DEVELOPMENT OF PLANTS 489 



bearing the head elongates considerably, so as to lift the fruit 

 above the surrounding vegetation — the extent of its elongation 

 depends upon the height of this vegetation. The corolla withers 

 away and the hair-like calyx grows out into a white, delicate 

 pappus, which is lifted upon a beak-like outgrowth of the ovary 

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

 tribution. 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 the most perfect type of parachute (Fig. 338, c). 



(6) The Thistle Family, Carduaceae. — This is the largest 

 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 arraj^ of plants, many 

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

 nonia), thoroughwort (Eicpatorium) , blazing star (Lacinaria), 

 golden-rod (Solidago), aster, fleabane {Erigeron), cat's-paw 

 (Antennaria) , everlasting {Gnaphalium) , rosinweed and com- 

 pass plant {Silphium), Spanish needles (Bidens), daisy (Chrys- 

 anthemum), groundsel (Senecio), thistle (Carduus). Cultivated 

 for their showy flowers: Tickseed (Coreopsis), cone flower (Rud- 

 beckia), sunflower (Helianthus), cornflower (Centaurea) , ciner- 

 arias, etc. Medicinal: Wormwood (Artemisia), one species of 

 which yields absinth, milfoil (Achillea), colt's foot (Tussilago), 

 tansy (Tanacetum), chamomile (Anthemis), arnica, burdock 

 (Arctium). 



The Ragweed familj', Ambrosiaceae, is a small group, princi- 



42 



