196 



ORGANOGRAPHY. 



Fig. 401. 



Fig. 401. Capitulum of Cotton Thistle {Onopordum Acanthium). 



a variety of forms is given to the heads of flowers. This 

 kind of indefinite inflorescence, as well as all others with 

 shortened or dilated primary axes, also exhibit a centripetal 

 order of expansion. This may be well seen in the capitulum 

 of the Scabious {Jig. 402), where the outer flowers, (or florets 

 as they are commonly called from their smallness), are fully ex- 

 panded, those within them less so, and those in the centre in an 

 unopened condition. Here therefore the order of expansion is 

 towards the centre — that is, centripetally. 



b. The Hypanthodiu7n. — This kind of inflorescence is but a slight 

 modification of the last. It is formed by a receptacle which is 

 usually of a fleshy nature becoming more or less incurved, and 

 thus partially, as in the Dorstenia {fig. 381), or entirely, as in 

 the Fig {fig. 380), enclosing the flowers which it bears upon its 

 surface. The flowers in this kind of inflorescence are usually 

 unisexual, and there is no involucre to them as is almost uni- 

 versally the case in the true Capitulum. 



c. The Umbel. — •AVhen the primary axis is shortened, and 

 gives oiF from its apex a number of secondary axes or pedicels 

 of nearly equal length, each bearing a flower, and the whole ar- 

 ranged like the ribs of an umbrella, an umhd'isiovmQdL {fig. 

 403) as in the Primrose and Cowslip. When the secondary axes 

 themselves divide, and form tertiary axes, which are also ar- 

 ranged in an umbellate manner, a compound umbel is produced 



