The hypocotyl is the part of the embryonic axis below the cotyledons and above the root. After the radicle emerges, the hypocotyl lengthens in a hook shape, pushes up through the medium, then straightens to raise the cotyledons into the light. Its length is controlled largely by light signals perceived by phytochromes and cryptochromes.
In practice
A long, thin, floppy hypocotyl is the classic "stretched seedling". It means too little light (or too little blue, or a lamp much too far away), sometimes combined with heat. Seedlings typically do fine around 100-300 µmol/m²/s at canopy; very dim windowsills give leggy hypocotyls.
Growers often bury a stretched hypocotyl deeper at transplant. Cannabis can root along a buried stem, and the plant gains support. Keep the cotyledons above the surface.
The hypocotyl is also the tissue most attacked by damping-off fungi, which pinch it at the soil line.
Don't confuse with
- Epicotyl: the stem above the cotyledons, carrying the first true leaves.
- Internode: the stem length between two nodes on an established plant.
- Radicle: the embryonic root below it.
One of the two rounded seed leaves packed in the embryo that open first after germination and feed the seedling until the true leaves take over.The embryonic root, the first structure to emerge from a germinating seed, which grows down to become the primary root or taproot.A family of plant photoreceptors that sense red and far-red light, measuring night length for flowering and shade for stem elongation.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.The collapse of seedlings or cuttings when soil-borne pathogens such as Pythium, Rhizoctonia solani and Fusarium rot the stem at the soil line.The first leaves produced by the shoot tip after the cotyledons, with the plant's characteristic serrated leaflets.The length of stem between two adjacent nodes; short internodes mean compact plants and tight bud stacking, long ones mean stretch.
