Seeds: Plants in Suspended Animation

Last month we talked about plant sex. If you missed that post, you can read it here. I’ll also post the same diagram from last time, from the University of Illinois Extension, so you can refer to it as we go along:flower12Imagine that the flowers in your garden are in full bloom. (I know, it’s still winter, but you can pretend.) The bees have been busy, and pollen from an anther has arrived at another flower’s stigma.  Now what?

Let’s start with pollen. Each grain of pollen contains two sperm (yes, they’re called sperm in plants, too). These sperm need to get to the ovules (= little eggs) at the base of a  flower’s pistil. In order to do that, the pollen grows a long tube, appropriately called a pollen tube, reaching down from the stigma, through the style, to the ovary where the ovules are. The pollen then sends its pair of sperm through the tube to the ovary full of ovules. Each pollen tube targets a specific ovule.

Corn silks_LAHA side note: Everyone who has ever shucked an ear of corn has been frustrated by the silks—all those clingy strings that are so hard to remove, and that get stuck between our teeth. Well, those silks are pollen tubes! Next time you prepare corn, notice how each silk goes to a specific kernel. Kernels that lack tubes never got pollinated. That’s why they remain stunted and inedible.

Now, what happens when those two sperm cells arrive at an ovule? Well, in the case of one sperm, it’s pretty much the same thing that happens when a human sperm meets a human egg. The sperm carries half the genetic information needed to form a new plant. The ovule carries a complementary set of genes. These merge, and the ovule begins to grow. Cells double, and double again, and eventually they develop into a tiny embryonic plant, with a stem, baby leaves, and budding root (see the bean seed photo below).

But what about that second sperm? It plays an equally important role by combining with two other structures in the ovule called the polar nuclei. (If you’re really into biology, note that the polar nuclei contain a full set of genes, so the resulting endosperm is triploid.)

This second sperm plus the polar nuclei fuse to become a single cell. This cell grows into support structures such as the seed coat for protection and a supply of starch to nourish the growing embryo.

The ovary in which all this is taking place also grows and becomes a fruit. (It’s called a fruit no matter how juicy and appetizing it is.) Thus, a fruit is actually a plant womb.

So here we have a fruit containing one or more plant embryos, which are each surrounded by a hard seed coat. They’re packaged with plenty of starchy food to keep them growing until they can push through the soil, spread their first leaves in the sunshine, and begin to make their own food through photosynthesis. Here’s a photo of a lima bean seed, with some parts labeled. (The seed coat is no longer hard and has slipped off the cotyledon, because I soaked the bean in water in order to open it.) As you can see, this seed contains a baby plant.

Labeled Lima seed LAHBut now the embryo enters a period of suspended animation. Instead of continuing to grow, this infant plant suddenly stops and goes dormant. Why doesn’t it just keep growing? There are several reasons.

For one, a single flower usually produces multiple seeds (not always—consider the peach pit), and there is usually more than one flower per plant. If all these seeds stayed in one spot, there wouldn’t be room for all the seedlings. They have to disperse so that they don’t compete for food and water. In fact, seeds are pretty much the only way for a stuck-in-the-mud plant to expand its territory.

Then, consider that most seeds form at the end of the growing season. If the baby plants start life then, they’d freeze. Instead, the seeds wait through the winter until conditions are right for growth. In fact, many temperate-climate plants produce seeds that cannot germinate right away. Either they must age first, or they most experience freezing temperatures.

But what about plants in the tropics? There, the growing season can last all year. Sure enough, seeds from tropical plants are often able to germinate as soon as they mature. In some cases, they have to grow right away; some are only viable for a few months at most.

As you go into your garden this spring, seed packet in hand, consider what an amazing thing a seed is. You’re sowing tiny embryos, plants in suspended animation, just waiting for the opportunity to grow.

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