A reference to the subjoined list of genera affected by this malformation, and the knowledge of its comparatively greater frequency in some than in others of them, will show that it is more often met with in plants having an indefinite form of inflorescence than in those having a definite one. The change may affect some only, or the whole of the flowers constituting an inflorescence; and though it is by no means a constant occurrence, it very frequently happens that the central or terminal flower in a definite inflorescence is alone affected, the others remaining in their ordinary condition, as in pinks (Dianthus); and in the indefinite forms of inflorescence, it is equally common that the uppermost flower or flowers are the most liable to be thus affected.
In those plants which present this deviation from the ordinary condition with the greatest frequency, it often happens that the axis is normally more or less prolonged, either between the various whorls of the flower, as in the case of the gynophore, &c., or into the cavity of the carpels, as in the instances of free central placentation. To bear out this assertion, the following instances taken from those genera having definite inflorescence, and which are very commonly affected with prolification, may be cited; thus, in Anemone and Ranunculus the thalamus is prolonged to bear the numerous carpels; in Dianthus there is a marked internode separating the carpels from the other parts of the flower; in Primulaceæ central prolification is very common, and this is one of the orders where the placenta seems from the researches of Duchartre and others, to be truly a production of the axis within the carpels;[123] in Thesium also, another genus with free central placenta, this malformation has been found.
So also among plants with indefinite inflorescence, prolification seems very frequently to affect those wherein the axis is normally prolonged; thus it is common in Dictamnus, which plant has an internode supporting the pistil; it is frequent among Umbelliferæ, where the carpophore may be truly considered an axile production; it is common among Rosaceæ and Ranunculaceæ, in many of which the axis or thalamus is well-marked, and it is by no means infrequent in the flowers of the Orange, where the floral internodes are also slightly elongated; on the other hand, there is no case on record in Magnoliaceæ, and some other orders where the floral part of the axis is at some point or other elongated; still, on the whole, there can be but little doubt that there is a real relation between prolification and the normal extension of the floral internodes.
Under these circumstances, those instances wherein the parts of the flower become separated one from the other by the elongation of the internodes (apostatis), constitute a lesser degree of the same change, which operates most completely in the formation of a new bud at the extremity of the prolonged axis. Some specimens of Geum rivale (a plant very liable to become prolified) in my possession show this very clearly. In the wild plant the thalamus is elevated on a short stalk; in the abnormal ones the thalamus is simply upon a longer stalk than usual, or in a more advanced stage of the deviation the lengthened thalamus takes the form of a branch provided with leaves and terminated by a flower; it is noticeable, also, in these specimens, that the sepals of the lower flower have assumed entirely the dimensions and appearance of leaves.
Median prolification has occasionally been recorded in flowers that have, in their ordinary condition, but one carpel, as in Leguminosæ and in Santalaceæ. In Leguminosæ, as also in Amygdalus, it would seem as if the adventitious bud were strictly a lateral and axillary production, and moreover that the carpel itself is not strictly terminal but lateral in position, though apparently terminal from the abortion of other carpels. In the only recorded instance that I am aware of, of this malformation affecting the genus Thesium, the pistil was altogether absent, and occupying its place was the new bud or branch.[124]
Fig. 61.—Daucus Carota, showing leafly carpels, prolification, &c.
As the carpels are not unfrequently absent in cases of median prolification, it has been thought that the pistil in such cases was metamorphosed into a stem bearing leaves or flowers. Setting aside the physiological difficulties in the way of accepting such an opinion, an examination of any number of cases is sufficient to refute it; for, as Moquin well remarks, the carpels may frequently be found either in an unaltered condition or more or less modified.