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.
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Fig. 189.—Head of Sunflower.
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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 ap-
