274 OBSERVATIONS ON THE NATURAL FAMILY 



compound spike or head, the same order of expansion 

 obtains, and it continues though the florets in each com- 

 mon calyx or involucrum should be lessened in number, or 

 even reduced to unity, as in Echinops and Bolandra. 

 98] 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, appear 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 

 CcBSulia. 



In Lagasca the capitulum, both from its form and the 

 appearance of its involucrum, might at first sight be con- 

 sidered as simple : on examination, however, it is found to 

 differ from all simple capitula, in each floret being fur- 

 nished with a tubular envelope, exactly resembling a iive- 

 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 sequalis. 

 Jacquin, who has published Lagasca under the name of 

 Noccaa mollis} also describes the envelope of each flower 

 as a proper perianthium, although aware of its tube being 

 distinct from the ovarium. Subsequent writers have, 

 indeed, more correctly referred the genus to Polygamia 

 segregata; but the terms involucellum and calyculus, 

 which they apply to the envelope in question, appear to me 

 objectionable, 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 perianthium, which in appearance it very 



' Fragm. Bot. p. .58, tab. 85. 



