Page:A Voyage to Terra Australis Volume 2.djvu/572

558 COMPOSITÆ. Of this family, which is the most extensive among Dicotyledones, upwards of 2500 speoies have been already described. About 300 are at present known in Terra Australis, in which therefore the proportion of Compositæ to its Dicotyledonous plants is considerably smaller than that of the whole order to Dicotyledones generally, and scarcely half that which exists in the Flora of South Africa. It is also inferior in number of species to Leguminosæ, like which it seems expedient to consider it as a class including several natural orders. Of these orders Cichoraceæ and Cinarocephalæ are comparatively very rare in Terra Australis, not more than ten species of both having hitherto been observed.

The class therefore chiefly consists of Corymbiferæ, which are very generally diffused, they are however evidently less numerous within the tropic, and their maximum appears to exist in Van Diemen's Island. Corymbiferæ may be subdivided into sections and the greater part of the genera peculiar to Terra Australis belong to that section which may be named Gnaphaloideæ, and exist either in the principal parallel or higher latitudes.

The whole of Compositæ agree in two remarkable points of structure, in their corolla; which, taken together at least, materially assist in determining the limits of the class. The first of these is its valvular æstivation, this, however, it has in common with several other families. The second I believe to be peculiar to the class, and hitherto unnoticed. It consists in the disposition of its fasciculi of vessels, or nerves; these, which at their origin are generally equal in number to the divisions of the corolla, instead of being placed opposite to these divisions and passing through their axes, as in other plants, alternate with them; each of the vessels at the top of the tube dividing into two equal branches running parallel to and near the margins of the corresponding laciniæ, within whose apices they unite. These, as they exist in the whole class, and are in great part of it the only vessels observable, may be called primary. In several genera, however, other vessels occur, alternating with the primary and occupying the axes of the laciniæ: in some cases these secondary vessels, being most distinctly visible in the laciniæ, and becoming gradually fainter as they descend the tube, may be regarded as recurrent; originating from the united apices of