=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=. [Illustration: FIG. 212.--LATERAL FLOWER OF AN ABUTILON. A greenhouse plant.] =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 outermost 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 sometimes 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) upwards (Fig. 213). The raceme may be _terminal_ to the main branch; or it may be _lateral_ to it, as in Fig. 214. Racemes often bear the flowers on one side of the stem, thus forming a single row. When a centripetal flower-cluster is long and dense and the flowers are sessile or nearly so, it is called a =spike= (Fig. 215). Common examples of spikes are plantain, mignonette, mullein. [Illustration: FIG. 213.--RACEME OF CURRANT. Terminal or lateral?] [Illustration: FIG. 214.--LATERAL RACEMES (in fruit) OF BARBERRY.] [Illustration: FIG. 215.--SPIKE OF PLANTAIN.] A very _short and dense spike_ is a =head=. Clover (Fig. 216) is a good example. The sunflower and related plants bear many small flowers in a very dense and often flat head. Note that in the sunflower (Fig. 189) the outside or exterior flowers open first. Another special form of spike is the =catkin=, which usually has scaly bracts, the whole cluster being deciduous after flowering or fruiting, and the flowers (in typical cases) having only stamens or pistils. Examples are the “pussies” of willows (Fig. 182) and flower-clusters of oak (Fig. 180), walnuts (Fig. 204), poplars. [Illustration: FIG. 216.--HEAD OF CLOVER BLOSSOMS.] [Illustration: FIG. 217.--CORYMB OF CANDY-TUFT.] When a loose, elongated centripetal flower-cluster has some primary branches simple, and others irregularly branched, it is called a =panicle=. It is a branching raceme. Because of the earlier growth of the lower branches, the panicle is usually broadest at the base or conical in outline. True panicles are not very common.