Lorantheæ. A species of Loranthus, parasitical on the Acacia nilotica, was observed very commonly from Fezzan to Bornou.
Leguminosæ. Of this class the herbarium contains thirty-three species, among which there are hardly more than two undescribed, and these belonging to a well-established genus.
Of the order or tribe Mimoseæ only three species occur, namely, Acacia nilotica, Mimosa Habbas, and Inga biglobosa, or a species very nearly related to it. Of this last named plant, I judge merely from ripe fruits adhering to the singular club-shaped receptacle, or axis of the spike. The specimens were collected in Soudan, and belong to a tree of considerable importance to the inhabitants of that country, by whom it is called Doura. According to Captain Clapperton, “The seeds are roasted as we roast coffee, then bruised, and allowed to ferment in water; when they begin to become putrid, they are well washed and pounded; the powder made into cakes, somewhat in the fashion of our chocolate; they form an excellent sauce for all kinds of food. The farinaceous matter surrounding the seeds is made into a pleasant drink, and they also make it into a sweetmeat.” The Doura of Captain Clapperton is probably not specifically different from the Nitta mentioned by Park, in his First Journey; nor from Inga biglobosa of the Flore d’Oware of M. de Beauvois, according to whom it is the Nety of Senegal; and he also well remarks, that Inga biglobosa, described by Jacquin as a native of Martinico, has probably been introduced into that island by the Negroes, as he himself found it to have been in St. Domingo.
Inga Senegalensis of M. De Candolle (Prodr. 2. p. 442) may also belong to the same species.
It is possible, however, that some of the plants here mentioned, though very nearly related to each other, and having all the same remarkable club-shaped spike, may be specifically distinct; for it appears from specimens collected at Sierra Leone by Professor Afzelius, that two plants having this form of spike are known in that colony; and two species, with similar inflorescence, probably distinct from those of Africa, are described in the manuscript Flora Indica of Dr. Roxburgh. All these plants possess characters fully sufficient to distinguish them from Inga, to which they have hitherto been referred. The new genus which they form, one of the most striking and beautiful in equinoxial Africa, I have named Parkia[103], as a tribute of respect to the memory of the celebrated traveller, by whom the fruit of this genus was observed in his first journey, and who, among other services rendered to botany, ascertained that the plant producing Gum Kino is a species of Pterocarpus[104]. I have formerly endeavoured to distinguish Mimoseæ from Cæsalpineæ, by the valvular æstivation of both its floral envelopes, and by the hypogynous insertion of its stamina. Instances of perigynous insertion of stamina have since been noticed by MM. Kunth and Auguste de St. Hilaire; but no exception has been yet pointed out to the valvular æstivation of their calyx and corolla. Parkia, however, differs from other Mimoseæ not only in its æstivation, which is imbricate, but in the very manifest irregularity of its calyx, and in the inequality of its petals, which, though less obvious, is still observable.
Erythrophleum, another genus indigenous to equinoxial Africa, which I have elsewhere[105] had occasion to notice, and then referred to Cæsalpineæ, more properly belongs to Mimoseæ, although its stamina are perigynous. In this genus, both calyx and corolla are perfectly regular, and their æstivation, if not strictly valvular, is at least not manifestly imbricate, though the flower-buds are neither acute nor angular. In Erythrophleum and Parkia, therefore, exceptions to all the assumed characters of Mimoseæ are found, and there is some approach in both genera to the habit of Cæsalpineæ. It is still possible, however, to distinguish, and it will certainly be expedient to preserve, these two tribes or orders. Abandoning divisions strictly natural, and so extensive as the tribes in question, merely because we may not be able to define them with precision, while it would imply, what is far from being the case, that our analysis of their structure is complete, would, at the same time, be fatal to many natural families of plants at present admitted, and among others to the universally received class to which these tribes belong. No clear character, at least, is pointed out in the late elaborate work of M. De Candolle[106], by which Leguminosæ may be distinguished from Terebintaceæ and Rosaceæ, the orders supposed to be the most nearly related to it. It is possible, however, that such characters, though hitherto overlooked, may really exist; and I shall endeavour to show that Leguminosæ, independent of the important but minute differences in the original structure and developement of its ovulum, may still be distinguished at least from Rosaceæ.
In the character of Polygaleæ, which I published in 1814[107], I marked the relation of the parts of the floral envelopes to the axis of the spike, or to the subtending bractea. I introduced this circumstance chiefly to contrast Polygaleæ with Leguminosæ, and to prove, as I conceived, that Securidaca, which had generally been referred to the latter family, really belonged to the former.
M. de Jussieu, who soon after published a character of Polygaleæ, entirely omitted this consideration, and continued to refer Securidaca to Leguminosæ. M. De Candolle, however, in the first volume of his Prodromus, has adopted both the character and limits of Polygaleæ, which I had proposed, though apparently not altogether satisfied with the description he himself has given of the divisions of the calyx and corolla.
The disposition of the parts of the floral envelopes, with reference to the axis of the spike, in Polygaleæ, namely, the fifth segment of the calyx being posterior or superior and the fifth petal anterior or inferior, is the usual relation in families the division of whose flower is quinary. This relation is in some cases inverted; one example of which I have formerly pointed out in Lobeliaceæ[108], as I proposed to limit it, and a similar inversion exists in Leguminosæ. But this class also deviates from the more general arrangement of the parts of the flower with regard to each other. That arrangement consists, as I have long since remarked[109], in the regular alternation of the divisions of the proximate organs of the complete flower. To this arrangement, indeed, many exceptions are well known; and M. De Candolle has given a table of all the possible deviations, but without stating how many of these have actually been observed[110].
In Leguminosæ the deviation from the assumed regular arrangement consists in the single pistillum being placed opposite to the lower or anterior segment of the calyx.