This order is sufficiently distinguished from both the others by the hypogynous insertion and valvular æstivation of its corolla, which being perfectly regular differs in this respect also from the greater part of Lomentaceæ and from all the Papilionaceæ.
Nearly the whole of the Australian species of the Linnean genus Mimosa belong to Acacia of Willdenow, as it is at present constituted; and about nine tenths of the Acaciæ to his first division of that genus, described by him as having simple leaves, but which is in reality aphyllous; the dilated foliaceous footstalk performing the functions of the true compound leaf, which is produced only in the seedling plant, or occasionally in the more advanced state in particular circumstances, or where plants have been injured.
The great number of species of Acacia having this remarkable economy in Terra Australis forms one of the most striking peculiarities of its vegetation. Nearly 100 species have already been observed; more than half of these belong to the principal parallel, at both extremities of which they appear to be equally abundant; they are, however, very generally diffused over the whole country, existing both on the north coast of New Holland, and at the south end of Van Diemen's Island. But though the leafless Acaciæ are thus numerous and general in Terra Austrahs, they appear to be very rare in other parts of the world; none of the Australian species are found in other countries, and at present I am acquainted with only seven additional species, of which five are natives of the intratropical Islands of the southern hemisphere; the sixth was observed in Owhyhee, and is said to be the largest tree in the Sandwich Islands; the seventh is Mimosa stellata of Loureiro, upon whose authority it entirely rests.
The second order, Lomentaceæ or Cæsalpineæ, comprehends all the genera having perigynous stamina, a corolla whose æstivation is not valvular, and which though [552 generally irregular is never papilionaceous. To these characters may be added the straight embryo, in which they
