316 



THE PTERIDOPHYTES 



Both sexual organs (antheridia and archegonia) are found on 

 the same prothallium if it is well developed. But when pro- 

 thallia are crowded or grown under other unfavorable conditions 

 they remain small and stunted and become irregular in form 

 (Fig. 278, A). Such dwarf gametophytes only develop antheridia. 



FIG. 277. 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: z, apical cell. D, mature prothallium, 

 showing group of archegonia on the cushion just back of the notch, and anther- 

 idia 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 



On well-nourished prothallia the antheridia are formed first on 

 the edge and lower surface of the back portions. The arche- 

 gonia are developed last when the prothallium is quite large, and 

 are only found on the thickened region, called the cushion, 

 directly back of the notch, or growing point. 



The antheridia. The antheridia (Fig. 278, B, C) are very much 

 smaller than those of the bryophytes. They develop from a 



