POLLINATION 373 



tion. As with the young Fern, however, its independence is 

 soon established. In contrast to Gymnosperms, the most striking 

 features are connected with the very efficient arrangements for 

 the protection and nourishment of the developing embryos and 

 seeds, and the highly perfected mechanism for pollination by 

 virtue of which the most intimate relation often obtains between 

 the flower and the pollinating agent. Not all flowers, however, 

 are equally highly adapted, and almost all grades of specialisation 

 can be found in relation to pollination by the agency of insects, 

 wind, or water. The more advanced of the insect-pollinated 

 flowers usually exhibit a fusion of both sepals and petals, a 

 zygomorphic corolla, and a reduced output of pollen. In the 

 most extreme cases only one type of insect is effective in pollina- 

 tion, or is most efficient in the pollination of that particular 

 species. Wind-pollinated flowers are usually inconspicuous, pro- 

 duce an abundance of pollen, and often possess richly branched 

 stigmas. Water-pollinated plants, which like wind-pollinated 

 ones are commonly unisexual, exhibit floating devices either in 

 the female flowers or pollen. 



The pollen grains and embryo sacs of Angiosperms are ob- 

 viously comparable to the microspores and megaspores of Gym- 

 nosperms and Selaginella, and within them divisions take place 

 which lead to the formation of the male and female gametes 

 respectively. The contents of the embryo sac may therefore be 

 regarded as a female prothallus, and the contents of the micro- 

 spore as a still more reduced male prothallus. By those who 

 find such comparisons profitable, an alternation of generations, 

 analogous to that of Pteridophyta and Bryophyta, is thus 

 recognised. 



