Fig. 39. Dahlia, with storage roots packed with food.
Although it is general for the roots to hold the plant firmly in the ground, in some cases they grow out of the stem in the air and help to hold it up against a tree or wall, or some support, as in the case of ivy. If you pull off a branch of ivy which is climbing up a tree you will find that all along the back of the stem there are tufts of short thick rootlets which often come away holding a piece of the bark of the supporting tree. These roots, you will see, do not come out in the usual way from the main root or base of the stem, but come out all along the stem itself (see fig. 40). Such special roots are called “Adventitious,” and they grow from the stem wherever they are needed.
Fig. 40. Adventitious roots growing out from the stem of Ivy between the leaf stalks.
Adventitious roots may also come out from a wounded plant which has had its true roots cut away. For example, take a piece of Forget-me-not stem without any roots, and slit it at the base, and put it in a glass of fresh water. After a week or so you should see little white roots growing out from the stem into the water, and if you let them get strong you may then plant the sprig and get a new forget-me-not plant from it. In this and all “cuttings” adventitious roots growing out from the stem do the usual work of roots. There are many other kinds of adventitious roots, but we must only mention the orchids, some of which have long tufts of roots which grow out irregularly from the stem and hang in the air. These are special air-roots, and grow on many orchids, but also on some other plants which live attached to trees and absorb the water out of the air instead of from the soil (see fig. 41).
Fig. 41. Tufts of air roots of an Orchid.