142 



PLANT BIOLOGY 



pearance 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 chrysan- 

 themum. In some species, as dandelion, all the florets 

 naturally have rays. Syngenesious arrangement of an- 

 thers is the most characteristic single feature of the 

 composites. 



Double Flowers. Under the stimulus of cultivation and 

 increased food supply, flowers tend to become double. 



True doubling arises 

 in two ways, mor- 

 phologically: (i)sta- 

 mens or pistils may 

 produce petals (Fig. 

 191) ; (2) adventi- 

 tious or accessory 

 petals may arise in 

 the circle of petals. 

 Both of these cate- 

 gories may be pres- 

 ent in the same 

 flower. In the full 

 double hollyhock the petals derived from the staminal col- 

 umn 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 snow- 

 ball is another case. In the wild snowball the external 

 flowers of the cluster are large and sterile. In the culti- 



FIG. 191. PETALS ARISING FROM THE STAMI- 

 NAL COLUMN OF HOLLYHOCK, and accessory 

 petals in the corolla-whorl. 



