274 OBSERVATIONS ON THE NATURAL FAMILY
compound spike or bead, 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. 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, 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 T select two genera, Lagasca and Ccesulia.
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 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 sequalis. Jacquin, who has published Lagasca under the name of Nocccea 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
1 Fragm. Bot. p. 58, tab. 85.
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