INDETERMINATE INFLORESCENCE. 



219 



bracts borne by the Birch, Poplar, Willow, and, as to one of the 

 two sorts of flowers, by the Oak, Walnut, and Hickory, which are 

 accordingly called amentaceous trees. Catkins usually fall off in 

 one piece, after flowering or fruiting, especially the sterile cat- 

 kins. 



392. The Head, or Capitlllum, is a globular cluster of sessile flow- 

 ers, like that of the Button Bush, the balls of the Buttonwood or 

 Plane-tree, &c. It is a many-flowered centripetal inflorescence, 

 in which neither the primary axis nor the secondary axes are at all 

 lengthened. We may conceive it to originate, either from the 

 non-development of the pedicels of an umbel (Fig. 232), or the 

 non-elongation of the axis of a spike. In other words, the head 

 differs from a spike only in its shortness. So what is at first a 

 head frequently elongates into a spike as it grows older ; as in 

 many species of Clover, &c. In all these forms, the blossoms ne- 

 cessarily expand from the base to the apex, or from the circum- 

 ference to the centre (387). 



393. The base both of the head and the umbel is frequently 

 furnished with a number of imper- 

 fect leaves or bracts, crowded to- 

 gether, or forming a whorl (236, 



Fig. 232), termed an INVOLUCRE. 

 The involucre assumes a great va- 

 riety of forms ; sometimes resem- 

 bling a calyx ; and sometimes (as 

 in Cornus Florida, or the common 

 Dogwood, and C. Canadensis, Fig. 

 240), becoming petal-like, and 

 much more showy than the blos- 

 som itself. It is, however, distin- 

 guished from the calyx or corolla 

 by including a number of flowers. 

 Sometimes, however, as in the 

 Mallow Family and Hibiscus, the 

 involucre forms a kind of outer 

 calyx to each flower. 



394. The axis, or rachis (382), of a head is called the RECEPTA- 

 CLE. Frequently, instead of being globular or somewhat pro- 



FIG. 240. Cornus Canadensis ; with its petal-like four-leaved involucre surrounding a head 

 of flowers : a, a separate flower from the head, enlarged. 



