darker green, and to generate leaves.
This change in growth pattern is caused by exposure to light, and is called “photomorphogenesis”–which literally means “light-change-development,” or a change in development caused by light.
Try This At Home
There are a couple of fun experiments you can try to see these amazing powers of germination.
First, to see photomorphogenesis, get two plastic cups, poke some holes in the bottom for drainage, and then fill them with dirt. Place the cups on saucers or in bowls to catch the water that drains out, then plant a bean seed in each one. Place one cup in a window that gets sunny, and place the other cup in a light-proof box or in a closet. Water the plants regularly, and at the end of two weeks, compare the two plants.
To see gravitropism in action, take your healthy bean plant (hopefully the one from the window,) and lay the cup down sideways, so the plant is poking out horizontally. Watch it over the next few days to see what happens to the plant.
Conclusion
ConclusionSo, now you know more about the magic of germination! If you want to see some great time lapse movies of germination in action, check out the Plants in Motion website at Indiana University.
If you liked today’s episode, you can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.comcreate new email.
Photo of sprouting seeds courtesy of Shutterstock.
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About the Author
Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech.
