Happens when seed: a concise orientation before we get practical.
Happens when seed: Quick notes
It’s amazing what happens inside a seed before and after it germinates. And it’s a process that I believe every gardener should see at least once and understand, because learning the science behind germination will help you become a better gardener!
I’m endlessly fascinated by how bushels of juicy tomatoes will come from a single seed smaller than the diameter of a pencil eraser, and how specks of basil seeds will turn into mounds of fragrant herbs that grow over 3 feet tall.
How does all that life happen, and in a relatively short period of time? Here’s a closer look at the cycle from seed to seedling.
Anatomy of a seed
When you have a seed in front of you, you’re looking at the seed coat, or testa. Seed coats can be soft and thin (like beans) or fleshy and thick (like squash).
Seed coats vary in thickness and texture across different seeds
Think of the testa as armor for the future plants inside. Seed coats protect against the elements until external conditions present themselves in just the right manner for germination to take place.
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Seed coats are adapted to their environments, and they function as a barrier to damage or injury, heat or cold, bacteria or fungi, and even stomach acidity (if ingested by animals).
With all this bomber protection in place, it’s no wonder we sometimes have trouble getting a seed to sprout! (Soaking your seeds first can help with this, or in the case of tomato seeds, you can ferment them to speed up germination.)
Seed coats can be tougher than they appear
On some seeds, you can see the scar, or hilum, that was left behind when the seed became detached from the mother plant (similar to a human belly button).
Hilum are most visible on bean seeds, where the scars as known as eyes—as in black-eyed peas. Others are less noticeable and look like little nubs or dents on the seed coat.
Hilum (seen here as eyes) on pea and bean seeds
Hilum (seen here as a small indentation) on a tomato seed
Inside the seed coat is the embryo (baby plant), the endosperm (nutritive tissue), the cotyledons (leaf-like structures), and the beginnings of the root and shoot.
Look closely at this wet seed and you’ll actually see the green cotyledons tucked tightly inside and curled up in fetal position. They are just waiting to be released! (Or at least, that’s what I hope every time I start some seeds.)
The cotyledon (green leaf-like structure) is visible inside this wet seed
The start of germination
Germination occurs when all the proper variables are in place for that particular variety (oxygen, temperature, light or darkness) and the seed coat absorbs water, causing it to swell and rupture.
Different types of seeds have their “ideal variables” built in genetically. Some may require prolonged exposure to cold before they sprout (a process known as vernalization). Some seeds won’t sprout if temperatures are too cool, while other seeds won’t sprout unless the soil cools down. Then there are seeds like hot peppers that need very warm temperatures to trigger germination.
Related: Find the ideal soil temperatures for germinating seeds
But once all the stars finally align, the first sign of life comes from the radicle, a little white tail that eventually becomes the primary root of the plant.
The radicle (primary root) emerges first when a seed germinates
A green stem (hypocotyl) starts to appear after the primary root (radicle) as the seed continues to sprout
The role of the radicle is to anchor the plant in the ground and start absorbing water. Once it absorbs water, a rudimentary stem called a hypocotyl emerges and the cotyledons start to unfurl, sometimes taking the seed coat with them as they rise above the soil.
(I like to call them seed hats, as sometimes they’re never shed from the cotyledons and end up looking like little berets.)
A radicle anchors the seedling and starts absorbing water before cotyledons unfurl above the hypocotyl
Cotyledons emerging from the seed coat
Shown above from left to right: Cotyledons, stem, seed coat, and radicle
Are cotyledons the first leaves?
Cotyledons look like leaves but are actually not leaves at all. Sometimes they’re called seed leaves, because they’re part of the seed or embryo of the plant. The function of these leaf-like structures is to absorb all the endosperm and become the temporary stores of the plant’s initial supply of nutrients.
There can be just one cotyledon (monocotyledon, as is the case with onions and corn) or there can be two (dicotyledons, which are the majority of your vegetables).
Monocotyledons (on onion seedlings)
Dicotyledons (on mustard seedlings)
Because cotyledons are not the “true leaves” on a plant, most sprouts from the same family look alike at birth, even if they don’t resemble one another as they mature (like these tomatoes and peppers below).
(This is why it’s important to label your seed trays, otherwise you’ll be waiting weeks for the seedlings to develop further before you can identify them.)
The cotyledons feed the plant until all the nutrients are used up. Once they’re spent, they naturally wither and fall off the stem as new “true leaves” form.
Cotyledons vs. true leaves
In general, it takes one to three weeks for true leaves to appear once the cotyledons emerge. (The timeframe is dependent on the type of plant, as well as environmental factors like sunlight, moisture, and temperature.)
So how can you tell the difference?
Cotyledons are always the first leaf-like structures to form when a seed germinates. Most cotyledons are nondescript and tend to look similar within a plant family.
For example, radishes, broccoli, and kale (members of the brassica family) begin life with two stubby, heart-shaped cotyledons. On the other hand, tomato, pepper, and eggplant seedlings (members of the nightshade family) all start out with a pair of long, narrow leaves with slightly pointed tips.
Once the true leaves develop, they start to resemble actual leaves on the mature plant—just tinier versions of them. The first set of true leaves usually emerges as a pair, growing opposite each other on the stem. Second and subsequent true leaves may grow one at a time, depending on the species.
True leaves grow above the cotyledons and take over the job of supporting the plant for the rest of its lifecycle. Eventually, the cotyledons dry up and fall off as they’re no longer needed.
After the first few sets of leaves sprout and the roots dig deeper into the soil, the seedling draws energy from photosynthesis above ground and nutrients found below ground. This is when it enters its vegetative state, and your once-little seedling is on its way to maturity (bud, flower, fruit, and seed).
They grow up so fast, don’t they?
Frequently asked questions
This post updated from an article that originally appeared on March 19, 2014.
A short mention of Happens when seed helps readers follow the flow.
We reference Happens when seed briefly to keep the thread coherent.
