The mesocotyl or basal portion of the coleoptile elongates when the seed germinates in soil, and in darkness. It pushes the coleoptile above the soil surface.
Fig. 5 - Mesocotyl pushing the coleoptile above the soil surface.
The first seedling leaf, or primary leaf, emerges from the growing seed. It is green and shaped like a cylinder. It has no blade. The second leaf is a complete leaf. It is differentiated into a leaf blade and a leaf sheath.