488 THE CAMPANULALES 



of this type of flower is more effective than any as yet noticed, 

 and it would appear impossible for any of the numerous insects 

 which frequent these flowers to miss crossing a score of them at 

 a single visit. Crossing between the flowers of a head is effected 

 in some genera by the movements of the flowers. The inner 

 leaves of the involucre fold over the flowers at night and in rainy 

 weather, protecting them like a perianth. This results in crowd - 



FIG. 340. Inflorescence of the bur-marigold (Bidens), a common ray form 

 of flower of the Thistle family: in, involucre. 



ing of the flowers together, and the stigmas are often brought in 

 contact with the spores that have been dusted upon the various 

 parts of adjoining flowers. The outward curving of the stylar 

 lobes may also bring the stigmas in contact with the spore-covered 

 parts of adjoining flowers with a like result. Autogamy is 

 brought about in the dandelion, as in nearly all other members 

 of the order, by the curvature of the lobes of the style which 

 continue to bend back until the stigmatic surface is brought in 

 contact with the spore-covered style (Fig. 341, D). The bloom- 

 ing of the flowers progresses from the margin to the center of 

 the head so that during several days new sets of flowers are 

 exposed for crossing. When the period of bloom is passed the 

 involucre closes over the head and remains in this condition until 

 the fruit is mature (Fig. 338, &). During this period, the stalk 



