natural Family of Plants called Composita. 113 
and besides these, one or two minute processes are generally ob- 
servable. Stigmata revolute *. 
In the 12th edition of Systema Nature, Linneus added to his 
genus Calea a fourth species, namely Calea scoparia; for what rea- 
son it would be difficult to discover, as it does not resemble, either 
* In theremarkable character of its re-curved florets, as well as in some other respects, 
this species of Salmea agrees with Spilanthus arboreus of George Forster (in Commentat. 
Gotting. ix. p. 66.), of which he originally formed his genus Laxmannia ; from a very er- 
roneous view of its structure, however, having described the Nectarium or glandula epigyna 
as a * germen superum ;” the real, though imperfect, germen with its two aristæ as a 
e perianthium bidentatum," and consequently referring the genus to Polvgamia segregata. 
When he afterwards corrected these errors and reduced Laxmannia to. Spilanthus, he 
did not discover that he had only the imperfect hermaphrodite or male plant before him. 
That Spilanthus arboreus is really dioecious, I have ascertained from the examination of 
numerous specimens collected by Sir Joseph Banks in the Island of St. Helena, where it 
forms a small tree called by the inhabitants White-wood. It is Bidens arborea and per- 
haps also Spilanthus tetrandrus of Dr. Roxburgh's List of Plants appended to General Beat- 
son's Tracts on St. Helena; the former being probably the female, the latter as tarved 
variety of the male plant. 
In re-establishing Spilanthus arboreus as a genus, sufficiently distinct from Bidens, Spi- 
lanthus, and Salmea, it will not, I conclude, be considered expedient to recur to Forster's 
name Laxmannia, which as far as relates to this plant is connected only with a series of 
blunders, was abandoned by the author himself, and has since been applied to another ge- 
nus already generally adopted. It may be —— by the following character, sat 
named 
PETROBIUM. 
Bia polyphyllum subduplici serie: exteriore breviore, foliolis paucioribus 4 
. ceptaculum paleaceum, planiusculum. Flosculi dioici, tubulosi, 4-fidi :  Masculi : Bn 
ris exsertis ; Stigmatibus acutis hispidulis : Feminei : Staminibus sterilibus ; ; resin 
acutis recurvis. Achenium v. parallelo compressum v. creen ae (2-3) aristatis : : 
aristis persistentibus, antrorsum. denticulatis. - ; 
Arbor (Insulæ Ste Helenz). Folia opposita, indivisa. EN idrednalis, brachiata. In- 
.. volucrum oblongum. Paleæ receptaculi squamis involucri subsimiles. Corollulæ ochroleu- 
cæ, tubo arcuato-recurvo (ut capitulum primo intuitu radiatum videatur). Mas. Antheris 
nigricantibus, basi emarginatis, appendice apicis brevissimo, acuto; loculis vestigio septi 
longitudinalis instructis. Fem. Staminibus sterilibus distinctis, antheris sagitiatis cassis. 
VOL. XII. Q in 
