By Jennifer Langille
As gardeners, we spend these late winter days dreaming of seeds—sorting them, ordering new ones, and imagining the life they’ll bring. But before any green shoot breaks through the soil, there’s a quiet miracle at work: the cotyledon. You might have come across the term ‘cotyledon’ and wondered what it is, what its function is, and why it matters.
For several of my gardening years – despite having learned about cotyledons in grade 6 science class – I didn’t truly understand their importance. Once I did, it opened up a whole new way of seeing how plants begin.
The word cotyledon (pronounced coat-uh-lee-dun) comes from the Greek words ko and tyle, meaning “cup-shaped cavity” – a fitting description for something that cradles new life.
What is a Cotyledon?
Cotyledons are often called seed leaves or embryonic leaves. They are part of the basic structure of a seed, formed while the seed is still developing inside the parent plant. They contain stored food reserves to nourish the embryo as it begins to grow. In this way, cotyledons are a bit like the placenta in mammals, responsible for helping new life begin before it can feed itself.
In fact, the term cotyledon is also used in human biology. The human placenta is made up of lobed sections called cotyledons, each one helping deliver nutrients and oxygen from mother to baby. Though very different in form, both plant and human cotyledons serve a similar purpose: to sustain life during its earliest and most delicate stage.
The First to Appear
Once a seed germinates, the cotyledons are typically the first parts to emerge from the soil. They often look nothing like the true leaves of the plant. Sometimes they turn green and photosynthesize, sometimes they wither and fall off quickly, and other times they remain much longer.
Photosynthesis is the process by which green parts of a plant capture light energy and convert it into chemical energy stored in sugars. Cotyledons may contribute to this process until the true leaves can take over.
Dicots and Monocots: One Cotyledon or Two?
Different plants use cotyledons in slightly different ways, and one of the most basic distinctions in plant biology is based on how many cotyledons a seed has. There are two broad types of flowering plants: dicotyledons (dicots) and monocotyledons (monocots). Dicots have two cotyledons in their seeds, while monocots have just one. Beans, tomatoes, and sunflowers are dicots, while grasses, corn, and lilies are monocots. This difference can be seen right when seedlings emerge. Dicots show two leaf-like cotyledons, and monocots just one.
The difference starts right from the seed. Watch the germinations of a dicot and a monocot here:
Above and Below: Epigeal vs Hypogeal Germination
Some seedlings lift their first leaves above the soil, while others keep them hidden underground. There are two styles of emergence:
- In epigeal germination, the cotyledons rise above the soil and often turn green to photosynthesize. Beans, cucumbers, and squash are examples.
- In hypogeal germination, the cotyledons stay below ground, safe from harsh weather and hungry critters. This is common in peas, lentils, and runner beans.
You might be wondering—does this match up with monocots and dicots? The answer: not exactly.
Some dicots (with two cotyledons) have epigeal germination—like beans and squash. Others, like peas, are dicots with hypogeal germination. Monocots (with one cotyledon) can also go either way: corn shows epigeal germination, while barley is hypogeal.
In other words, epigeal vs. hypogeal doesn’t depend on whether a plant is a monocot or dicot. It’s just another variation in the way plants have adapted to grow—and a great reminder that nature doesn’t always colour inside the lines.
While the evolutionary reasons aren’t fully understood, each method has its own advantages. In epigeal germination, where cotyledons emerge above ground, the seedling benefits from additional photosynthetic power early on. In hypogeal germination, the cotyledons stay protected underground, offering greater resilience in harsh weather or if grazing animals nibble the tops of seedlings. Both are fascinating glimpses into how plants begin their journey.
Tiny Leaves with a Big Job
So next time you’re admiring a tray of new seedlings, take a closer look at those first little leaves. They’re more than just a sign of life—they’re the plant’s very first helpers on its journey to the sun. Small, but mighty. Isn’t plant science cool?
April 2025