FOOTNOTES:
[522] 'El. Ter. Veg.,' p. 132.
[523] Spinosæ arbores cultura sæpius deponunt spinas in hortis, 'Linn. Phil. Bot.,' § 272.
[524] Mr. Selby, in his 'History of British Forest Trees,' p. 465, gives the following account of the formation of this peculiar growth:—"In the autumn the parent aphis deposits her eggs at the base of the embryo leaves, within the bud destined to produce the shoots of the following year. When these begin to burst and expand in spring, the leaves, at whose bases the eggs have been deposited, instead of increasing in length, enlarge at the base, and form a cell or cyst whose mouth is at first closed by a red velvety-looking substance. If opened in this state a nest of small greenish aphides is distinctly visible, and at a certain period, or when they have acquired maturity, which is towards the end of the summer, the mouth of the cell opens and the insects fly off to inflict a similar injury upon the nascent buds of the year. In some instances the leaves of only a portion of the circumference of a shoot are affected, in which case, though a slight distortion may take place, the branch is not prevented from elongating; but in others, where the whole of the leaves around the shoot are converted into nidi, elongation is prevented and distortion to a great extent takes place."
[525] See Cramer, 'Bildungsabweich.,' pp. 53, 64, for further references.
[526] 'El. Ter. Veg.,' p. 124.
[527] Schlechtendal, 'Bot. Zeit.,' 1857, vol. xv, p. 67.
[528] On the subject of this paragraph the reader may consult A. Braun, "Ueber abnorme Blattbildung," &c., in 'Verhandl.,' d. 35, 'Naturforscherversammlung;' Jaeger, 'Flora,' 1850. p. 481, tab. 4, Digitalis.
[529] 'Org. Veget.,' i. p. 286.
[530] 'Bull. Soc. Bot. France, vol. viii, 1861, p. 710.
[531] 'Linnæa,' 1830, vol. v, p. 492.
[532] 'Mus. Senkenb.,' ii. p. 45.
[533] 'Bull. Acad. Roy. Belg.,' 1851, t. xviii. part i, p. 275.
[534] 'Bull. Soc. Bot. France,' vol. viii, 1861, p. 147.
[535] See Darwin, 'Variation of Domest. Anim. and Plants,' ii, 165.
[536] Gay, 'Ann. Sc. Nat.,' ser. i, 1824, t. iii, p. 44.
[537] See De Candolle, 'Mem. Legum.,' tab. 3, f. 1; Wyville Thomson, 'Trans. Bot. Soc. Edinb.,' 1851, July 10th; Berkeley, 'Gardeners' Chronicle,' June 22nd, 1867, p. 654. A similar case is described by Dr. Robb, in Sir W. Hooker's 'Journal of Botany,' 1841, vol. iii, p. 99, with illustrative figures. The specimens there described were produced at New Brunswick, where plum trees flower very freely, but seldom produce ripe fruit. Dr. Robb's account is as follows:—"In the summer of 1839 I had an opportunity of watching the process of destruction among the plums, and it was as follows—Before or soon after the segments of the corolla had fallen off, the ovarium had become greenish yellow, soft, and flabby. As the fruit continued to increase in magnitude, its colour grew darker and of a more ruddy yellow, and at the end of a fortnight or three weeks the size of the abortive fruit rather exceeded that of a ripe walnut. In fact, an observer might imagine himself to be walking amongst trees laden with ripe apricots, but, like the fabled fruit on the banks of the Dead Sea, these plums, though tempting to the eye, when examined, were found to be hollow, containing air, and consisting only of a distended skin, insipid, and tasteless. By-and-bye a greenish mould is developed on the surface of the blighted fruit; then the surface becomes black and shrivelled, and at the expiration of a month from the time of flowering the whole are rotten and decomposed. The flower appears about the beginning of June, and before August there is hardly a plum to be seen. It is curious that where two flower-stalks arise from one point of the branch, one will often go on to ripen in the normal way, while the other will become abortive, as above described."
In a specimen described by Mr. Berkeley there were two distinct ovules of equal size close to the apex of the fruit, connected with the base by vessels running down the walls. It should be observed that there is a worthless variety of plum, Kirke's stoneless, or Sans Noyau, in which the kernel is not surrounded by any bony deposit.
