226 



GENERAL BIOLOGY 



S VON 



. ; , Fig. 122. 

 I. The Fern Prothallium and Archegonium. 



A., stages in the germination of the spore. B., young prothallium, showing 

 first appearance of wedged-shaped, apical cell x. C., tip of prothallium beginning 

 to take on the heart-shaped form ; x., apical cell. D., mature prothallium, showing 

 group of archegonia on the cushion just back of the notch, and antheridia further 

 back: rh., rhizoids. E., an open archegonium with egg ready for fertilization, 

 and two sperms near the entrance of the neck. (A., B., C., E., after Campbell; 

 D., after Schenck.) 



II. The Antheridium and Sperms of a Fern (Onoclea) . 



A., small prothallium with many antheridia an. : s., old spore wall. B., 

 antheridium, showing cover cell c., ring cell r., and basal cell b., inclosing the sperm 

 mother cells. C., antheridium opening. D., sperms. (After Campbell.) 



III. Diagram of a cytological life-cycle, based on a hypothetical fern with four 

 chromosomes in the sporophyte. The nuclear phenomena are based on those of the 

 thread- worm (Ascaris) . Each chromosome is designated by a characteristic mark 

 so that it may be traced throughout the diagram. (After R. F. Griggs). 



o long vitality, and makes it possible for the embryo to develop so far 

 within its protective covering that it can take root and establish itself 

 r.eadily when the time is ripe. Then, too, the seed is a storage organ 

 oi condensed food for the embryo. 



j( uo rjrhe pollen grain of seed plants produces a male gametophyte which 

 bears either sperm or sperm nuclei. 



-ojjj-Jn the ovule of seed plants there is a megaspore which produces an 

 sac in which the egg is formed. The pollen grain produces an 



