28 ELEMENTS OF BOTANY. 



the ribs of an umbrella. It is simple when the secondary axes bear 

 flowers (Fig. 41) ; compou nd when they divide before flowering (Fig. 42). 

 In the latter case the ultimate clusters of flowers are termed umbel lets, 

 or partial umbels. 



In compound umbels the bracts at the base constitute the involucre, 

 and those at the base of the umbellets the involucel, or partial in- 

 volucre. 



Fio. 45. A panicle. 



A spike is an inflorescence in which the flowers are sessile on the 

 primary axis (Fig. 43). It is compound when secondary axes rise from the 

 primary one and bear sessile flowers (Fig. 44). In many cases these pedi- 

 cels are long and form panicles (Fig. 45). 



Catkins (Figs. 46 and 47) and cones are forms of spikes in which 

 the flowers are incomplete, as will appear later. 



A head is an inflorescence in which the primary axis is depressed 



