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A TEXT-BOOK OF BOTANY. 



the prothallus which is frequently dorsiventral and in a number 

 of cases somewhat heart-shaped, but varies considerably in out- 

 line, being sometimes more or less tuberous. The prothallus is 

 frequently but a few millimeters in diameter, and the cells usually 

 contain chloroplasts. On the under or ventral surface rhizoids 

 are usually present (Fig. 50, rh). The sexual organs usually 

 arise on the lower surface (Fig. 50), but they may develop on the 

 upper or dorsal surface or even laterally. A single prothallus 

 gives rise to both kinds of organs, unless stunted in its growth, 

 when it produces antheridia only. 



B 



FIG. 51. A, B, development of archegonia of a fern (Pteris) showing the neck (h), 

 the neck-canal cells (k), and oosphere (e). After Strasburger. 



C, development of antheridium in the Venus-hair fern (Adiantum Capillus-Veneris): 

 prothallus (p), antheridium (a), sperm (s), sperm mother cell with starch grains (b) ; 

 I, immature state of antheridium, II, sperms developed, and III, discharge of sperm mother 

 cells and escape of coiled and pluriciliate sperms. After Sachs. 



The antheridia either develop upon or are sunk in the tissues 

 of the prothallus. The archegonia (Fig. 51) are not flask-shaped 

 as in the Bryophytes. The venter containing the oosphere or egg- 

 cell (Fig. 51, c) is embedded in the thallus, the structure being 

 surmounted by a few-celled neck (Fig. 51, // ). The inner cells of 

 the neck are known as canal cells (Fig. 51, &), and these at the 

 time of ripening of the egg swell and exit through the opening of 

 the archegonium, through which then the sperms enter, one of 

 which unites with the egg, thus effecting fertilization. The fer- 

 tilized egg or oospore takes on a cellulose membrane. 



