320 HETEROMORrHY. 



&c., the degree of twisting being dependent to a great 

 extent on the roughness of the surface around which 

 the stem twines.^ 



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 accom- 

 panied by displacement of the leaves, whorled leaves 

 becoming alternate, and opposite or whorled leaves 

 becoming arranged on one side of the stem only. Fre- 

 quently also this condition is associated with fasciation, 

 or, at least, with a distended or dilated state. An 

 illustration of this in Asparagus has been figured at 



p-14. .... 



Very often the leaves are produced in a spiral hue 

 round the stem, as in a specimen of Dracocephalmn 

 speciosum described and figured by 0. Morren. The 

 leaves of this plant are naturally rectiserial and de- 

 cussate, but, in the twisted stem the leaves were curvi- 



6 

 serial, and arranged according to the plan. Now, 



JLo 



referring to the ordinary notation of alternate leaves, 



we shall have the first leaf covered by the fifth, with 



two turns of the spiral ; since decussate leaves result 



from two conjugate Hues, the formula will be neces- 



2.5 

 sarily . The fi'action - hence comes regularly into 



O i-O 



2 . /2 3 5^ 

 the -K- series /-, -j \. Thus, the leaves in assuming 



a new phyllotaxy, take one quite analogous to the 

 normal one. 



One of the most curious instances that have fallen 

 under the writer's own observation occurred in the 

 stem of Dipsacus fullonum. (See * Proceedings of 



' See Darwin " On Climbing Plants," ' Joum. Linn. Soc. Botany,' 

 vol. ix, p. 6. 



