348 BOTANY: PRINCIPLES AND PROBLEMS 



into the sac one of these unites with an egg. The fertihzed 

 egg now divides and grows into a young sporophyte, the embryo, 

 possessing a primitive root, stem, and leaves. The embryo soon 

 stops its growth and becomes dormant, embedded in the tissues 

 of the sac which are by now filled with reserve food or endosperm. 

 The integument in the mean time has developed into the tough 

 seed coat and the whole structure soon separates from the mother 

 plant as a mature seed. Under favorable conditions of tempera- 

 ture and moisture this seed will germinate, the embryo breaking 

 out through the seed coats and estabhshing itself in the soil 

 as a new plant which grows for a time at the expense of the stored 

 food but soon becomes independent. 



The Advances from Pteridophytes to Seed Plants. — The essen- 

 tial advances made by seed plants over the higher pteridophytes 

 are therefore: (1) The retention of the megaspore within the 

 megasporangium and its germination there into the female 

 gametophyte; (2) the enclosure of the sporangium and sac by a 

 new structure, the integument; (3) the transference of the 

 reduced male gametophyte directly to the vicinity of the female 

 gametophyte, to which the male gametes obtain access by another 

 new structure, the pollen-tube; (4) the development of the young 

 sporophyte in contact with, and at the expense of, the parent 

 sporophyte, and (5) its final release, dormant, well supplied with 

 food, and protected by a heavy coat. It is noteworthy that the 

 reversal of the reproductive situation as we find it in the bryo- 

 phytes is now complete, for instead of the sporophyte being 

 attached to the gametophyte, the gametophyte (and even the 

 succeeding sporophyte) is here attached to the parent sporo- 

 phyte. Indeed, in the most advanced types both gametophytes 

 are so much reduced that little beside the sexual cells remains, 

 and the alternation of generations has practically disappeared, 



The Flower. — The sporophylls of seed plants tend to be 

 arranged in distinct clusters on short branches. In the lower 

 members of the group these clusters are entirely similar to the 

 cones of some of the pteridophytes, but higher up a very special- 

 ized shoot, commonly called the flower, has been evolved. In 

 its fully developed form this contains not only the stamens and 

 carpels, but modified leaf -like structures for protection of, the 

 sexual organs and for attraction of insects. From the possession 

 of this structure the spermatophytes are sometimes called the 

 "flowering plants." 



