Various gardening tasks use the appearance of the first “true leaves” as a start time – when a seedling can be up-potted fertilized, or grafted, for instance. So it’s good to know what the first true leaves are.
The first green leaves that emerge are called “seed leaves”, or cotyledons. They are generally rounded, not at all like the plants normal leaves. There are only two seed leaves for dicots (hence, di-) and the very next leaves that come out are the true leaves. They may take a bit of time, but they look exactly like what the normal leaves for the plant look like. This is when the plant starts photosynthesizing and all other general plant processes.
So my question is: how mature do the first set of true leaves have to be before grafting, for instance?