See Schauer's translation of Moquin-Tandon, 'El. Terat. Veget.,' p. 245, adnot., and 'Al. Braun Polyembryonie.'
Increased number of the cotyledons.—Although the presence of one or of two cotyledons in the embryo is generally accepted as a valuable means of separating flowering plants into two primary groups, yet, like all other means of discrimination, it occasionally fails, and, indeed, almost always requires to be taken in conjunction with some other character. There are cases among flowering plants where the embryo is homogeneous in its structure, there are others in which the number of the cotyledons is more than two. Thus, in some seeds of Cola acuminata the cotyledons vary in number from two to five. I have not been able to ascertain precisely whether this multiplication of the cotyledons is characteristic of all the seeds of particular trees, or whether some only are thus affected. Some fruits that I examined bore out the latter view, as in the same pod were seeds with two, three, and four cotyledons respectively.
I have also seen three cotyledons present in embryo-plants of Correa, Cratægus Oxyacantha, Dianthus sinensis, Daucus Carota, Cerasus Lauro-cerasus. De Candolle alludes to a case of the kind in the bean, and figures a species of Solanum with three cotyledons.[420] Jaeger alludes to a similar instance in Apium Petroselinum;[421] Ehrenberg to one in the marigold (Calendula);[422] Reinsch to an analogous appearance in the beech (Fagus), associated with a union of the margins of two out of the three cotyledons, and of those of two out of the three leaves next adjacent.[423] This fusion seems frequently to accompany increase in the number of cotyledons. It was so in the Correa, and in the Cratægus previously mentioned. Some of these cases may be accounted for by chorisis or by a cleavage of the original cotyledons, as happens, according to Duchartre,[424] in some Coniferæ, which he considers to be improperly termed polycotyledonous. Whether this holds good in the Loranths, where (Nuytsia, Psittacanthus) an appearance of polycotyledony exists, is not stated. In the case of the rue (Ruta) figured by M. A. de Jussieu[425] this splitting of one cotyledon into two is sufficiently evident, as is also the case in the sycamore (Acer pseudo-platanus), seedlings of which may often be met with divided cotyledons.
In other instances a fusion of two embryo plants may give rise to a similar appearance, as in the Euphorbia and Sinapis found by M. Alph. de Candolle ([see ante, p. 56]).
Pleiotaxy or multiplication of whorls.—In the preceding section notice has been taken of the increased number of parts in a single whorl, but an augmentation of the number of distinct whorls is still more frequently met with. Many of the so-called double flowers owe their peculiarity to this condition. The distinction between the two modes in which the parts of the flower are increased in number has been pointed out by Engelmann, Moquin, and others, and the two seem to require distinctive epithets; hence the application of the terms polyphylly and pleiotaxy, as here proposed.
Pleiotaxy in the bracts.—An increase in the number of bracts has been met with very constantly in a species of Mæsa, and in a peculiar variety of carnation, called the wheat-ear carnation.[426] In some of these cases the increase in the number of bracts is attended by a corresponding suppression in the other parts of the flower. Such a condition has been frequently met with in Gentiana Amarella, where the bracts are increased in number, coloured purple, and destitute of any true floral organs. A similar condition exists in some varieties of Plantago major (var. paniculata), as has been previously stated, p. 109.
Fig. 187.—Wheat-ear carnation. The appearance is due to the multiplication of the bracts and the suppression of the other parts of the flower.