RUBIACEÆ.[1] As this order is now constituted it appears to me impracticable to distinguish it from Apocineæ, by characters taken from the fructification alone; and even if the Stellatæ or Asperuleæ be excluded, and the remarkable stipulation of its remaining sections be taken into account, it will not then, perhaps, admit of a definition entirely free from exceptions. It must also, I think, be allowed that Rubiaceæ, Apocineæ, Asclepiadeæ, and certain genera at present referred to Gentianeæ, form but one great natural class. In this class the leaves are uniformly simple, perfectly entire, and, with a very few exceptions, occurring in Asclepiadeæ and Apocineæ, also opposite; while in the parts of fructification there are hardly any characters that are not liable to exceptions, unless the monopetalous regular corolla, and stamina alternating with its laciniæ and not exceeding them in number.
The order Rubiaceæ, admitting it as it is at present established, is chiefly æquinoctial. In Terra Australis its maximum is also within the tropic, where, however, it is not very numerous; and the most remarkable Australian part of the order, consisting of Opercularia and Pomax, is chiefly found in the principal parallel. Jussieu is very unwilling to admit these two genera into Rubiaceæ, and is rather disposed to consider them as a distinct family; chiefly on account of their single-seeded ovarium. To prove that this character alone, however, is not of such importance as to separate plants into different natural orders, it is sufficient to advert to Proteaceæ, Amaranthaceæ, and Epacrideæ, all of which contain genera with one, two, and even an indefinite number of seeds: and as Operculariæ entirely agree with many genera of Rubiaceæ in other points of structure of fructification, in habit, and especially in their remarkable stipulation, I think there can be no doubt that they ought to be referred to the same order, of which they may form a section, characterised not only by its single-seeded ovarium, but by the peculiar dehiscence of its compound fruit.
APOCINEÆ.[2] I have already observed[3] that this order is very nearly