Here's why:

* Hypocotyl: The Stem Below the Cotyledons The hypocotyl is the part of the stem of a seedling that is located between the root and the cotyledons (seed leaves). It's responsible for lifting the cotyledons above the soil surface during germination.

* Adult Plant Development: As a plant grows, the hypocotyl elongates and becomes part of the main stem. The cotyledons usually fall off as the true leaves develop. The hypocotyl essentially merges with the rest of the stem structure.

Therefore, the hypocotyl, as a distinct structure, is not found in an adult plant.