456 



THE C A A I P AXULALES 



339, F) . The fruit is now mature and in a position for distri- 

 bution. The involucre opens, the hairs of the pappus expand, 

 loosening the akenes so that the least touch or breath of air floats 

 off the fruit as a most perfect type of parachute (Fig. 338, c). 

 (b) The Thistle Family, Cardnaeeae. — This is the largest 



Fig. 340. Inflorescence of the bur-marigold (Bidcns), a common ray- 

 form of flower of the thistle family : in, involucre. 



family of the angiosperms. The flowers have the same arrange- 

 ment and structure as in the Cichoriaceae, save that the corollas 

 are either all tubular, as in the thistle and ironweed (Fig. 342), 

 or the marginal flowers of the head may be ligulate, thus in- 

 creasing the conspicuousness of the inflorescence, as in the asters, 

 daisy, etc. (Figs. 340, 341). The marginal ligulate flowers are 

 termed ray flowers to distinguish them from the inner tubular or 

 disc flowers. The ray flowers may be sterile, though more fre- 

 quently imperfect and provided with pistils. The calyx may be 

 wanting, but more often assumes the form of silky or plumose 

 pappus or of membranous scales for wind transportation or of 

 barbed bristles of various kinds for distribution by animals. In 

 the case of the burdock (Arctium) the involucre is covered with 

 hooks. The Carduaceae includes a great array of plants, many 

 of which are among our troublesome weeds: Ironweed (ler- 

 nonia), thoroughwort (Eupatorium) , blazing star (Laciiiaria), 



