The part of a cannabis plant seedling that develops into a root, the root tip.
Merriam-Webster Online Dictionary
radicle (noun)1.
the lower part of the axis of a plant embryo or seedling
a) the embryonic root of a seedling
b) - hypocotyl
c) the hypocotyl and the root together
2.
- radical
Sprout, Seedling, Embryonic root, Primary root, Germinal root