36 



ZINNIA. 



[CHAP. 



regular corollas, occupying the centre of the head, form 

 the disk. 



In describing the structure of flower-heads {capitula), it is 

 well to examine the ray and disk florets separately. Neither 

 of these appear, at first sight, to have a cal}TC. Analogy, 



Fig. ao. Vertical section of a capitulum of Garden Zinnia, showing 

 two ray- florets and one disk-floret. 



however, aftbrds sufficient reason to conclude that each 

 floret has its own calyx, but it is wholly adherent to the 

 ovary. It is superior and gamosepalous. In many plants 

 related to the Zinnia — the Thistle, for example 

 — the upper free portion {limb) of the calyx 

 exists as a crown of fine bristles surrounding 

 the top of the ovary. The corolla of the ray- 

 florets is gamopetalous and irregular ; of the 

 disk-florets, gamopetalous and regular. In 

 both it is inserted apparently upon the top 

 of the ovary, and is consequently termed 

 epigynous. The stamens in the ray-florets are 

 absent or imperfect ; in the disk-florets they 

 are five in number (pentandrous), and inserted upon the 

 corolla. In consequence of this adhesion to the corolla 

 they are termed epipetalous. 



Fig. 21. Fruit of 

 Thistle, with 

 sessile, plumose 

 pappus. 



