320 HETEROMORPHY. 
&e., 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 line 
round the stem, as in a specimen of Dracocephalum 
speciosum described and figured by C. Morren. The 
leaves of this plant are naturally rectiserial and de- 
cussate, but, in the twisted stem the leaves were curvi- 
~ 
serial, and arranged according to the . plan. Now, 
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 lines, the formula will be neces- 
sarily = The fraction 3 hence comes regularly into 
3 *) 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 fullonwm. (See ‘ Proceedings of 
the : series a 
1 See Darwin “On Climbing Plants,” ‘Journ. Linn. Soc. Botany,’ 
vol. ix, p. 5. 
