SUMMARY OF THE SPERMATOPHYTES 401 



3. The development of the pollen tube from the microspore 

 (pollen grain) as a result of the habit of pollination, by which 

 microspores enter the micropyles of the ovules in gymnosperms, 

 and fall upon a receptive structure, the stigma, in angiosperms. 



4. The degeneration of the male gametophyte until it is 

 hardly more than a group of nuclei, with accompanying pro- 

 toplasm, in the pollen grain and its tube. The degeneration of 

 the motile sperms until they are represented by two sperm 

 nuclei alone (cycads and Ginkgo excepted, Sec. 348), which are 

 carried by the pollen tube into the embryo sac. 



5. The development and retention of the embryo sporophyte 

 within the embryo sac, and the ripening of the ovule into 

 the seed. 



6. The massing of the sporophylls on the shoot, accompanied 

 by envelopes which constitute the perianth of the flower. The 

 development in the angiosperms of the megasporophyll, or carpel, 

 into the simple pistil, and the grouping of carpels through zonal 

 growth (syncarpy) to form the compound pistil, so that the 

 ovules become inclosed in an ovule case (ovary). The differ- 

 entiation of a receptive surface, the stigma, on the pistil upon 

 which the pollen grain may germinate. 



7. The differentiation of the parts of the perianth into sepals 

 and petals, and their grouping through zonal growth, together 

 with the stamens, to give perigyny, epigyny, sympetaly, syn- 

 sepaly, and epipetaly. The development of bilateral symmetry 

 and dorsiventrality. 



8. A general development of the sporophytes in many par- 

 ticulars, giving them much greater complexities of tissue struc- 

 ture, growth, and form than those of the pteridophytes. 



