natural Family of Plants called Composita. 93 
-The absolute constancy in the order of expansion of the simple 
capitulum from circumference to centre, and the more or less 
complete inversion of this order in the compound capitulum, ap- 
pear to afford tests of the real structure in certain cases where 
the degree of composition, and consequently the proper names of 
some of the parts, might otherwise be doubtful. 
To illustrate this I select two: genera, Lagasca and Casilia. 
In Lagasca the capitulum, both from its form and the appear- 
ance of its involucrum, might at first sight be considered as sim- 
ple : on examination, however, it is found to differ from all simple 
capitula, in each floret being furnished with a. tubular envelope, 
exactly resembling a five-toothed | perianthium, but which does 
not in any state cohere with the included ovarium. - 
Cavanilles, by whom the genus was established, regarded this 
envelope as a genuine perianthium, and erroneously described its 
tube as cohering with the ovarium ; an error which is copied in 
Persoon's Synopsis Plantarum, where the genus is consequently 
placed in. Polygamia æqualis. Jacquin, who has published La- 
gasca- under the name of Noccea mollis *, also describes the en- 
velope of each flower as a proper perianthium, although aware 
| of its tube being distinct from the ovarium. Subsequent writers 
have, indeed, more correctl y referred the genus to. Polygamia 
segregata ; but the terms involucellum and calyculus, which they 
apply to the envelope in question, appear to me —— 
for a reason that will presently be given... + 
_. Three suppositions may be formed respecting the: nature of this 
| envelope, namely, either that it is an involucrum reduced, as in 
Echinops, to a single flower ; secondly, that it is a proper perian- 
: thium, which in appearance it very much resembles ; or thirdly, 
Ee Fragm. Bot. p: 98, tab. 85. : 
: that 
