412 



GENERAL BOTANY 



parts of which are united to form a tube. The calyx is absent 

 in the yarrow, but in many other Compositae it is split into a 

 large number of fine hairs which together are called the pappus. 

 The essential organs comprise five stamens and a single com- 

 pound pistil composed of two sporophylls, or carpels. The ovary 

 bears a single seed and forms an achene in fruit. The style splits 

 at the apex to form two lobes, the inner faces of which are 

 roughened to form the stigmatic surfaces. The apex of each 



Stigma 



FIG. 262. Flower and fruit of the yarrow and of the dandelion (Taraxacum) 



a, young tubular flower of yarrow, with stigma still within the stamen tube ; 6, older 



flower, with the style and stigma protruding ; c, ligulate flower of the dandelion and 



its parts ; d, fruit with pappus and beak 



stylar lobe is also furnished with a brush of hairs which together 

 constitute the stylar brush. The stamens are attached to the 

 corolla by slender filaments, and the anthers are united to form 

 a tube around the style. 



Pollination. The umbellate inflorescences of the yarrow form 

 a continuous flat-topped floral mass which is rendered conspicu- 

 ous by the white or pink ray flowers. Since there may be as 

 many as one hundred heads in the entire inflorescence, with from 

 fifteen to twenty flowers in each head, a visiting insect is certain 

 to come in contact with a large number of separate flowers, all 

 of which are especially adapted to secure cross-pollination. The 

 mechanism of cross-pollination is concerned with the structure 



