Seeds are essentially embryos. Cotyledons form the embryonic leaves. Botanists classify plants based on the number of cotyledons or leaves. I think. Sorry, I am not a botanist. What I can tell you for tomato seedlings is that the First Leaves to emerge are not True Leaves. They break the surface and are part of germination geared towards establishing the tomato. They are simply called first leaves. In this case a picture is worth a thousand words.
|First Leaves but Not True Tomato Leaves - The Rusted Garden Blog|
These first leaves to emerge do not have a basic tomato leaf shape. The edges are not scalloped or broken in any way. True leaves for a tomato seedling are the second set of leaves to develop. They typically show up 10 days after germination or 14-21 days from planting the seeds. True Leaves for a tomato typically have the tomato leaf shape we are accustomed to seeing. Here is another picture to show you the actual true leaves of tomato seedlings. Notice the difference in shape.
|True Leaves of a Tomato Seedling - The Rusted Garden Blog|
True leaves let you know it is time to start considering transplanting the seedlings into larger containers. When you decide to transplant and how you transplant them is a personal choice of technique. Typically, tomatoes are transplanted 3-5 weeks from the original seed planting date.
Another 60 Second or Sow Video on Identifying First Leaves and True Leaves on Tomato Seedlings
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