How do cotyledons function during seed germination?

Cotyledons, or seed leaves, are essential structures within a seed that support a plant embryo during germination until it becomes self-supporting.1

Their primary functions include:

• Nutrient Storage and Mobilization: Cotyledons often serve as the site for nutritional reserves, such as starch, proteins, and lipids. In many eudicots, these reserves are stored directly within the cotyledons. In other plants, the cotyledon acts as an absorbing organ that secretes enzymes to mobilize and transfer nutrients from the endosperm to the developing embryo.1
• Photosynthesis: In some species, such as the castor bean, cotyledons expand upon exposure to light to become the seedling's first photosynthetic organs.1
• Germination Support: Depending on the species, cotyledons facilitate growth through two patterns:
  • Hypogeal: The cotyledons remain underground, transferring stored reserves to the growing radicle and epicotyl.1
  • Epigeal: The hypocotyl elongates, lifting the cotyledons aboveground, where they may either perform photosynthesis or degenerate after exhausting their stored nutrients.1

Typically, these embryonic leaves do not persist long after the seedling establishes itself as an independent organism.1