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 276 OBSERVATIONS ON THE NATURAL FAMILY

There is here, therefore, a nearer approach to a true perianthium than in the involncrnni of Lagasca ; but the expansion of the flowers being, as in that genus, from centre to circumference of the capitulum, I consider the envelope of Ccesulia as unquestionably an involucrum, and the genus consequently belonging to Polygamia segregata.

I may here remark, that the name Polygamia segre- gata, invented by Linnaeus for those genera of Compositae with densely aggregate capitula, is calculated to give an erroneous idea of the nature of the structure ; the opposite term Polygamia congregata being, according to the view now taken, obviously more proper for those genera, at least, whose involucra contain several flowers. It is not unlikely, indeed, that Linnaeus himself was aware of the true nature of the inflorescence of these genera ; but the term Poly- gamia congregata would not have suited the artificial arrangement which he adopted in his subdivisions of the class, nor his including in it the order Monogamia ; for with this order the single-flowered genera of Polygamia segregata must then have been confounded.

It is a curious circumstance, that the order of expansion in Compositae does not depend on the number of flowers actually existing, but on the effort, if I may so term it, made to produce them, manifested by the presence of an involucrum or common calyx, which is in some cases reduced to a single flower. The fact at the same time con- tributes to prove, that the whole natural class is formed on 96] that plan of dense aggregation of flowers, for which I have already attempted to show that certain parts of the structure of a syngenesious floret are peculiarly well adapted.

The circumstance, however, is not confined to Compo- sitae, but exists in an equally remarkable degree in Gra- nt in ece.

I have formerly 1 considered the gluma, or what Linnaeus has termed calyx, in this family of plants, as an involucrum.

In those genera where this gluma or involucrum con-

1 \_Vol. i,p. 55.]

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