lants either the whole of the upper part of the stem is thus twisted,
or a portion only: thus Reinsch[360] cites a case in _Equisetum
Telmateia_, where the upper and lower portions of the stem were normal,
while the intermediate portion was twisted spirally. In this instance
the whorl next beneath the spiral had twenty-eight branchlets, and that
immediately above it thirty. Along the course of the spire there were
two hundred and three; dividing this latter number by the mean of the
two preceding, it was seen that the spire included the constituents of
seven ordinary verticils.

[Illustration: FIG. 173.--Stem of _Galium_ spirally twisted. From a
specimen communicated by Mr. Darwin.]

Here also may be mentioned a curious bamboo, the stem of which is
preserved in the British Museum, and in which the internodes, on the
exterior, and the corresponding diaphragms and cavities within are
spiral or oblique in direction.

The root is also subject to the same malformation, the inducing cause
being usually some obstruction to downward growth, as when a plant has
been grown in a small pot, and becomes, as gardeners say, pot-bound.

[Illustration: FIG. 174.--Showing "pot-bound" root twisted spirally
(from the 'Gard. Chron.,' 1849).]

The axial portion of the flower, the thalamus, is also occasionally
twisted in a spiral direction, the lateral parts of the flower being in
consequence displaced. Morren spoke of this displacement of the floral
organs as "speiranthie."[361]

Morren draws a distinction between spiral-torsion or spiralism and the
less regular torsion spoken of in the preceding section; in the former
case not only is the axis twisted, but its constituent fibres also. The
condition in question in some cases seems to be inherited in the
seedling plants.

The following is a list of the plants in which spiral torsion of the
stem or branches has been most frequently observed. (See also under
Fasciation and Contortion.)

Hesperis matronalis.
Dianthus barbatus.
Pyrus Malus.
torminalis.
Cercis siliquastrum!
Punica Granatum.
Robinia pseudacacia!
Rubia tinctorum.
Dipsacus fullonum!
pilosus.
Gmelini.
Scabiosa arvensis.
*Valeriana officinalis!
dioica!
Galium aparine!
* Mollugo!
verum!
Hippuris vulgaris!
Veronica spicata.
longifolia.
Hyssopus officinalis.
Thymus Serpyllum.
Lamium purpureum!
Dracocephalum speciosum.
M