SPIRAL TORSION. 319 
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 unfre- 
quently, 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.’ 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 
Alstremeria, 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 conyolvulus, passion flower, 
' See Goethe, ‘ Ueber die spiral Tendenz.’ 
