Fig 767.—Leaf of Nepenthes.
Fig. 766.—Sarracenia.
“Efforts have been made to determine the laws to which these various modes of leaf-arrangement may be referable. The result is found in the doctrine of ‘Phyllotaxy,’ as it is called, the fundamental principle of the whole being that Nature, in the disposition of the leaves upon the stem, works upon precisely the same idea as that which is set forth so distinctly and elegantly in the common pine-cone; and, on a minor scale, in the beautiful cone of the female hop; not to mention the quasi-cones of many species of tropical palm, such as the Sagus and the Mauritia; nor to mention either, the very delicate repetition of the whole series in the florets of the Rudbeckia and the ripening fruits of Chaucer’s daisy. In every one of the flower and fruit arrangements mentioned, the idea is the spiral,—the same sweet old fashion which we have had in the twining stems of the convolvulus, the woodbine, and the scarlet bean; which comes out again in many a sea-shell, and in human ringlets; and this idea, according to ‘Phyllotaxy,’ governs the position of the leaves. Following alternate leaves up the stem, their sequence is clearly spiral. Through the non-development of internodes, they are brought closer and closer together; and even when the entire mass of foliage is concentrated and condensed into the rosulate form, as in the houseleek and the Echeverias, the spiral prototype is still distinguishable. The whole matter has been reduced to one of arithmetical exactitude; and for those who love calculations and “fractions,” the determination of the spirals, their continuity and intermixture, supplies abundance of curious entertainment. All three modes of leaf-arrangement are found in certain herbaceous plants, none disclosing this particular kind of playfulness more plainly than the common pyramidal Loosestrife, Lysimachia vulgaris, and the purple Lythrum of the waterside, in each of which very handsome wild flowers, alternate, opposite, and whorled leaves may be found in near neighbourhood. Alternate and opposite leaves are also met with, side by side, in various species of Myrtaceæ; and imperfectly, upon young shoots of the common ash-tree. The rule is, nevertheless, that there shall be uniformity, and in many of the largest natural orders the rule is never broken. In the Rosaceæ the stem leaves are invariably alternate; in the Gentianaceæ they are invariably opposite.”
Fig. 768.—Branch of horse chestnut.
