Cotyledon

Cotyledons form during embryogenesis, along with the root and shoot meristems, and are therefore present in the seed prior to germination.

True leaves, however, form post-embryonically (i.e. after germination) from the shoot apical meristem, which generates subsequent aerial portions of the plant.

Gnetophytes, cycads, and ginkgos all have 2, whereas in conifers they are often variable in number (multicotyledonous), with 2 to 24 cotyledons forming a whorl at the top of the hypocotyl (the embryonic stem) surrounding the plumule.

As these reserves are used up, the cotyledons may turn green and begin photosynthesis, or may wither as the first true leaves take over food production for the seedling.

[citation needed] Hypogeal plants have (on average) significantly larger seeds than epigeal ones.

[5][6] The ultimate development of the epigeal habit is represented by a few plants, mostly in the family Gesneriaceae in which the cotyledon persists for a lifetime.

Cotyledon from a Judas-tree ( Cercis siliquastrum , a dicot) seedling
Comparison of a monocot and dicot sprouting. The visible part of the monocot plant (left) is actually the first true leaf produced from the meristem ; the cotyledon itself remains within the seed
Schematic of epigeal vs hypogeal germination
Peanut seeds split in half, showing the embryos with cotyledons and primordial root
Two-week-old Douglas fir (a conifer) with seven cotyledons
Mimosa pudica (a dicot) seedling with two cotyledons and the first "true" leaf with six leaflets