FOOTNOTES:
[350] Moore, 'Nature Printed Ferns,' 8vo edition, vol. ii, p. 154, et p. 173.
[351] 'Flora (B. Z.),' 1821, vol. iv, p. 717, c. tab.
[352] Chavannes, 'Mon. Antirrh.'
[353] 'Bull. Soc. Bot. France,' t. vii, 1860, p. 877.
[354] Ibid., t. iv, 1857, p. 759.
[355] Jaeger, "De monstrosa folii Phœnicis dactyliferæ conformatione a Goetheo olim observata," 'Act. Acad. Leop. Car. Nat. Cur.,' vol. xvii, suppl., p. 293, c. tab. color. iv.
[356] See Goethe, 'Ueber die spiral Tendenz.'
[357] See Darwin "On Climbing Plants," 'Journ. Linn. Soc. Botany,' vol. ix, p. 5.
[358] 'Ephem. Nat. Cur.,' dec. 2, ann. 1, 1683, p. 68, fig. 14.
[359] 'Ann. des Scienc. Nat.,' third series, vol. i, 1844, p. 292.
[360] 'Flora' Feb. 4, 1858, p. 69, tab. ii, f. 3, and also 'Flora,' 1860, p. 737, tab. vii, f. 9.
[361] 'Bull. Acad, Belg.,' t. xvii, p. 196, "Lobelia," p. 53, c. tab.
[362] Moore, 'Nature-printed Ferns,' 8vo edition, vol. ii, p. 183.
[363] 'Bull. Soc. Bot. Fr.,' 1860, vol. vii, p. 461. See also Naudin, 'Ann. Sc. Nat.,' 4 ser., t. iv, p. 5. Clos, 'Bull. Soc. Bot. Fr.,' t. iii, p. 546.
[364] London's 'Magazine Nat. Hist.,' vol. ii, p. 463.
[365] C. Morren, 'Bull. Acad. Belg.,' 1852, t. xix, part iii, p. 444.
CHAPTER II.
POLYMORPHY.
Usually the several organs of the same individual plant do not differ to any great extent one from another. One adult leaf has nearly the same appearance and dimensions as another; one flower resembles very closely another flower of the same age and so on. Nevertheless it occasionally happens that there is a very considerable difference in form in the same organs, not only at different times, but it may also be at the same time. Descriptive botanists recognise this occurrence in the case of leaves, and apply the epithet heterophyllous to plants possessed of these variable foliar characters. In the case of the flower, where similar diversity of form occasionally exists, the term dimorphism is used.
As these phenomena appear constantly in particular plants, they are hardly to be looked on, under such circumstances, as abnormal, but where they occur in plants not usually polymorphic, they may be considered as coming within the scope of teratology.
Heterophylly.—As a general rule, the leaves or leaf-organs in each portion of a plant, from the rhizome or underground axis, where it exists, to the carpellary leaf, have their own special configuration, subject only to slight variations, dependent upon age, conditions of growth, &c. The cotyledons are very uniform in shape in each plant, and are scarcely ever subject to variation. The leaves near the base of the stem, the root-leaves as they are not unfrequently called, sometimes differ in form from the stem-leaves; these again differ from the bracts or leaves in proximity to the flower. The floral envelopes themselves, as well as the bud-scales, all have their own allotted form in particular plants, a form by which they may, in most cases, be readily recognised. Hence, then, in the majority of plants there is naturally very considerable difference in the form of the leaf-organs, according to the place they occupy and the functions they have to fulfil; but, in addition to this, it not unfrequently happens that the leaf-organs in the same portion of the stem are subject to great variation in form. This is the condition to which the term heterophylly properly applies. The variation in form is usually dependent on a greater or less degree of lobing of the margin of the leaf; thus, in the yellow jasmine, almost every intermediate stage may be traced from an ovate entire leaf to one very deeply and irregularly stalked. Broussonettia papyrifera, and Laurus Sassafras, and the species of Panax, may be mentioned as presenting this condition. Sometimes in the last-named genus, as also in Pteridophyllum, every gradation between simple and compound leaves may be traced. The horse-radish (Cochlearia Armoracia) may also be instanced as a common illustration of polymorphism in the leaves. In ferns it is likewise of frequent occurrence, markedly so in Scolopendrium D'Urvillei, in which plant every gradation from a simple oblong frond to an exceedingly divided one may be found springing from the same rhizome at the same time.
Fig. 177.—Syringa persica laciniata, showing polymorphous leaves.
A similar protean state, but little less remarkable, occurs in many of our British ferns, notably in Scolopendrium vulgare, of which Mr. Moore enumerates no fewer than 155 varieties,[366] many of the forms occurring on the same plant at the same time. Cultivators have availed themselves of this tendency to produce multiform foliage, not only for the purposes of decoration or curiosity, as in the many cut-leaved or crisped-leaved varieties, but also for more material uses, as, for instance, the many varieties of cabbages, of lettuces, &c. Most of these variations are mentioned under the head of the particular morphological change of which they are illustrations.
The effect of a change in the conditions of growth in producing diversity in the form of the leaf may be here alluded to. Ficus stipulata, a plant used to cover the walls of plant-stoves in this country, and growing naturally on walls in India, like ivy, produces leaves of very different form, size, and texture, when grown as a standard, from what it does when adhering to a wall. Marcgraavia umbellata furnishes another example of a similar nature, as indeed, to a less extent, does the common ivy.
