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 the 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.
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 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 centre 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.
Fig. 192.—Narcissus or Daffodil. Single flower at the right.
Suggestions.—145. If the pupil has been skilfully conducted through this chapter by means of careful study of specimens rather than as a mere memorizing process, he will be in mood to challenge any flower that he sees and to make an effort to understand it. Flowers are endlessly modified in form; but they can be understood if the pupil looks first for the anthers and ovaries. How may anthers and ovaries always be distinguished? 146. It is excellent practice to find the flowers in plants that are commonly known by name, and to determine the main points in their structure. What are the flowers in Indian corn? pumpkin or squash? celery? cabbage? potato? pea? tomato? okra? cotton? rhubarb? chestnut? wheat? oats? 147. Do all forest trees have flowers? Explain. 148. Name all the monœcious plants you know. Diœcious. 149. What plants do you know that bloom before the leaves appear? Do any bloom after the leaves fall? 150. Explain the flowers of marigold, hyacinth, lettuce, clover, asparagus, garden calla, aster, locust, onion, burdock, lily-of-the-valley, crocus, Golden Glow, rudbeckia, cowpea. 151. Define a flower.