CHAPTER II.
MULTIPLICATION OF FOLIAR ORGANS.
The cases referrible to this head may be ranged under two sections according as the increase is due to plurality of ordinarily single organs, or to an increase in the number of verticils or whorls.
When, in place of a single leaf organ two or more are really or in appearance present the occurrence may be due to one of several causes; among them may be mentioned an actual formation of parts in unwonted number, hypertrophy or enation, chorisis or fission, disjunction, adhesion of one leaf to another or to the stem, as in some of the leaves called "geminate," wherein the two leaves, though apparently in juxtaposition, yet originate from different parts of the stem, but by coalescence or lack of separation produce the impression as if they sprang from the same node. In the adult state it is not always possible to ascertain with certainty to which of these causes the increase in the number of leaves is due, though a clue to the real state of things may be gained from attention to the distribution of the veins, to the arrangement or phyllotaxy of the leaves, the size and position of the supernumerary organs, &c.
The term "phyllomania," as ordinarily used, is applied to an unwonted development of leafy tissue, as in some begonias where the scales or ramenta are replaced by small leaflets, or as in some cabbage leaves, from the surface of which project, at right angles to the primary plane, other secondary leafy plates; but these are, strictly speaking, cases of hypertrophy (see Hypertrophy).
Those instances in which the actual number of leaves is increased, so that in place of one there are more leaflets, may be included under the term "pleiophylly," which may serve to designate both the appearance of two or more leaves in the place usually occupied by a single one, and also those normally compound leaves in which the number of leaflets is greater than usual.
The increased number of leaves in a whorl may well be designated as "polyphylly," using the word in the same sense as in ordinary descriptive botany, while "pleiotaxy" may be applied to those cases in which the number of whorls is increased.
Fig. 183.—Supernumerary leaflet, Ulmus campestris.
Pleiophylly.—As above stated, this term is proposed to designate those cases in which there is an absolute increase in the number of leaves starting from one particular point, as well as those in which the number of leaflets in a compound leaf is preternaturally increased. The simplest cases are such as are figured in the adjacent cuts, wherein, in place of a single leaf, two are produced in the elm. In the one case the new leaflet springs from the apex of the petiole and partially fills the space consequent on the obliquity of the base of the leaf. In the other it would seem as if two distinct leaves emerged from the stem in juxtaposition. This is probably due to a lateral chorisis or subdivision of the primitive tubercle or growing point, followed by a like subdivision of the vascular bundle supplying it. There are certain varieties of elm that very generally present this anomaly on their rank, coarse, growing shoots. In these cases the new growths have the same direction as the primary one, but in other cases the supplementary production is exactly reversed in direction. Thus, in the common hazel (Corylus) a second smaller leaf proceeding from the end of the leaf-stalk at the base of the primary one may frequently be seen. M. Germain de Saint Pierre records an instance in a mulberry leaf, from the base of which proceeded a large leafy expansion divided into two tubular, horn-like projections, and in the centre a thread-like process representing the midrib and terminated by a small two-lipped limb.[392] Dr. Ferdinand Müller speaks of a leaf of Pomaderris elliptica as bearing a secondary leaf on its under surface.[393]