Allusion has been already made to the occasional persistence of forms in adult life, which are commonly confined to a young state, as in the case of some conifers which present on the same plant, at the same time, two different forms of leaves. Mention has also been made of the presence of adventitious buds on leaves and in other situations. The leaves that spring from these buds are usually of the same form as the other leaves of the plant, but now and then they differ. Of this a remarkable illustration is afforded by a fern, Pteris quadriaurita, in which the fronds emerging from an adventitious bud are very different from the ordinary fronds.
Fig. 178.—Portion of a frond of Pteris quadriaurita, with an adventitious bud, the form of the constituent foliage of which is very different from that of the parent frond.
Dimorphism.—This term, applied specially to the varied form which the flowers or some of their constituent elements assume on the same plant, is an analogous phenomenon to what has been above spoken of as heterophylly, and, like it, it cannot, except under special circumstances, be considered as of teratological importance. A few illustrative cases, however, may here be cited.
Sir George Mackenzie describes a variety of the potato[367] (Solanum tuberosum), which produces first double and sterile flowers, and subsequently single fertile ones; the other portions of the plant do not differ much.
Stackhousia juncea, according to Clarke, has mixed with its perfect flowers a number of apetalous blossoms destitute of anthers.[368]
This peculiarity is well exemplified in the tribe Gaudichaudieæ of the order Malpighiaceæ. A. de Jussieu, in his monograph, speaks of these flowers as being very small, green, destitute of petals, or nearly so, with a single, generally imperfect anther; the carpels also are more or less imperfect, but not sufficiently so to prevent some seeds from being formed. A similar production of imperfect flowers has been noticed in many other orders, e.g. Violaceæ, Campanulaceæ, &c. In some cases these supplementary blossoms are more fertile and prolific in good seeds than are the normally constructed flowers. M. Durieu de Maisonneuve alludes to a case where flowers of this description are produced below the surface of the ground. The plant in question is Scrophularia arguta, and it appears that towards the end of the summer the lowest branches springing from the stem bend downwards, and penetrate the soil; the branches immediately above the lowest ones also bend downwards, but do not always enter the earth. These branches bear fertile flowers: those which are completely below the soil are completely destitute of petals; those which are on the surface have a four-lobed corolla whose divisions are nearly equal, like those of Veronica.[369]
To Sprengel, and specially to Darwin, physiologists are indebted for the demonstration of the relation of di- and trimorphic flowers to fertilisation. In certain genera of orchids, such as Catasetum, &c., flowers of such different form are produced that botanists, without hesitation, considered them as belonging to different genera, until the fact of their occasional production on the same plant showed that they were not of even specific importance. It was reserved for Mr. Darwin to show experimentally that these very different flowers are really sexual forms of one and the same species, ordinarily occurring on different plants, i.e. diœcious, but occasionally formed on the same spike. The same excellent observer has demonstrated that the di- and trimorphic forms of Primula, of Linum, Lythrum, and other plants—forms differing mainly in the relative length of the stamens and styles, are also connected with striking differences in the number of perfect seeds produced. The most perfect degree of fertility is obtained when the stigma of one form is fertilised by the pollen taken from stamens of a corresponding height. On the other hand, when the union is, as Mr. Darwin states, illegitimate, that is, when the pollen is taken from stamens not corresponding in length to the style, more or less complete sterility ensues in the progeny, sometimes even utter infertility, such as happens when two distinct species are crossed, so that, in point of fact, the offspring of these illegitimate unions correspond almost precisely to hybrids.[370]
Mere variations of form arising from hybridisation or other causes hardly fall within the limits of this work, though it is quite impossible to say where variations end and malformations begin. There are, however, two or three cases cited by Mr. Darwin[371] from Gallesio and Risso to which it is desirable to allude. Gallesio impregnated an orange with pollen from a lemon, and the fruit borne on the mother tree had a raised stripe of peel like that of a lemon both in colour and taste, but the pulp was like that of an orange, and included only imperfect seeds. Risso describes a variety of the common orange which produces "rounded-oval leaves, spotted with yellow, borne on petioles, with heart-shaped wings; when these leaves fall off they are succeeded by longer and narrower leaves, with undulated margins, of a pale green colour, embroidered with yellow, borne on foot-stalks without wings. The fruit whilst young is pear-shaped, yellow, longitudinally striated and sweet; but, as it ripens, it becomes spherical, of a reddish-yellow, and bitter."
Sports or bud variations.—These curious departures from the normal form can only be mentioned incidentally in this place, as they pertain more to variation than to malformation.
The occasional production of shoots bearing leaves, flowers, or fruits of a different character from those found on the normal plant, is a fact of which gardeners have largely availed themselves in the cultivation of new varieties. The productions in question have been attributed to various causes, such as cross-breeding, grafting, budding, dissociation of hybrid characters, or reversion to some ancestral form, all of which explanations may be true in certain cases, but none of them supply the clue to the reason why one particular branch should be so affected, and the rest not; or why the same plant, at the same time, as often happens in Pelargoniums, should produce two, three, or more "sports" of a different character.
These bud variations may be perpetuated by grafts or by cuttings, sometimes even by seed. With reference to cuttings a curious circumstance has been observed, viz., that if taken from the lower part of the stem, near the root, the peculiarity is not transmitted, but the young plant reverts to the characters of the typical form (Carrière). This circumstance, however, is not of universal occurrence.
For further particulars on this interesting subject the reader is referred to Darwin's 'Variation of Animals and Plants,' i, p. 373, where numerous references are given, and wherein certain well-known and highly remarkable instances, such as the Cytisus Adami, the trifacial orange, &c., are discussed.