Fig. 192. Opposite leaves of Euonymus, or Spindle-tree, showing the successive pairs crossing each other at right angles.
[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.
[194.] Vernation or Præfoliation, 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, ["Æstivation, or Præfloration."]
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. [194]), as in the Maple and Currant. If rolled, it may be so either from the tip downwards, as in Ferns and the Sundew (Fig. [197]), when in unrolling it resembles the head of a crosier, and is said to be Circinate; or it may be rolled up parallel with the axis, either from one edge into a coil, when it is Convolute (Fig. [195]), as in the Apricot and Plum; or rolled from both edges towards the midrib,—sometimes inwards, when it is Involute (Fig. [198]), as in the Violet and Water-Lily; sometimes outwards, when it is Revolute (Fig. [196]), in the Rosemary and Azalea. The figures are diagrams, representing sections through the leaf, in the way they were represented by Linnæus.
Section VIII. FLOWERS.
196. Flowers are for the production of seed ([16]). Stems and branches, which for a time put forth leaves for vegetation, may at length put forth flowers for reproduction.