Fig. 184.—Supernumerary leaf, Ulmus montana.
Fig. 185.—Supernumerary leaf of hazel.
The leaves of Heterocentron macrodon have likewise been observed occasionally to produce leaflets from their upper surface.
To this production of leaves from leaves the late Professor Morren applied the term "autophyllogeny."[394] The Belgian botanist figures a small perfect leaf springing from the nerves of the upper surface of the primary leaf in a species of Miconia. As in the hazel, the direction of the adventitious leaf is inversely that of the primary one, the upper surface of the supernumerary leaflet being turned towards the corresponding surface of the normal leaf. A similar occurrence took place in Gesnera zebrina, but the new growth in this case sprang from the lower face of the leaf. Morren explains the appearances in question by supposing that the supplementary leaf is one of a pair belonging to a bud borne on a slender stalk. This stalk and one of the bud-leaves are supposed to be inseparably united with the primary leaf. But there is no reason at all for supposing the existence of adhesion in these cases; no trace of any such union is to be seen. A much more natural explanation is that, from some cause or another, development at the apex of the petiole or on the surface of the nerves, instead of taking place in one plane only, as usual, takes place in more than one, thus showing the close relationship, if not the intrinsic identity, between the leaf-stalk and its continuation, the midrib, with the branch and its subdivisions. The form of the leaf-stalk and the arrangement of the vascular bundles in a circle in the case of the hazel, before alluded to, bear out this notion. Such cases are significant in reference to the notion propounded by M. Casimir de Candolle, that the leaf is the equivalent of a branch in which the upper portion of the vascular circle is abortive.[395]
Compound leaves, as has been stated, occasionally produce an extra number of leaflets; one of the most familiar illustrations of this is in the case of the four-leaved shamrock (Trifolium repens), which was gathered at night-time during the full moon by sorceresses, who mixed it with vervain and other ingredients, while young girls in search of a token of perfect happiness made quest of the plant by day. Linné, who in this matter, at any rate, had less than his usual feeling for romance, says of the four-leaved trefoil that it differs no more from the ordinary trefoil than a man with six fingers differs from one provided with the ordinary number. It should be stated that five and six adventitious leaflets are found almost as frequently as four.
Walpers describes a case where the leaf of T. repens bore seven leaflets. Schlechtendal alludes to a similar increase in number in Cytisus Laburnum, and many other instances might be cited.