GAMETOPHYTE AND SPOROPHYTE. 



245 



when the fertile leaves are expanding into the sterile ones, proto- 

 nemal outgrowths occur among the aborted sporangia on the 

 leaves of the sporophyte. Similar rudimentary protonemal 

 growths sometimes occur on the leaves of the common brake 

 (pteris) among the sporangia, and some of the rudimentary spo- 

 rangia become changed into the protonema. In some other 

 ferns, as in asplenium(A. filix-fcemina, var. clarissima), prothallia 

 are borne among the aborted sporangia, which bear antheridia 

 and archegonia. In these cases the gametophyte develops from 

 the tissue of the sporophyte without the intervention or necessity 

 of the spores. This is apospory. 



478. The sporophyte may develop directly from the tissue 

 of the gametophyte. — In some of the ferns, Pteris cretica for 

 example, the embryo fern sporophyte arises directly from the tissue 

 of the prothallium, without 

 the intervention of sexual 

 organs, and in some cases 

 no sexual organs are de- 

 veloped on such prothallia. 

 Sexual organs, then, and 

 the fusion of the spermato- 

 zoid and egg nucleus are 

 not here necessary for the 

 development of the spo- 

 rophyte. This is apogamy. 

 Apogamy occurs in some 

 other species of ferns, and 

 in other groups of plants as well, though it is in general a rare 

 occurrence except in certain species, where it may be the general 

 rule. 



479. Perhaps there is not a fundamental difference between 

 gametophyte and sporophyte. — This development of sporo- 

 phyte, or leafy-stemmed plant of the fern, from the tissue of the 

 gametophyte is taken by some to indicate that there is not such a 

 great difference between the gametophyte and sporophyte of plants 

 as others contend. In accordance with this view it has been 



Fig. 323- 

 Apogamy in Pteris cretica. 



