It exceeds in extent the herbarium formed by Mr. Ritchie near Tripoli, and on the Gharian hills, which, however, though containing only fifty-nine species, includes twenty-seven not in Dr. Oudney’s herbarium.
The specimens in Mr. Ritchie’s collection are carefully preserved, the particular places of growth in most cases given, and observations added on the structure of a few; sufficient at least to prove, that much information on the vegetation of the countries he visited might have been expected from that ill-fated traveller.
In these two collections united, hardly more than five species are contained not already published in the works that have appeared on the botany of North Africa; particularly in the Flora Atlantica of M. Desfontaines, in the Flore d’Egypte of M. Delile, and in the Floræ Libycæ Specimen of Professor Viviani, formed from the herbarium of the traveller Della Cella.
The plants collected in the Great Desert and its oases, between Tripoli and the northern confines of Bornou, and which somewhat exceed a hundred, are, with about eight or ten exceptions, also to be found in the works now mentioned. And among those of Bornou and Soudan, which fall short of one hundred, very few species occur not already known as natives of other parts of Equinoctial Africa.
A complete catalogue of the herbarium, such as I have now described it, even if the number and condition of the specimens admitted of its being satisfactorily given, would be of but little importance, with reference to the geography of plants. Catalogues of such collections, if drawn up hastily, and from imperfect materials, as must here have been the case, are indeed calculated rather to injure than advance this department of the science, which is still in its infancy, and whose progress entirely depends on the scrupulous accuracy of its statements. To produce confidence in these statements, and in the deductions founded on them, it should in every case distinctly appear, that in establishing the identity of the species enumerated, due attention has been paid to the original authorities on which they depend, and, wherever it is possible, a comparison actually made with authentic specimens.
In the account which I am now to give of the present collection, I shall confine myself to a slight notice of the remarkable known plants it contains, to characters or short descriptions of the more interesting new species, and to some observations on such of the plants as, though already published, have either been referred to genera to which they appear to me not to belong, or whose characters require essential alteration.
In proceeding on this plan, I shall adopt the order followed in the botanical appendix to Captain Tuckey’s Expedition to the River Congo. And as there will seldom be room for remarks on the geographical distribution of the species I have to notice, I shall chiefly endeavour to make my observations respecting them of some interest to systematic botanists.
Cruciferæ. Fifteen species belonging to this family exist in the collection, one of which only appears to be undescribed, and of this the specimens are so imperfect, that its genus cannot with certainty be determined. Of those already published, however, the generic characters of several require material alterations, some of which suggest observations relative to the structure and arrangement of the natural order.
Savignya Ægyptiaca, (De Cand. Syst. 2. p. 283,) is the first of these. It was observed near Bonjem, by Dr. Oudney, whose specimens slightly differ from those which I have received from M. Delile, by whom this plant was discovered near the pyramid of Saqqârah, and who has well figured and described it in his Flore d’Egypte, under the name of Lunaria parviflora. By this name it is also published by M. Desvaux. Professor Viviani, in giving an account of his Lunaria libyca, a plant which I shall presently have occasion to notice more particularly, has remarked[87], that Savignya of De Candolle possesses no characters sufficient to distinguish it as a genus from Lunaria; and still more recently, Professor Sprengel has referred our plant to Farsetia[88]. The genus Savignya, however, will no doubt be ultimately established, though not on the grounds on which it was originally constituted; for the umbilical cords certainly adhere to the partition, the silicule, which is never absolutely sessile, is distinctly pedicellated in Dr. Oudney’s specimens, the valves are not flat, and the cotyledons are decidedly conduplicate. In describing the cotyledons of his plant as accumbent, M. De Candolle has probably relied on the external characters of the seed, chiefly on its great compression, its broad margin or wing, and on the whole of the radicle being visible through the integuments. It would appear, therefore, that the true character of the cotyledons of Savignya has been overlooked, chiefly from its existing in the greatest possible degree. To include this degree of folding, in which the margins are closely approximated, and the radicle consequently entirely exposed, a definition of conduplicate cotyledons somewhat different from that proposed in the “Systema Naturale” becomes necessary. I may here also observe, that the terms Pleurorhizæ and Notorhizæ, employed by M. De Candolle, to express the two principal modifications of cotyledons in Cruciferæ, appear to me so far objectionable, as they may seem to imply that in the embryo of this family, the position of the radicle is variable, and that of the cotyledons fixed. It is at least deserving of notice, that the reverse of this is the fact; though it is certainly not necessary to change these terms, which are now generally received.
On the subject of Savignya, two questions naturally present themselves. In the first place: Is this genus, solely on account of its conduplicate cotyledons, to be removed from Alyssineæ, where it has hitherto been placed, to Velleæ, its affinity with which has never been suspected, and to whose genera it bears very little external resemblance? Secondly: In dividing Cruciferæ into natural sections, are we, with M. De Candolle, to expect in each of these subdivisions an absolute uniformity in the state of the cotyledons? As far as relates to the accumbent and flatly incumbent states, at least, I have no hesitation in answering the latter question in the negative; and I believe that in one case, namely Hutchinsia, these modifications are not even of generic importance; for it will hardly be proposed to separate H. alpina from petræa, solely on that ground. I carried this opinion farther than I am at present disposed to do, in the second edition of Mr. Aiton’s Hortus Kewensis, where I united in the genus Cakile plants which I then knew to differ from each other, in having accumbent and conduplicate cotyledons; and I included Capsella bursa pastoris in the genus Thlaspi, although I was aware, both from my own observations, and from Schkuhr’s excellent figure[89], that its cotyledons were incumbent. I am at present, however, inclined to adopt the subdivision of both these genera, as proposed by several authors, and received by M. De Candolle; but to this subdivision the author of the Systema Naturale must have been determined on other grounds than those referred to; for in these four genera, in which the three principal modifications of cotyledons occur, he has taken their uniformity for granted.