The term adventitious refers to a structure arising from an unusual place, such as stems from roots or roots from stems. Normally roots grow underground at the base of a plant. But some plants produce roots from their stems and branches. With ivy, for example, adventitious roots provide nourishment and enable the plant to cling onto other plants or buildings for support.
Other examples of adventitious roots
The root system in this, as in other cereals, consists of adventitious roots from the lowest nodes of the stem.
This is a tree that grows in muddy river mouths in the tropics. The original root, which is embedded in the mud, dies from lack of oxygen soon after the stem has grown up. Then adventitious roots grow down from the stem and branches, penetrating the mud. The parts of these roots which remain above the mud are submerged in water at high tide, but at low tide are exposed to the air, and so can absorb oxygen.
Banyan tree (Ficus bengalensis)
Aerial roots (which are, of course, adventitious roots) grow down to the ground from the long horizontal branches of the tree. They penetrate the ground and thicken to form additional "trunks," so that the tree gradually covers a wider and wider area, eventually forming a small wood or forest on its own.