=Compositous Flowers.=--The head (anthodium) or so-called “flower” of sunflower (Fig. 189), thistle, aster, dandelion, daisy, chrysanthemum, goldenrod, _is composed of several or many little flowers, or_ =florets=. These florets are inclosed in a more or less dense and usually green _involucre_. In the thistle (Fig. 190) this involucre is prickly. A longitudinal section discloses the florets, all attached at bottom to a common torus, and densely packed in the involucre. The pink tips of these florets constitute the showy part of the head. [Illustration: FIG. 190.--LONGITUDINAL SECTION OF THISTLE HEAD; also a FLORET OF THISTLE.] Each floret of the thistle (Fig. 190) is a complete flower. At _a_ is the ovary. At _b_ is a much-divided plumy calyx, known as the =pappus=. The corolla is long-tubed, rising above the pappus, and is enlarged and 5-lobed at the top, _c_. The style projects at _e_. The five anthers are united about the style in a ring at _d_. Such anthers are said to be =syngenesious=. These are the various parts of the florets of the Compositæ. In some cases the pappus is in the form of barbs, bristles, or scales, and sometimes it is wanting. The pappus, as we shall see later, assists in distributing the seed. Often the florets are not all alike. The corolla of those in the outer circles may be developed into a _long, straplike, or tubular part_, and the head then has the appearance of being one flower with a border of petals. Of such is the sunflower (Fig. 189), aster, bachelor’s button or cornflower, and field daisy (Fig. 211). These long corolla-limbs are called rays. In some cultivated composites, all the florets may develop =rays=, as in the dahlia and chrysanthemum. In some species, as dandelion, all the florets naturally have rays. Syngenesious arrangement of anthers is the most characteristic single feature of the composites. [Illustration: FIG. 191.--PETALS ARISING FROM THE STAMINAL COLUMN OF HOLLYHOCK, and accessory petals in the corolla-whorl.] =Double Flowers.=--Under the stimulus of cultivation and increased food supply, flowers tend to become double. True doubling arises in two ways, morphologically: (1) _stamens or pistils may produce petals_ (Fig. 191); (2) _adventitious or accessory petals may arise in the circle of petals_. Both of these categories may be present in the same flower. In the full double hollyhock the petals derived from the staminal column are shorter and make a rosette in the center of the flower. In Fig. 192 is shown the doubling of a daffodil by the modification of stamens. Other modifications of flowers are sometimes known as doubling. For example, double dahlias, chrysanthemums, and sunflowers are forms in which the disk flowers have developed rays. The snowball is another case. In the wild snowball the external flowers of the cluster are large and sterile. In the cultivated plant all the flowers have become large and sterile. Hydrangea is a similar case.