The Spiral Disposition of the Leaves. bi^ 

 to one of the ordinary types of leaf arrangement, (e. g., 



%). 



In the rosettes of radical leaves, where the internodes 

 do not lenginen, the spiral disposition of the leaves does 

 not involve any further disturbance. The leaves simply 

 grow out and retain their original position. But when, 

 in the second year, the young internodes begin to elon- 

 gate, this cannot happen equally on all sides of the stem, 

 because the line of attachment of the leaves acts as a 

 check. In consequence of this the stem must twist and 

 unwind the leaf-spiral, the angle of divergence between 

 successive leaves becoming gradually smaller. The num- 

 ber of windings decreases, and on the other hand, the 

 numbers of leaves on a single section of the spiral (i. e., 

 from a given point on the stem to another vertically 

 above it) increases, as our Fig. 123 clearly shows. 



Spiral torsion is, therefore, a mechanical result of the 

 loss of a single character, the decussate arrangement of 

 the leaves. Once this is lost, the ancestral spiral dispo- 

 sition steps In, but now accompanied by peculiarities in 

 the structure of the basal parts of the leaves, which on 

 normal plants never occur Independent of an arrangement 

 in whorls, since they can only, so to speak, agree with the 

 normal structure of plants with this arrangement. There- 

 fore, In my opinion spiral torsion is due to a retrogressive 

 transformation of the decussate arrangement of leaves. 



That It Is a mechanical result can be proved by ex- 

 periment; all that Is necessary Is to remove the cause of 

 the twisting at an early stage, by cutting through the 

 leaf spiral. If this is done carefully, the general growth 

 of the plant Is not Interfered with, but the torsion will 

 be locally inhibited, or, more strictly speaking, does not 

 appear at the place operated on. Thus a straight inter- 



