HERBACEOUS AND WOODY DICOTYLEDONS 413 



of the flower already described and with the relation of the anther 

 tube and the stigmatic surfaces in young and old flowers. The 

 flowers are protandrous, and in young flowers the pollen is shed 

 while the style is short and the stigmatic surfaces closely approxi- 

 mated (Fig. 262, a). As the flower matures the style elongates 

 and the stylar brush pushes the pollen .out of the anther tube, 

 where it may come in contact with an insect's body passing over 

 the flower. When the end of the style and the stylar brush 

 emerge from the corolla tube, the two halves into which the tip 

 is divided separate, and the stigmas are exposed for pollination 

 with pollen from another flower (Fig. 261, D, and Fig. 262, 6, c). 

 In case cross-pollination is not effected self-pollination may be 

 brought about by pollen falling from the stylar brush upon the 

 outer portion of the stigmatic surface in the same flower. 



Fruit. The fruit is an achene, that is, a ripened ovary with 

 one inclosed seed. In the yarrow there is no special device for 

 seed dissemination, but in many of the Compositae, such as 

 the dandelion (Taraxacum), Spanish needles (Bidens), and 

 thistle {Carduus), special outgrowths in the form of the pappus 

 or of spines facilitate the wide dispersal of seeds by animals or 

 wind (Fig. 262, c, d). The large number of flowers that are so 

 perfectly adapted to cross-pollination, and the special devices for 

 disseminating the seeds, enable the Compositae to spread and to 

 increase with extreme rapidity wherever opportunity is offered. 



4 



DICOTYLEDONS AND MONOCOTYLEDONS 



The angiosperms which we have studied thus far have all 

 belonged to the dicotyledons, or plants like the bean, mandrake, 

 and marigold, which have two cotyledons in the embryo. The 

 monocotyledons have but one cotyledon in the embryo, a 

 condition which is supposed to have arisen by the loss of one 

 cotyledon in the ancestral dicotyledons. The close relationship 

 of the two groups is further emphasized by the great similarity 

 between them in the structure of the flower and in the game- 

 tophyte plants resulting from the germination of the spores, and 

 also by the occurrence of species with intermediate characters. 



