In the germination ofthe emerges from the protecting -coat before the shoot. It then grows down into the ground to get on with its work of absorption and anchorage. The elongation of the radicle is the tap- , the first root-form of every plant.
All young forest trees have a well-defined tap-root. In most cases this either dies away, sooner or later, or its growth in length is arrested and the branch, spreading out horizontally in all directions, form the whole root system of the plant.
In time the spreading branch roots may form considerable ridges above the soil. When they first occupied theirthey were young and thin. Each succeeding year saw an increase in their girth, and as they thickened they exerted considerable pressure on the soil in which they lay. The solid ground below them was in this way pressed downwards. The lighter layer above them was raised and pushed aside, so that the roots appeared above the ground-level. Still further denudation may be caused by wind and rain, and sometimes the lower, as well as the upper, part of the root becomes exposed, and the branch root makes a sort of elongated archway a little distance above the ground.
In manyof plants it is the normal thing for the first root to die away and for its place to be taken by roots that grow from the . This is the case in all Grasses. Roots that grow from any other part of the plant than the primary root system are called adventitious roots. In the Grasses the adventitious roots are threadlike. A clump of these, growing from the base of one grass-stem, is known as a fibrous root.
A fibrous root, then, is one in which the primary root has died away ; all the branch roots are thin and threadlike and grow from the base of the stem.
A third form of root is the tuberous root, characteristic of the. In this case, too, the primary root dies away and adventitious roots grow from the base of the stem. These branches are, however, extremely swollen, because of the large amount of food material they store.
It is on the growth of adventitious roots that the” striking” ofdepends. Torn or cut of , Geraniums, and , when brought into contact with damp earth, or even with water, are, in some way that is not understood, stimulated to put out adventitious roots. Poplar and Willow twigs, kept in water in the laboratory, produce numbers of such roots in a very short time.
plants are readily obtained by tearing or the and pinning them closely to damp sand or soil, so that the prominent veins of the under-surface of the are embedded in the soil. Adventitious roots grow out from the angles formed by the veins ; a shoot grows upward at the same time and thus a new plant arises without the intervention of a seed.
Such methods ofare vegetative as opposed to sexual reproduction.
The generally accepted definition of a root is that it is that part of a plant that does not bear leaves ; the lateral appendages resemble the axis from which they spring. This definition does not hold good in all cases.
It is a difficult matter to keep a lawn free from Dandelions. Spudding down to a considerable depth still leaves some of the tap-root in the ground. Later a new rosette of leaves occupies the position of the old one. The root has actually formed a bud from which the-rosette has grown up above the ground. The explanation of this very tiresome habit may be that, after the mutilation, it is useless for branch roots to grow from the small portion of tap-root remaining in the ground, and that therefore the contained sap, designed for their production, is so diverted that leaf-buds are developed in their stead.
Such buds are termed radical buds, and the shoots to which they give rise are radical shoots. They are by no means rare, but occur alike on the roots of herbaceous plants and of shrubs. All gardeners are familiar with them as suckers in the Raspberry, the, and the .