the left, as
shown in Fig. 191. For an account of the way in which the character of
the phyllotaxy may be deduced from the secondary spirals, see Structural
Botany, Chapter IV.

[Illustration: Fig. 191. A young plant of the Houseleek, with the leaves
(not yet expanded) numbered, and exhibiting the 13-ranked arrangement;
and showing secondary spirals.]

193. =Phyllotaxy of Opposite and whorled Leaves.= This is simple and
comparatively uniform. The leaves of each pair or whorl are placed over
the intervals between those of the preceding, and therefore under the
intervals of the pair or whorl next above. The whorls or pairs alternate
or cross each other, usually at right angles, that is, they _decussate_.
Opposite leaves, that is, whorls of two leaves only, are far commoner
than whorls of three or four or more members. This arrangement in
successive decussating pairs gives an advantageous distribution on the
stem in four vertical ranks. Whorls of three give six vertical ranks,
and so on. Note that in descriptive botany leaves in whorls of two are
simply called _opposite_ leaves; and that the term _verticillate_ or
_whorled_, is employed only for cases of more than two, unless the
latter number is specified.

[Illustration: Fig. 192. Opposite leaves of Euonymus, or Spindle-tree,
showing the successive pairs crossing each other at right angles.]

194. =Vernation or Praefoliation=, the disposition of the leaf-blades in
the bud, comprises two things; 1st, the way in which each separate leaf
is folded, coiled, or packed up in the bud; and 2d, the arrangement of
the leaves in the bud with respect to one another. The latter of course
depends very much upon the phyllotaxy, i. e. the position and order of
the leaves upon the stem. The same terms are used for it as for the
arrangement of the leaves of the flower in the flower-bud. See,
therefore, "AEstivation, or Praefloration."

195. As to each leaf separately, it is sometimes _straight_ and open in
vernation, but more commonly it is either _bent_, _folded_, or _rolled
up_. When the upper part is bent down upon the lower, as the young blade
in the Tulip-tree is bent upon the leaf-stalk, it is said to be
_Inflexed_ or _Reclined_ in vernation. When folded by the midrib so that
the two halves are placed face to face, it is _Conduplicate_ (Fig. 193),
as in the Magnolia, the Cherry, and the Oak. When folded back and forth
like the plaits of a fan, it is _Plicate_ or _Plaited_ (Fig.