The phenomenon is not confined to woody plants, but has been met with in chicory, in _Antirrhinum_, and other herbaceous species. It is very difficult in some cases to separate these instances of irregular torsion from those in which the twisting takes place in a more or less regular spiral direction. In the former case the fibres of the plant are only indirectly involved, but in the latter the fibres themselves are coiled spirally from right to left, or _vice versa_ (spiral torsion), while not unfrequently both conditions may be met with at the same time. The leaves also are subject to similar deformities, of which a notable illustration has been recorded in the case of the date palm, _Phoenix dactylifera_, originally observed by Goethe, and figured and described by Jaeger;[355] the leaves are folded and twisted in every direction, in consequence of the fibrous band or cord which surrounds the leaves, and which generally breaks as the leaflets increase in size, remaining from some cause or other unbroken, and thus serving to restrain the growth. A similar irregularity of growth occurs, not unfrequently, in the case of crocus leaves, when in the course of their growth, as they push their way through the soil, their progress becomes checked either by a stone or even by frost. =Spiral torsion.=--Growth in a spiral direction, and the arrangement of the various organs of the plant in a spiral manner, are among the most common of natural phenomena in plants.[356] Fibres are coiled spirally in the minute vessels of flowering plants, and are not wholly wanting even among fungi. The leaf-organs are very generally spirally arranged; the leaf-stalks are often so twisted as to bring leaves on one plane which otherwise would occupy several. In the leaf itself we have a spiral twist taking place constantly in _Alstroemeria_, in _Avena_, and other plants. A similar tendency is manifested in the flower-stalks, as in _Cyclamen_ and _Vallisneria_, and the whole inflorescence, as in _Spiranthes_. Even the bark and wood of trees is often disposed spirally. This is very noticeable in some firs, and in the bark of the sweet chestnut (_Castanea_), of _Thuja occidentalis_, and other trees. The knaurs or excrescences which are sometimes found on the roots or stems of trees afford other illustrations of this universal tendency. These bodies consist of a number of embryo buds, which, from some cause or other, are incapable of lengthening. On examination every rudimentary or undeveloped bud may be seen to be surrounded by densely crowded fibres arranged spirally. The axes of nearly all twining plants are themselves twisted, and twisted in a direction corresponding to the spontaneous revolving movement exhibited by these plants, as in the hop, the convolvulus, passion flower, &c., the degree of twisting being dependent to a great extent on the roughness of the surface around which the stem twines[357].