142 



PRACTICAL COURSE IN BOTANY 



i6i. Indeterminate inflorescence is always axillary, 

 since the production of a terminal flower would stop further 

 growth in that direction and thus terminate the development 

 of the axis. The raceme is the typical 

 flower cluster of the indefinite sort. In 

 such an arrangement the oldest flowers 

 are at the lower nodes, new ones appear- 

 ing only as the axis lengthens and pro- 

 duces new internodes. The little scale or 

 hract usually found at the base of the pedi- 

 cel in flower clusters of this sort is a re- 

 duced leaf, and the fact that the flower 

 stalk springs from the axil shows it to be 

 of the essential nature of a branch. 

 When the flowers are sessile and crowded 

 on the axis in various degrees, the cluster 

 produced may be a spike, as seen in the 

 plantain, knotweed, etc., or a head, like 

 that of the clover, buttonwood, and syca- 

 more. The catkins that form the characteristic inflorescence 

 of most of our forest trees are merely pendant spikes. The 

 corymb is a modification 

 of the raceme in which 

 the lower pedicels are 

 elongated so as to place 

 their flowers on a level 

 with those of the upper 

 nodes, making a convex, 

 or more or less flat- 

 topped cluster, as in the 

 wall-flowor and haw- 

 thorn. The umbel dif- 

 fers from the corymb in 

 having the pedicels with 

 their bracts all gathered 



at the top of the pe- Fig, leS. — Catkins of aspen. 



Fi(i. 167. — Raceme 

 of milk vetch (Astraga- 

 lus). 



