Ch. VI, 11] MORPHOLOGY OF CLUSTERS 



337 



Both racemes and cymes often become compound by the 

 branching of the main flower stalks, and the two types occur 

 intermingled in the more complicated clusters, such as the 

 pyramidal thyrsus of the Lilac and Horse-chestnut and the 

 much looser panicle of the Meadow Rue, and most of the 

 loose-topped Grasses. In the other direction, the clusters 

 become very compact. Thus racemes sometimes have so 

 many flowers on such short 

 stalks as to form collectively 

 a spike (Fig. 232), as familiar 

 in Mullein, while if bracts in 

 a spike are more prominent 

 than petals, as so commonly 

 occurs in wind-pollinated 

 trees, we have a catkin, 

 familiar in Birches (Fig. 197) 

 and " pussy willows . " If the 

 main stem remains short, 

 bringing the flowers all close 

 together, the cluster is a head, 

 as familiar in Clover and 

 Button Bush (Fig. 233). 



The clusters thus far noted 

 are little more than aggrega- 

 tions of similar flowers, but 

 more highly developed kinds 

 show a marked approach to 



the aspect of single large flowers. The tendency is first 

 manifest in the production of flat-topped clusters. Thus, 

 if the main stem and the stalks of the lower flowers of a 

 raceme all elongate at about the same rate, there results 

 a flat-topped corymb (Fig. 234). When, further, the 

 main stem remains still shorter, or undeveloped, and the 

 flower stalks have all about equal lengths, there results a 

 characteristic umbel (Fig. 235), a very common form of 

 cluster, and one which prevails through, and has given name 



Fig. 235. — A typical umbel, of 

 Agapanthus umbellatus. (From 

 Bailey.) 



