support; but as the penultimate internodes grew in length, they
pushed themselves up for a considerable space (ascertained by
coloured marks on the shoot and on the support) round the stick, and
the spire became more open. {13}

It follows from this latter fact that the position occupied by each
leaf with respect to the support depends on the growth of the
internodes after they have become spirally wound round it. I mention
this on account of an observation by Palm (p. 34), who states that
the opposite leaves of the Hop always stand in a row, exactly over
one another, on the same side of the supporting stick, whatever its
thickness may be. My sons visited a hop-field for me, and reported
that though they generally found the points of insertion of the
leaves standing over each other for a space of two or three feet in
height, yet this never occurred up the whole length of the pole; the
points of insertion forming, as might have been expected, an
irregular spire. Any irregularity in the pole entirely destroyed the
regularity of position of the leaves. From casual inspection, it
appeared to me that the opposite leaves of Thunbergia alata were
arranged in lines up the sticks round which they had twined;
accordingly, I raised a dozen plants, and gave them sticks of various
thicknesses, as well as string, to twine round; and in this case one
alone out of the dozen had its leaves arranged in a perpendicular
line: I conclude, therefore, Palm's statement is not quite accurate.

The leaves of different twining-plants are arranged on the stem
(before it has twined) alternately, or oppositely, or in a spire. In
the latter case the line of insertion of the leaves and the course of
the revolutions coincide. This fact has been well shown by
Dutrochet, {14} who found different individuals of Solanum dulcamara