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."