346 



MORPHOLOGY. 



684. The gametophyte may develop directly from the tissue 

 of the sporophyte. — If portions of the sporoi^hyte of ct-rtain 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 Avill 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 rudimeulary pr(jtonemal 

 growths sometimes occur on tlie lea\"es of the conmion brake 

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

 rangia become changed into the jirotonema. In some other 

 ferns, as in aspleniuni( A. filix-fcemina, \ar. clarissima ), prothallia 

 are borne among the aborted sjiorangia, which liear anthcridia 

 and archegonia. In these cases the gamctophyte de\'elops from 

 the tissue of the sporophyte without the intervention or necessity 

 of the spores. This is iipospuiy. 



685. The sporophyte may develop directly from the tissue 

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

 example, the embryo fern spjoroiihyte arises dire<'th' 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- 

 rojihyte. This \^ apngamy. 

 Apogamy occurs in some 

 other species of ferns, and .ApoK.imy in I'm-is ci-ctio. 



in other grou])s of plants as well, though it is in general a rare 

 occurrence except in certain spi-cies, where it may be the general 

 rule. 



