The Forms of Flower Clusters. 91 



is the lowest. Evidently the spike, shown at b, is a raceme of 



Fig. 71. Forms of Axillary Inflorescence, a. Raceme, b. Spike, c. Corymb, d. Umbel, e. Head. 



stemless (sessile) flowers. When the peduncles of the lower 



flowers, by growing, keep pace with the central stem a flat- 



topped cluster called a corymb is formed. 



An umbel is formed when the internodes 



do not grow. The bracts then are all at 



the end of the peduncle and the pedicels 



belonging to them (subtended by them) 



all start from the same place. Shorten- 



ing the pedicels in an nmbel, thus bring- 



ing the flowers to nearly or quite a sessile 



condition, changes it to a head. The 



whorl of bracts in an umbel or a head is 



called an involucre. Racemes and spikes 



may be dense or close, loose or lax, 



drooping, erect, etc. A spike of apetalous 



n 11 . 



nowers growing on a shrub or tree is 



called a catkin or ament (Fig. 72). The peculiar spike of a 



Fig . 72 . tatkinsoraments 



flowers (lower end of a cat- 



kin three inches long). 



