Thus the production of adventitious roots on the stem of the vine is considered to be due to untoward circumstances impairing the proper action of the ordinary subterranean roots. So, too, the formation of roots on the upper portions of stems that are more or less decayed below, as in old willows, is to be considered as an attempt to obtain fresh supplies through a more vigorous and healthy channel. A similar occurrence often arises as a consequence of some injury. Virgil had this circumstance in view when he wrote "_Quin et, caudicibus sectis, mirabile dictu, Truditur e sicco radix oleagina ligno._"--'Georg.' Bk. ii. I have seen many specimens of adventitious roots produced on the olive in the way just mentioned. In the 'Gardeners' Chronicle,' January 8th, 1853, p. 21, is described a curious formation of roots in the fissure between two divisions of a laburnum stem. In the same journal, January 1st, 1853, p. 4, Mr. Booth mentions the case of a Cornish elm, the trunk of which was divided at the top into two main divisions, and from the force of the wind or from some other cause the stem was split down for several feet below the fork. Around the edges of the fracture, layers of new bark were formed, from which numerous roots issued, some measuring an inch in diameter and descending into the cleft portion of the tree: similar instances must be familiar to all observers. It may happen that these roots sent down into the cavity of a decaying trunk may, after a time, become completely concealed within it, by the gradual formation and extension of new wood over the orifice of the cavity formed by the death and decay of the old wood. Such is presumed to be the explanation of a specimen of this kind in the possession of the writer, and taken from a cavity in an apparently solid block of rosewood; externally there were no marks to indicate the existence of a central space, but when the block was sawn up for the use of the cabinet-maker, this root-like structure was found in the centre and attached to one end of the cavity. The production of roots which ultimately serve as props to support the branches, or as buttresses to compensate for the increasing weight of branches and foliage, is also a familiar occurrence. The huge gnaurs and burrs met with occasionally on some trees often produce great quantities, not only of adventitious buds, but of roots also. [Illustration: FIG. 71.--Production of adventitious roots from leaf stalk of celery.]