In the herbarium there are three species of the genus Cleome. Two of these, C. pentaphylla and arabica, are in many respects well known plants; the third I believe to be an undescribed species, but nearly related to monophylla.
If the very natural group, formed by the Linnæan genus Cleome, is not to be preserved entire, its subdivision must be carried much farther, and established on other grounds, than has been done by M. de Candolle, whose genera and sections appear to me to have been equally founded on partial considerations. Thus, his Polanisia, uniting all the Cleomes whose stamina exceed six, contains in its first section, in addition to the species from which the genus was formed, at least two sets of plants, having very little affinity either with each other, or with the original species, whose only congener is placed in a second section.
Gynandropsis also consists of two groups not very intimately connected: the first is composed of species belonging to South America, and having the usual æstivation of the family: the second, of which C. pentaphylla may be taken as the type, is chiefly African, and is readily distinguished by its very different æstivation,—the great peculiarity of which consists in the petals not covering the stamina at any period. To this mode of æstivation of petals, which has never before been noticed, though it equally exists in Crateva and in Resedaceæ, I shall apply the term aperta. It is constantly conjoined, and, perhaps, necessarily connected with the early opening of the calyx, whose segments are originally connivent and slightly imbricate: for it may here be remarked, that in all the modifications of what I have termed imbricate æstivation of petals, they are, I believe, in the very early stage in like manner erect, and the sexual organs equally exposed.
If the expediency of preserving the genus Cleome entire were admitted, a question which I do not pretend at present to decide, it would still be of the greatest importance to arrange its numerous species according to their affinities, and carefully to distinguish the subordinate groups that compose it. To such inferior groups, whether termed subgenera or sections, names, in fact, have been of late years very generally assigned, both by zoologists and botanists.
It has not yet been proposed, however, that these subgeneric names should form an essential part of the name of the species; although, by employing them in this manner, while the principal groups would be kept in view, their subdivision would be carried to the same extent, and the subordinate groups as well expressed as if they had been actually separated into distinct genera.
The adoption of this method, which would not materially disturb names already existing, would probably lead to a greater consistency in the formation of genera, with reference to the natural orders of which they are subdivisions. In this way also the co-operation of two classes of naturalists, at present opposed to each other on the question of the construction of genera, might to a certain extent be expected, and greater uniformity in nomenclature consequently secured.
These advantages appear to me so important, that some expedient for obtaining them will, I am persuaded, at no distant period, be generally adopted.
In favour of the present plan it may be remarked, that it is analogous to the method followed by the Romans in the construction of the names of persons, by which not only the original family, but the particular branch of that family to which the individual belonged was expressed. Thus the generic name corresponds with the nomen (Cornelius), the name of the section with the cognomen (Scipio), and that of the species with the prænomen (Publius).
Without attempting at present to obviate the objections to which the proposed innovation is no doubt liable, I shall proceed to apply it to Cleome pentaphylla. According to my view the genus Cleome would include Gynandropsis, a name which, as that of a section, may be continued to those species of M. de Candolle’s genus belonging to equinoctial America, and having the common æstivation of the family: while Gymnogonia, derived from its remarkable æstivation, may be employed for the section that includes C. pentaphylla, of which the name might be given in the following manner:—
Cleome (Gymnogonia) pentaphylla. This plant, the earliest known species of Cleome, and that on which the genus was chiefly constituted, was found in Bornou. The species is regarded by M. de Candolle as a native of the West India islands, and he doubts whether it may not also belong to Egypt and India. On the other hand I consider it a native of Africa and India, and am not satisfied with the evidence of its being also indigenous to the American islands, where, though now very common, it has probably been introduced by the negroes, who use it both as a potherb and in medicine. It is not unlikely that M. de Candolle, in forming his opinion of the original country of this plant, has been in part determined by finding several species of his Gynandropsis decidedly and exclusively natives of the new continent. But if I am correct in separating these species from the section to which Cleome (Gymnogonia) pentaphylla belongs, this argument, which I have formerly applied to analogous cases[96], would be clearly in favour of the opinion I have here advanced; those species of the section with which I am acquainted being undoubtedly natives of Africa or of India.