514 A TEXTBOOK OF BOTANY [Ch. XII] 



single nucleus. They are normally one to a megasporan- 

 gium, which is the nucellus, the coats being a new growth 

 apparently not represented in Pteridophytes unless by the 

 indusium in Ferns. Although but one megaspore develops, 

 four are usually formed, as in Selaginella, in connection with 

 the reduction division, but three are aborted. The micro- 

 spores are the pollen grains, in the early stage with a 

 single nucleus. They also are formed in tetrads in micro- 

 sporangia, which make the anthers. The sporophyUs are the 

 carpels and stamens, which are megasporophylls and mi- 

 crosporophylls respectively (page 327), though it is still 

 uncertain whether petals and sepals represent sterilized 

 sporophylls or specialized foliage leaves. The gameto- 

 phytes, advancing a stage beyond Selaginella, develop 

 wholly inside the spores, in great contrast to the true Ferns, 

 where the gametophyte grows out from the spore, turns 

 green, and becomes the independent prothallus. The game- 

 tophyte produced by the megaspore is represented by the 

 organized contents of the embryo sac, appearing ultimately 

 as the food substance or endosperm, which is therefore the 

 prothallus, while the archegonium, no longer necessary in 

 this situation, becomes rudimentary and finally disappears, 

 leaving the egg cell only. The gametophyte produced by 

 the microspore is represented by the organized contents 

 of the pollen grain after the internal cell divisions, which 

 in some forms , produce rudimentary prothallial cells (Fig. 

 366), and always two sperm nuclei which are carried by the 

 pollen tube to the embryo sac. 



Thus the gametophytes in Spermatophytes are reduced 

 very greatly from their former high estate as an inde- 

 pendent generation in the true Ferns, and as the main plant 

 in all groups below them, while the sporophytes, originally 

 the dependent spore cases in lower forms, have become 

 stately green plants (Fig. 361). This remarkable overturn 

 in the relative importance of the two generations appears to 

 have resulted from the fact that the vertical position of 



