GAMETOPHYTE AND SPOROPHYTE. 



245 



477. The gametophyte may develop directly from the tissue 

 of the sporophyte. — If portions of the sporophyte of certain of 

 the mosses, as sections of a growing seta, or of the growing 

 capsule, be placed on a moist substratum, under favorable condi- 

 tions some of the external cells will grow directly into protonemal 

 threads. In some of the ferns, as in the sensitive fern (onoclea), 

 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 protoiiema. In some other 

 ferns, as in asplenium(A. filix-foemina, 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 directh- 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. 

 Apogani)' 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. 



Fig. 323- 

 Apogamy in Pteris cretica. 



