_must next be covered with a paper bag to prevent the access of pollen_ (Figs. 208, 209). If the stigma is not receptive at the time (as it usually is not), the desired pollen is not applied at once. The bag may be removed from time to time to allow of examination of the pistil, and when the stigma is mature, which is told by its glutinous or roughened appearance, the time for pollination has come. If the bag is slightly moistened, it can be puckered more tightly about the stem of the plant. The time required for the stigma to mature varies from several hours to a few days. (_c_) When the stigma is ready, an unopened anther from the desired flower is crushed on the finger nail or a knife blade, and the pollen is rubbed on the stigma by means of a tiny brush, the point of a knife blade, or a sliver of wood. The flower is _again covered with the bag_, which is allowed to remain for several days until all danger of other pollination is past. Care must be taken completely to cover the stigmatic surface with pollen, if possible. The seeds produced by a crossed flower produce _hybrids_, or plants having parents belonging to different varieties or species. =165.= One of the means of securing new forms of plants is by making hybrids. Why? [Illustration: FIG. 208.--A PAPER BAG, with string inserted.] [Illustration: FIG. 209.--THE BAG TIED OVER A FLOWER.] [Illustration: FIG. 210.--FIG. The fig is a hollow torus with flowers borne on the inside, and pollinated by insects that enter at the apex.] CHAPTER XX FLOWER-CLUSTERS =Origin of the Flower-cluster.=--We have seen that branches arise from the axils of leaves. Sometimes the leaves may be reduced to bracts and yet branches are borne in their axils. Some of the branches grow into long limbs; others become short spurs; _others bear flowers_. In fact, a flower is itself a specialized branch. Flowers are usually borne near the top of the plant. Often they are produced in great numbers. It results, therefore, that flower branches usually stand close together, forming a cluster. The shape and arrangement of the flower-cluster _differ with the kind of plant_, since each plant has its own mode of branching. [Illustration: FIG. 211.--TERMINAL FLOWERS OF THE WHITEWEED (in some places called ox-eye daisy).] Certain definite or well-marked types of flower-clusters have received names. Some of these names we shall discuss, but the flower-clusters that perfectly match the definitions are the exception rather than the rule. The determining of the kinds of flower-clusters is one of the most perplexing subjects in descriptive botany. We may classify the subject around three ideas: =solitary flowers=, =centrifugal or determinate clusters=, =centripetal or indeterminate clusters=.