276 OBSERVATIONS ON THE NATURAL FAMILY 



There is here, therefore, a nearer approach to a true 

 perianthium than in the involucrum 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 CcBsulia 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 Compositse 

 w^ith 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 Compositse 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, w^hich 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- 

 sitse, but exists in an equally remarkable degree in Gra- 

 minecs. 



I have formerly^ 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- 



' IFol.ip. 55.] 



