274 PLANT LIFE. 



The male organ was called the antheridium, from the idea 

 that it was like the anther of seed-plants, which was once 

 supposed to be the male organ of the flower. There is no 

 special objection to the irame, but a more appropriate one for 

 it is the spermarv, since tliese male cells are known as the 

 spermatozoids or, briefly, the sperms. 



The final step in the development of sexuality is the restric- 

 tion of the formation of sex organs to a certain phase in the 

 life history of the plant, which is therefore known as the 

 sexual phase, or gametophyte, the remaining phase or phases 

 being called, for the sake of distinction, non-sexual, and con- 

 stituting the sporopliyte. The gametophyte alternates with 

 the sporophyte, giving rise to the phenomenon known as the 

 "alternation of generations." (See 'j 55. ) 



378. Directive agents. — To secure the union of the male 

 and female cells, the male gamete must be directed to the 

 female. PSy what means this is accomplished is not fully 

 known. Organic acids and sugar exercise such an influence 

 on certain sperms that they swim towards the source of these 

 substances. The wide distribution of such compounds sug- 

 gests that probably their presence in the female gamete may 

 render it attractive. If tliis is true, the sperms exhibit a spe- 

 cial irritability towards these materials, whose diffusion acts 

 as a stimulus. 



Isogamy. 



Sexual reproduction, as de\'eloped among existing plants, 

 shows two main types, known as isogamy and Iiclcrngaiiiv. 



379. Isogamy is that mode of sexual union in «hich the 

 size and form of the gametes is alike. In some cases tire 

 behavior also of both male and female is alike, while in others 

 the male shows a greater power of movement. "When both 

 are eipially motile and escape from the cell, conjugation occurs 

 wherever they happen to come in contact. The form is usually 



