Page:An introduction to physiological and systematical botany (1st edition).djvu/486

 456 can never afford a generic character, provided there be no corresponding genus without them. This must be determined by experience and observation. They are indeed to be considered as a very secondary mark, the most essential in this Class being derived from the receptacle, crown of the seed, and calyx. These Gærtner has illustrated with the greatest accuracy and skill, but even these must not be blindly followed to the destruction of natural genera.

4. Polygamia necessaria. Florets of the disk furnished with stamens only, those of the margin, or radius, only with pistils; so that both are necessary to each other. This is well seen in the common Garden Marigold, Calendula, in whose calyx, when ripening seed, the naked and barren disk is conspicuous. Othonna, ''Curt. Mag. t. 306, 768, Arctotis, Osteospermum and Silphium'', not rare in gardens, are further examples of this Order, which I believe is constant and founded in nature. We have no British specimens either of it or the following. Filago, at least as far as our