Seeds & growth
Cotyledon (seed leaves)
Also known as: seed leaves
The first leaves to emerge from a germinating seed, usually simple in shape and different from the plant true leaves.
Cotyledons, often called seed leaves, are the very first leaves a plant produces as it emerges from a seed. They are already formed inside the seed before you sow it, packed away ready to unfold the moment germination begins. As the shoot pushes up through the compost, these are usually the first green you see.
Their job: a packed lunch
A cotyledon's main role is to feed the young seedling until it can fend for itself. In many seeds they act as a built-in food store, holding the energy reserves that powered the seed through germination. In others they unfurl, turn green and start photosynthesising straight away, making the first sugars the seedling needs to grow. Either way, they are a temporary support system: a packed lunch that keeps the plant going during its most vulnerable first days, before it has proper roots and leaves to earn its own living.
How they differ from true leaves
The key thing to recognise is that cotyledons look nothing like the leaves the plant will go on to make. They are usually plain and simple in shape, often a smooth oval or a pair of slim straps, and they appear in a set number, typically two for the broad-leaved plants most of us grow. The leaves that follow, the true leaves, have the characteristic shape and texture of the plant: the jagged, scented leaf of a tomato, the lobed leaf of a courgette, the divided frond of a carrot. Once the true leaves appear, the seedling is properly up and running and the cotyledons start to do less.
Why it matters when you're sowing
Knowing the difference is genuinely useful at the seed tray. Many growers wait for the first pair of true leaves before pricking out or potting on, because a seedling with only its seed leaves is still very young and easily checked by the move. Spotting cotyledons also helps you tell your wanted seedlings from weeds, since a tray of evenly spaced, identical seed leaves is a sign your sowing has come up well.
In time the cotyledons usually yellow, shrivel and drop off once the plant can support itself, and that is perfectly normal rather than a sign of trouble. So when you see those first plain leaves appear on a UK windowsill in spring, you are looking at the seedling's starter rations, not the real thing. The proper plant is the one that comes next.
In a UK garden
On a UK windowsill in early spring, the cotyledons are usually the first green you'll see a few days after sowing tomatoes, courgettes or beans indoors, well before the recognisable true leaves arrive.
Example
A tomato seedling pushes up two slim, smooth seed leaves first; the familiar jagged tomato leaves only appear a week or so later.