156 



PLANT BIOLOGY 



kinds of flower-clusters is one of the most perplexing sub- 

 jects in descriptive botany. We may classify the subject 

 around three ideas : solitary flowers, centrifugal or deter- 

 minate clusters, centripetal or indeterminate clusters. 



Solitary Flowers. In many cases flowers are borne 

 singly ; they are separated from other flowers by leaves. 

 They are then said to be solitary. The solitary flower may 



be either at the end of the 

 main shoot or axis (Fig. 211), 

 when it is said to be terminal ; 

 or from the side of the shoot 

 (Fig. 212), when it is said to 

 be lateral or axillary. 



Centripetal Clusters. If 

 the flower-bearing axils were 

 rather close together, an open 

 or leafy flower-cluster might 

 result. If the plant continues 

 to grow from the tip, the 

 older flowers are left farther 

 and farther behind. If the 

 cluster were so short as to be 

 flat or convex on top, the out- 

 ermost flowers would be the 

 older. A flower-cluster in which the lower or outer flowers 

 open first is said to be a centripetal cluster. It is some- 

 times said to be an indeterminate cluster, since it is the 

 result of a type of growth which may go on more or less 

 continuously from the apex. 



The simplest form of a definite centripetal cluster is a 

 raceme, which is an open elongated cluster in which the 

 flowers are borne singly on very short branches and open 

 from below (that is, from the older part of the shoot) 



FIG. 212. LATERAL FLOWER OF 

 AN ABUTILON. A greenhouse 

 plant. 



