THE DICOTYLEDONES 



1961 



autumn, and in October it is not uncommon to see the flowers covered with 

 Tortoiseshell, Red Admiral and Peacock butterflies. 



As an example of the pollination mechanism in the tribe Helenieae we 

 may describe the condition in Chrysanthemum segetum (Fig. 1890) (Corn 

 Marigold). The diameter of the golden-yellow head in this species is 

 nearly 5 cm., of which about one-third is occupied by the disc. The twelve 



Fig. 1890. — Chrysanthemum segetum. Corn Marigold. A, The blunt ends 

 of the stigmatic lobes act like a piston in extruding the pollen. 

 B, Later stage, with stigmatic lobes expanded and receptive. C, 

 Female ray floret. 



to sixteen ray florets are female and the stigma projects only slightly from 

 the corolla tube. There are about 300 disc florets each with a corolla tube 

 surmounted by a bell-shaped corolla. In the first stage of anthesis the style, 

 covered with pollen, projects beyond the corolla bell, while later, after the 

 pollen has been distributed, the branches of the style separate to expose 

 the stigmatic papillae. The mechanism therefore in this example is essen- 

 tially similar to that in Aster. 



The pollination mechanism in the tribe Arctotideae is interesting. Small 

 and von Minden investigated this case and found that the style is sensitive 

 to touch. The capitulum consists of about half a dozen rings of male 

 florets which are tubular and in which the style functions only as a pollen 

 presenter, there being no functional ovary. Outside this there is a ring of 

 hermaphrodite tubular florets which may be partly male and partly female, 

 for the fertility varies in difl^erent florets. Outside this again are true 

 hermaphrodite florets which can all produce viable seeds. There is one 

 row of ray florets which are ligulate and possess no stamens. 



In the first stage in anthesis the style emerges from the tubular corolla. 

 In good growing conditions this takes about five minutes. The upper part 

 of this style is covered on the outside with pollen. If this region is touched it 

 reacts rapidly, turning in the direction of the impact. It soon regains its 