[538] 'Bull. Soc. Bot. Fr.,' 1862, vol. ix, pp. 37 et 291.
[539] Carl Schimp, 'Fl. Friburg,' vii, p. 745; Hook, fil., 'Journ. Linn. Soc.,' vi, p. 9.
[540] 'Linnæa,' vol. v, 1830, p. 493.
[541] Moquin-Tandon, 'El. Ter. Veg.,' p. 325.
[542] Alph. De Candolle states that the position of the abortive ovules affords a good character for discriminating between certain species of Quercus, 'Bibl. Univ. Genev.,' 1862, t. xv, p. 929.
[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.
Hence, in such plants there is a tendency to a separation of the sexes, while, from what has been before stated, it might be expected that rudiments of the male or female organs would be found, and also as a result of the operation of the law of inheritance. On the same principles it is easy to understand the occasional presence of the perfect in place of the rudimentary organs, as in Dianthus.
In some instances the assumption of a scale-like form by any organ is attended by a change in texture, the organs becoming dry and scarious, or fleshy. Moquin cites in illustration of the first phenomenon the flower of a Vicia, in which the petals were thick and fleshy, like the scales of a bulb; and of the second the leaves of a Chrysanthemum, which were replaced by small, glossy scales, like those which invest ordinary leaf-buds. Sometimes the entire flower is replaced by accumulations of small, acute, green scales. Cases of this kind, wherein the flowers of a pea and of the foxglove were replaced by collections of small ovate green scales packed one over the other till they resembled the strobile of a hop, have been already alluded to. Most of these scales are represented as having had other accumulations of scales in their axils.
Similar collections of scales may frequently be met with in the birch and in the oak, and probably represent abortive leaf-buds. Other cases of a like kind in Gentiana Amarella, where the scales are coloured, are mentioned elsewhere.
In some kinds of Campanula a similar change is not uncommon.
Formation of hairs, spines, &c.—The adventitious production of hairs is likewise frequently due to an arrested growth, in some cases arising from pressure impeding the proper development of the organ. In other cases the formation of hair seems to accompany the diminished development of some organ, as on the barren pedicels of the wig plant, Rhus Cotinus. A similar production of hair may be noticed in many cases where the development of a branch or of a flower is arrested, and this occurs with especial frequency where the arrest in growth is due to the puncture of an insect, or to the formation of a gall. In such cases the hairs are mere excrescences from the epidermis.
Prickles differ but little from hairs save in their more woody texture, but true spines or thorns are modifications either of a leaf or of a branch. Their presence seems often dependent on the soil in which the plants grow, or on other external circumstances.
They occur normally in the sepals of Paronychia serpyllifolia and other plants.
Formation of glands.—Under this name are associated a number of (generally) rudimentary organs very different in their morphological nature and significance, and also in their functions. Some are truly glandular or secreting organs, while others have no visible office. Anything like a complete account of these structures would be out of place, and reference is only made to them here on account of the occasional existence of intermediate forms, which throw light on the morphological significance of these structures. Thus, in Passiflora and Viburnum Opulus, the so-called glands on the sides of the petiole appear to represent leaflets, and are not unfrequently developed as such.
M. Dunal observed a flower of Cistus vaginatus in which some of the stamens were replaced by an hypogynous disc.[547] Moquin has seen similar instances in the flowers of a Rose, Hypericum, and Poppy.
M. Planchon[548] gives an account of some very curious malformations in Drosera intermedia, which go to show that the ovules are homologous with the glandular hairs on the margins of the leaves of these plants, an opinion corroborated by the researches of MM. Grönland and Trécul.[549]
Dr. Hooker shows that the pitcher of Nepenthes is due to a modification of a gland placed at the extremity of the midrib.[550]
Formation of tendrils.—These are of very varied morphological import; sometimes they are degenerated peduncles, as in passion-flowers, or vines; at other times they are of foliar origin; or, again, they may proceed from the segments of the perianth, as in Hodgsonia and some other cucurbitaceous plants. From their very different origin in different plants it is necessary to study the development in each case, and not apply to the generality what may be peculiar to one. In any case this formation in question generally belongs more to general morphology than to teratology.[551]
Kirschleger, however, has recorded the existence of a cirrhose sepal in Cucurbita Pepo.[552]