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 fiingi. 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 

 Ahtrcemeria, in Avena, and other plants. A similar 

 tendency is manifested in the flower-stalks, as in 

 Cyclamen and ValUsneria, 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 

 {Gastanea), of Thuja occidentalism 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 im developed 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 tliese 

 plants, as in the hop, the convolvulus, passion flower, 



' See Groethe, ' Ueber die spiral Tendenz.* 



