[Chap. L THE SEED PLANTS 689 



stamens and carpels. The floral organs are of many forms, colors, and 

 arrangements ( Chapter XXXII ) . 



The life cycle. The earlier chapters of this book include discussions of 

 the physiological processes, the structures, reproduction, and heredity 

 of flowering plants. The life cycle of angiosperms may now be briefly 

 compared with that of other great groups of green plants. As would be 

 expected, the details of the life cvcle are not the same in all plants. 



The sporophvte. The familiar plant of angiosperms, such as tulip, 

 wheat, or maple, is the sporophyte. Leaves, stems, roots, and flowers 

 are conspicuous in most species. A perianth, usually composed of calyx 

 and corolla, surrounds the microsporophylls (stamens) and the mega- 

 sporophylls ( carpels ) . 



The microspores form in the anther and become pollen grains upon 

 nuclear division. The dehiscence of the pollen sacs liberates the pollen, 

 and pollination may be effected in various ways. Just prior to being 

 shed, or sometimes shortly after, the pollen grain contains a tube nucleus 

 and Hvo sperms formed from a generative nucleus. The pollen with 

 the pollen tube is the male gametophyte. 



The megaspores form in the megasporangia (ovules), which are en- 

 closed in ovularies. Each ovule ordinarily has one megaspore which upon 

 nuclear division becomes the embryo sac. Subsequent nuclear divisions 

 result in an embryo sac with eight nuclei, one of which becomes the 

 egg; two unite and form a fusion nucleus, and the remainder usually 

 disintegrate. The embryo sac is the female gametophyte, but no struc- 

 tures comparable to archegonia occur. 



Fertilization. After pollination and germination of the pollen grain on 

 the stigma, the pollen tube penetrates the tissues of the style and enters 

 the ovule, often through the micropvle. When the pollen tube enters the 

 embryo sac and bursts, the two sperms are released and two fusions 

 occur: one sperm unites with the egg, resulting in a fertilized egg or 

 zvgote; the other unites with the fusion nucleus, forming the triple- 

 fusion nucleus. 



The embryo. The zygote germinates, and from it an embryo or voung 

 sporophyte develops. In some seeds an endosperm develops around the 

 embryo from the triple-fusion nucleus. The growth and hardening of 

 the ovule coats (integuments) complete the formation of the seed. 

 The embryo consists of a hypocotyl, a plumule, and cotyledons. 



Chromosome constitution. Reduction division in most angiosperms 

 occurs during the formation of megaspores and microspores. Each of 



