rving 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].

Considered as an exceptional occurrence, it occurs frequently in certain
plants, and, when it affects the stem or branches, necessarily causes
some changes in the arrangement of the parts attached to them; thus,
spiral torsion of the axial organs is generally accompanied by
displacement of the leaves, whorled leaves becoming alternate, and
opposite or whorled leaves becoming arranged on one side of the stem
only. Frequently also this condition is associated with fasciation, or,
at least, with a distended or dilated state. An illustration of this in
_Asp