What Are Cotyledons?

The word cotyledon is often brought up when discussing microgreens and is usually followed by the question "What are cotyledons?

Cotyledons are in essence microgreens. That being said, for many microgreens, you do wait until the first true leaves form before harvesting them; but still, the cotyledons are a large part of the harvestable microgreen.

The green "leaf" of the radish microgreens below are in actuality, radish cotyledons. 

What Are Cotyledons?

Cotyledons are the food source of the plant embryo and are also the first leaves that emerge from the soil as the plant germinates.

In a way, they are at first a battery or inverter and then the solar panels that energize the young plant.

Let's take a look at a bean seed, as most people have seen bean seeds whether they garden or not. Don't worry about the dicot label that will be discussed shortly.

There are a lot of terms I know but bear with me.

The radicle will be the roots. Don't worry about the hypocotyl, that is the connection between the roots and the cotyledon, the place where energy from the cotyledon is transferred to the roots.

The epicotyl is the main stem and where the first true leaves will form.

Back to my battery and solar panel analogy.

As the roots grow downward to anchor the plant, the leaves start to unfurl. The energy for this growth and movement if you will is stored in the cotyledons or the cotyledons transfer the energy stored in the endosperm.

This is why I think of cotyledons as batteries or at least the inverter.

The cotyledons are generally (not in all plants) pulled upward and turn green when they develop chloroplasts and create chlorophyll.

Since cotyledons are the first leaves on the plant, they are also called seed leaves. When the cotyledons start producing chlorophyll, they begin the process of photosynthesis.

Here's where the solar panel analogy comes in.

Cotyledons: Monocots or Dicots

Seeds can have one cotyledon and are classified as monocots, or they can have two and are categorized as dicots. 

The cotyledon in monocots usually doesn't grow up out of the soil. But most plants that we grow as microgreens are dicots. For microgreens, monocots are onions and leeks.

The cotyledons, or seed leaves, are easy to pick out, as they look nothing like the plant's true leaves.

Seed leaves usually are smooth, oval or heart shapes, and are not very large. They have no time to develop character as they need to get the plant established.

Cotyledons as Microgreens

Many of the microgreens we grow are consumed in the cotyledon stage, i.e. broccoli, cabbage, kohlrabi, radishes, sunflowers, etc.

Other microgreens like, cilantro, basil, leeks, and mustards we wait until the first true leaves form. However, the cotyledons are still present and are a large portion of the microgreen we consume.

The reason for this article is to know when to harvest the microgreen. Often, you'll see a statement saying "Wait until the first true leaves form before harvesting" or "Harvest while in the cotyledon stage."

What it comes down to is flavor, and sometimes nutrition. If we wait until the true leaves to form usually it because the true leaves have a better flavor than the plant's cotyledons. 

The opposite can also be true, that the true leaves taste more bitter, and in this case it's best to harvest during the cotyledon stage. Nutrition can also decrease as the plant matures, so it's best to harvest when the plant is still in the cotyledon stage.

Do Cotyledons Carry Out Photosynthesis?

Many websites state that cotyledons don't have the ability to carry out photosynthesis. 

That the only energy they contribute to the plant is the carbohydrates stored inside of them. 

However, a study by Zhang et al (2010) show that cotyledons of some plants do contribute energy to the plant by photosynthesis.

We believe that the cotyledons of many plants grown as microgreens do produce chlorophyl and help contribute more nutrition to microgreens. The main reason we believe that is how much the microgreens grow before producing their first true leaves.

Plants like tomatoes, whose cotyledons, shrivel and fall off soon after their true leaves form most likely don't carry out photosynthesis.