THE FLO WEE 



141 



ascending order of development appears as centripetal. That is, the 

 flowering proceeds from the margin or circumference regularly toward 

 the center; the lower flowers of the former answering to the outer 

 ones of the latter. 



296. In these three kinds of flower clusters, the flowers 

 are raised on conspicuous pedicels or stalks of their own. 

 The shortening of these pedicels, so as to render the flowers 

 sessile or nearly so, converts a raceme into a spike, and a 

 corymb or an umbel into a head. 



297. A Spike is a flower cluster with a more or less 

 lengthened axis, along which the flowers are sessile or nearly 

 so ; as in the Plantain (Fig. 245). 



298. A Head is a round or round- 

 ish cluster of flowers, which are sessile 

 on a very short axis or receptacle, as 

 in the Buttonball, Buttonbush (Fig. 

 246), and Red Clover. It is just what a spike 

 would become if its axis were shortened ; or an 

 umbel, if its pedicels were all shortened until 

 the flowers became sessile. The head of the 

 Buttonbush is naked; but that of the Thistle, 

 of the Dandelion, and the like, is surrounded 

 by empty bracts, which form an involucre. Two 



particular forms of the spike and the head have received particular 



names ; namely, the spadix and the catkin. 



299. A Spadix is a fleshy spike or head, with small 

 and often imperfect flowers, as in the Calla, Indian Tur- 

 nip (Fig. 173), Sweet Flag, etc. It is commonly sur- 

 rounded or embraced by a peculiar enveloping leaf, 

 called a SPATHE. 



300. A Catkin, or ament, is the name given to the 

 scaly sort of spike of the Birch (Fig. 247) and Alder, 

 the Willow and Poplar, and one sort of flower clusters 

 of the Oak, Hickory, and the like, the so-called amen- 

 taceous trees. 



301. Compound flower clusters 

 of these kinds are not uncommon. 

 When the stalks which in the 

 simple umbel are the pedicels of 

 single flowers themselves branch into an umbel, 

 a compound umbel is formed. This is the inflor- 

 escence of Caraway (Fig. 248), Parsnip, and 



almost all of the great family of umbelliferous (umbel-bearing) 



plants. 



The secondary or partial umbels of a compound umbpl are 



Jf 





247 



248 



