The Great Cannellini Experiment of 2013

I'm finally done with my experiments, and now we can talk about seeds. We don't interact with seeds much, except when we eat them, but I think people rarely ponder what a seed really is while they're chewing them to bits. Corn, beans, oats, wheat, coconuts, and even the peanuts I'm eating right now are seeds. They come in all shapes and sizes, but they all allow plants pass their genes on to a new generation, sometimes across vast stretches of time and space. They can survive much more extreme weather than their parent plants, and they pack enough nutrition to grow a baby plant's stalk and roots until it can start making its own food. How do they do this? Take a look at a dissected cannellini bean:

Well Howdy-do!

You may have just exclaimed, "Hey! That's a whole friggin' plant right there!" in which case you should apologize to anyone around you who was trying to work or study. Good? Good. You're right though, that is a plant, or rather, a plant embryo inside the bean. That's the whole point of seeds, to protect an already-developed embryo while it waits for environmental conditions that will allow it to grow successfully. Every slice of 12-seed bread you eat is covered in little baby plants.

That bean is in the process of germination, which occurs when that little plant goes through to break out of the seed and start growing. Seeds germinate when they have the right amount moisture and oxygen around them. Sometimes, they require other environmental qualities, like exposure to red light (which only reaches the ground when the tree leaf canopy is gone) or a long period of cold followed by warming soil (which would happen to a seed that fell in the fall). Basically, when the environment is right for the plant in the seed, it sets off a chain reaction that gives the embryo access to the energy stored in the seed so it can start to grow. Beans are pretty easy to please, and will germinate in a wet paper towel left in a dark, warm spot for a few days.

These beans have all germinated, as you can see by the roots sticking out of the seed casings. The roots are the first to grow, and reach down deep into the soil to secure a source of water for the embryo before the stalk starts to grow. These seeds were all oriented differently in the paper towel, so their roots are growing in different directions. If I had been able to take a picture of them without moving them first, all the roots would be pointing downward. Gravity affects hormones in the embryo which cause the root to grow downward and the shoot to grow upward.

We all know that plants make their own food to power their growth from sunlight, so how do the embryos grow if they're buried in the soil? The cotyledon (caht-uh-lee-dun) is packed full of food energy, which the plant stored in the seed before it fell off. A lot of plant seeds - like those of beans, tomatoes, squash, cherry, apple, and walnuts - have two cotyledons. In a bean, the cotyledons are the fleshy part of the seed - everything besides the embryo. Other plants have just one cotyledon, although some of those have an extra source of food inside as well, called the endosperm. Common monocots (plants with seeds with one cotyledon) include corn, all grasses, coconut, bamboo, date palms, sugarcane, and pineapple. Monocots usually have different qualities than dicots (two cotyledons). For example, they tend to be much more flexible, and stand up to wind better than dicots of the same size. 

After examining the beans after a few days in the paper towel, I sowed the ones that had germinated in a pot and left them on the window sill to grow. This photo shows the next stage in their development: emergence of the stalk from the soil. The stalk starts to grow toward the surface using energy from the cotyledons. They grow curved like that at first to protect the apical meristem (the point of growth at the tip of the stalk) from injury in the soil. As soon as sunlight hits the stalk, it starts to photosynthesize food to supplement that in the cotyledons, hence these stalks have turned green with chlorophyll. On each of these plants you can see a stalk and one or two distinct cotyledons, halves of the bean I planted days before. These will remain attached to the plant until their food energy is gone. You can also see the seed coats, thrown off almost like egg shells next to each plant. Fun Fact: At this point, the stalks were less than an inch tall, but the roots were starting to grown out the bottom of the 4-inch pot already!

Once the cotyledons are fully aboveground, the stalk starts to straighten up and we can see the embryonic leaves come out. These leaves are considered part of the cotyledons, since they were present in the embryo. True leaves usually look much different, as you'll see, and only grow after the plant has germinated. Embryonic leaves are usually very large to collect as much sunlight as possible to fuel the growth of true leaves. 

