Adventitious Root: A Root That Forms Where None Was Planned

Quick facts

What it is: any root that does not descend from the seedling root system but forms later from a stem, leaf or bud
Name: from Latin adventicius, arriving from outside
Where it matters: every root on a cutting or a layer is adventitious
Main regulator: auxin, though not on its own
Not to be confused with: a lateral root, which branches off an existing root

Where it comes from

Adventitious roots have no single tissue of origin. Documented sources include epidermis, cortex, phloem, cambium, xylem, pith and callus. In woody plants the commonest origin is young secondary phloem, the zone near the cambium. Which of these tissues supplies the root in any given case is still described in propagation textbooks as unresolved.

The sequence runs in stages: cells return to a dividing state, a root primordium forms from them, the primordium grows, and it finally breaks out through the cortex.

Preformed or wound-induced

Propagation science separates two cases. Some species carry dormant root initials in the stem that need only moisture; willow and pothos belong here, and their cuttings root almost by themselves. In other species the initial forms only after the cut, triggered by the wound itself. Cannabis belongs to the second group, but roots readily within it.

What controls it

Auxin is the central regulator, which is why rooting preparations are built on it. It does not act alone. In sunflower cuttings without boron, cells divided but no root primordia appeared, and in mung bean, indole-3-butyric acid worked only where borate was supplied as well. Cell division and root initiation are two separate steps, and the second can fail while the first proceeds.

Relevance to cannabis

All clonal propagation of cannabis rests on adventitious rooting. What has been studied in this species so far is leaf number, cutting position on the stock plant, rooting hormone, light spectrum, short-day pre-treatment and substrate. An anatomical study showing which tissue the roots on a cannabis cutting actually come from is missing.