HERBACEOUS AND WOODY DICOTYLEDONS 411 



roadsides and in waste places and is a convenient type for study. 

 The reproductive structures of the Compositae are so similar that 

 when one type is mastered the variations from this type are 

 easily understood. 



Inflorescence. The flowers of the yarrow are borne in dense 

 heads, each surrounded by a cluster of bracts, which together 

 form the involucre (Fig. 261, A). Each separate head of flowers 



FIG. 261. Inflorescence and flowers of yarrow (Acliillea) 



A, single head, or inflorescence; B, section of the same; C, ray flower; D, disk 



flower;' a, anther; ch, bract; d, disk flower; o, ovary; re, corolla of ray flower; 



s, stigma ; tc, corolla of tubular flower 



x 



thus represents an inflorescence with a broad, flat receptacle on 

 which the small flowers are closely crowded. The outer flowers 

 bear an outgrowth of the corolla which is strap-shaped, while 

 in the central flowers the corollas are tubular. The outer strap- 

 shaped flowers are the rav flowers of the head and serve to 

 render it more conspicuous. 



Flowers. The flowers are all epigynous, with the floral parts 

 borne above and upon the ovary, which is composed of two spo- 

 rophylls, or carpels. The perianth is composed of a corolla the 