Here are some pictures I took in the last few days to show the difference between embryonic leaves and true leaves. The plants are about 2 weeks old, that is, since they emerged from the soil. On the left, you can see the embryonic leaves have grown very large. If you look closely, it's clear that there are two embryonic leaves growing directly across from one another on each stem, each of these corresponds to a cotyledon.The leaves are very broad and spade shaped. On the right is the first true leaf to emerge from the stem. Bean leaves of this species have three leaflets on each leaf, so what you're seeing there is just one leaf. The apical meristem is within the small structure at the base of the leaf. The stalk will continue to grow from there, and send off another leaf higher up. This second true leaf won't be directly across from the first one, either, but something like 137 Degrees around the stem. The third one will be 137 degrees from the second and so on, and thus create a whorled pattern if viewed from above. This pattern lets the leaves absorb lots of sunlight without shading each other out.

And finally, here's what happens to the cotyledons as they get older. They still look like beans, don't they? I personally think it's amazing that they can become so green and soft after being dried out for so long. As their food energy gets used up, they shrivel because the starches are being broken down and transported to the roots for use. What's left of them is mostly structural cells, but they can still photosynthesize a little bit. In the next few days they'll shrivel more, the stem will form a sort of scab between them and itself, and they'll fall off and get consumed by bacteria in the soil, which will make even more of their nutrients available to the roots.

In addition to protecting plant embryos from harm, part of what makes seeds so valuable is their ability to lie dormant for long periods of time while waiting for the exact right conditions. This is why weeds are so difficult to get rid of, and why a forest that burns in a fire is so quick to recover. There are millions of seeds biding their time in any given plot of land, not germinating until they have the space and nutrients to survive. For weeds, this usually means that with every cultivation of a field, new seeds are circulated to the ideal soil depth and can then compete with the crop plants. An interesting idea to think about is how seed dormancy this affects the gene pool of a given species. Many plants can reproduce asexually - that is, clone themselves - if they are not pollinated before the season ends. This allows plants that fail to reproduce sexually in one time and place to try again potentially miles away and years down the line. More on that when I write about plant reproduction.

Of course, the longer a seed lies dormant, the less likely it will be to germinate successfully, although that varies from species to species. Some seeds can't last two years before they lose viability, others can survive hundreds to thousands of years in the right conditions. For example, I just read about a Judean Date Palm, which has been extinct as a species for since about 200 C.E., successfully germinating in 2005. The seed was apparently preserved in a very dry environment from about 45 B.C.E. until the 1960s, when it was discovered. This tree, Methuselah, was the only one to germinate out of all the specimens they found, but it flowered in 2008 and is therefore capable of reproduction. They plan to cross it with an Egyptian date palm species and harvest the first fruit around 2022. Crossing it with another species won't propagate it as a unique species, but it will reintroduce its genes into the date palm gene pool. The Judean Date Palm was widely used for medicinal purposes, which is probably why it went extinct. By letting Methuselah reproduce, we will be able to see what, if any, useful qualities this species has and hopefully create a hybrid species that can carry those qualities into the future.

I want to mention again before I sign off that it's only been 21 days since I started to germinate the cannellini beans. In that time, the embryos they contained have grown to hundreds of times their original size, and they're just getting started. The full bean plants will be 2 to 3 feet tall and have hundreds of leaves and hundreds of new seeds if they get pollinated. Take these banana seeds on the left here as another example. They'll grow into trees over ten feet tall! Whenever I spend time seeding crops, I marvel at the power of seeds to preserve their species. They contain all the genetic information needed to make a whole, new living organism, all inside a minuscule, near-indestructible pod. Next time you have one in your hand, take a good, hard look at it, and see if you can fathom that. You might be surprised at what you learn.

Thanks for Reading,
The Regular Farmer

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-All photos except those of Methuselah the Judean Date Palm and the Measured Banana Seeds are by Ryan Heisler and may be used and distributed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

-Photo Methuselah the Judean Date Palm: Author: Benjitheijneb at Wikimedia Commons. Creative Commons Attribution-Share Alike 3.0 Unported License
-Photo Measured Banana Seeds: Author: Firetwister at Wikimedia Commons. Creative Commons Attribution-Share Alike 3.0 Unported License