Adventitious Organ

in plants, an organ, such as a bud or root, that arises at sites where it ordinarily does not develop. For example, adventitious buds often arise not in the axils of leaves or at the stem apex, as usual, but on internodes, roots, or leaves. Adventitious roots may develop on the stem or leaf.

Adventitious organs always develop from secondary tissues (meristem), which arise in the parenchyma of a mature organ (in the leaf parenchyma, in the pericycle and phloem of the stem or root). Such organs have great biological significance. Large root suckers that ensure vegetative propagation are formed by the adventitious buds of the poplar, aspen, willow, and white acacia, as well as by those of herbs and shrubs (sowthistle, thistle, spurge, sorrel, horseradish, raspberry, dewberry, barberry).

Adventitious buds form least commonly on leaves. In Bryophyllum, shoots develop from adventitious buds on the denticles of the leaves. Rooted begonia slips develop such buds on the veins of the leaf blade. Adventitious buds may develop on a callus formed from the cutting and grafting of leaves.

Adventitious organs sometimes perform special functions. The ivy and the vanilla have aerial rootlets, and the adventitious roots of a number of Ficus species cling to supports. The aerial roots of tropical orchids, Bromeliaceae, and Araceae, absorb atmospheric precipitation. In some epiphytic orchids, the adventitious roots turn green and serve an assimilative function.