protocorm

from The Century Dictionary.

• noun A tuber-like body provided with rhizoids produced in the place of a hypocotyl with roots, in the germination of some species of Lycopodium. This is regarded by Treub (the author of the term) as the rudiment of a leafy shoot, but by Goebel as an arrested hypocotylous segment. Analogous formations are found in the related genus Phylloglossum and in some orchids and dicotyledons.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany A tuberous mass of cells that is the embryonic form of orchids and club mosses