=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. [Illustration: 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_. * * * * * NOTE TO THE TEACHER.—It cannot be urged too often that _the specimens themselves_ be studied. If this chapter becomes a mere recitation on names and definitions, the exercise will be worse than useless. Properly taught by means of the flowers themselves, the names become merely incidental and a part of the pupil’s language, and the subject has living interest. CHAPTER XIX THE FLOWER—FERTILIZATION AND POLLINATION [Illustration: FIG. 193.—_B_, POLLEN escaping from anther; _A_, pollen germinating on a stigma. Enlarged. ]