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A TEXTBOOK OP BOTANY 



[Ch. VI, 7 



the egg cell is no longer naked, but, in obvious correla- 

 tion with the danger which would attend the exposure of 

 its delicate, succulent substance to dry air, is inclosed 

 within a protective cover, so constructed that when the egg 

 cell is ready for fertilization and the surrounding conditions 

 are favorable, the cover opens, and not only permits, but 

 facihtates, the access of the free-swimming sperm cell, which 



Fig. 218. — Sccual reproduction of a Fern ; X 340. 

 The structures occur on the under side of the sexual or prothallus stage, 

 close to the ground. On the left, a section of the prothallus in which the 

 egg cell is buried and covered by the tubular " archegonium." On the 

 right, the free-swimming sperm cells escaping from the "antheridium." 

 When the sex cells are ripe, the access of water causes both structures to 

 open ; the archegonium releases into the water a substance (malic acid) 

 very attractive to the sperm cells, which swim towards it, and enter the tube, 

 when the first to reach the egg cell fuses there^vith and effects fertilization. 

 (After L. Kny.) 



itself develops in a special structure (Fig. 218) ; and then the 

 developing embryo is supplied with food by the parent 

 plant. Here is evidently represented still another stage in 

 the evolution of sex, in which have been tleveloped, above 

 the earlier differences, special and ilifferent .structiu'cs, which 

 protect the sex cells in ways to facilitate the access of the 

 free sperm cell to the fixed egg cell. 



VI. The highest development of sex in plants is repre- 

 sented in the construction of the flower, as described in an 



