DEVELOPMENT OF PLANTS 473 



the tube of the corolla while the anthers form a cylinder about 

 the style and discharge their spores before the flower opens. The 

 style begins to elongate as soon as the flower opens and pushes 

 out the spores which may be seen as little piles of dust at the 

 top of the anthers in freshly opened flowers. The filaments of 

 this genus are sensitive to touch and curve outward, pulling down 

 the sheath of anthers, thus assisting in sweeping out the spores 

 and exposing the style when an insect irritates them in his quest 

 for nectar. This device appears in other genera and is sometimes 

 the only means of exposing the microspores and styles. Autog- 

 amy is at first impossible owing to the fact that the two lobes of 

 the style are closely pressed together effectually excluding the 

 microspores from its inner, stigmatic, surface. Later the style 

 grows considerably above the anthers and the lobes separate, ex- 

 posing the stigmas for crossing (Fig. 339, E). The association 

 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 efl^ected 

 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- 

 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 ex- 

 posed 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, h). During this period, the stalk 

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



