[543] See Moore, 'Nature-Printed Ferns,' 8vo, for numerous illustrations both of depauperate and exindusiate ferns. Scolopendrium vulgare seems to be one of the ferns most commonly affected in this way. Moore, loc. cit., vol. ii, pp. 135, 147, 159, 165, &c.
[544] 'Bull. Acad. Belg.,' t. xvii, p. 38, t. 1; Lobelia, p. 85.
[545] Cited in 'Henfrey's Botanical Gazette,' i, p. 179.
[546] 'Origin of Species,' p. 450.
CHAPTER II.
DEGENERATION.
While the terms atrophy and abortion apply in the main to a mere diminution of size, as contrasted with the ordinary standard, degeneration may be understood to apply to those cases in which not only is the absolute bulk diminished, but the whole form is altered and depauperated. Degeneration, thus, is the result not so much of a deficiency in growth as of a perversion of development.
Under natural, i.e. habitual circumstances, the formation of pappus in place of a leafy calyx may be considered as an illustration of degeneration. It is evident, however, that no very decided line of demarcation can be drawn between cases of perversion and of arrest of development.
Formation of scales.—These may be mere epidermal excrescences, or they may be the abortive rudiments of leaves. Of this latter nature are the "cataphyllary" leaves which invest the root stocks of so many perennial plants, the perulæ of leaf-buds, or the paleæ on the common receptacle of composite flowers. Other illustrations of a like character are to be met with in the membranous scales that represent leaves in Ruscus, Asparagus, Pinus, &c. Similar productions are met with within the flower, where they may occur as the representatives of sepals, petals, stamens, or pistils, or as mere excrescences. (See Enation.) Whole families of plants, e.g. Sapindaceæ, are characterised by the presence of these organs, which are often of great interest to the morphologist as indicating the true symmetry of the flower, while they have acquired fresh importance since the publication of Mr. Darwin's work on the 'Origin of Species,' wherein we are taught to regard these rudiments as, in many cases, vestiges of organs that were more completely developed in the progenitors of the present race of plants, and the exercise of whose functions, from some cause or other, having been rendered impossible, the structures become, in process of time, proportionately stunted.
Thus, in diœcious plants we frequently find traces of stamens in the female flowers, and rudiments of the pistil in the male flower, indicating, according to the Darwinian hypothesis, that the ancestors of these plants were hermaphrodite (see Heterogamy).
Mr. Darwin has also shown that, in some cases, the utmost degree of fertility is attained, not from the action of the pollen on the stigma of the same flower, but on the influence of the male element of one blossom upon the female organs of another flower on another individual plant.