PTERIDOPHYTA 



267 



protoplasts are liberated from their cells before the spores are in the tetrad 

 stage (Fig. 223). These protoplasts fuse to form a multinucleate Plas- 

 modium that surrounds groups of spore mother cells and contributes 

 nourishment to them. These groups break up just prior to the formation 

 of tetrads. The Plasmodium disappears as the spores mature. Each 

 sporangium produces a large number of spores. Dehiscence occurs by 



Fig. 223. Longitudinal section through portion of young fertile spike of Ophioglossum 

 californicum, showing groups of spore mother cells surrounded by multinucleate Plasmo- 

 dium derived from the tapetum, X 130. 



means of a longitudinal slit in Helminthostachys and by a transverse slit in 

 the two other genera. 



Gametophyte. In all the Ophioglossales the prothallium is subterra- 

 nean, saprophytic, and without chlorophyll. An endophytic fungus is 

 always present. The prothallium of Ophioglossum is cylindrical and 

 either simple or branched (Fig. 22-iC, D). It may reach a length, in 

 Ophioglossum vulgatum, of 6 cm. Rhizoids are wanting. The antheridia 

 and archegonia are scattered and intermixed. The gametophyte of 

 Botrychium is tuberous, dorsi ventral, and flattened (Fig. 224A). It 

 reaches a length, in some species, of 18 mm., in others, of only 3 mm. 

 The surface may be smooth or covered with rhizoids, according to the 

 species. The antheridia are borne on a median dorsal ridge, while the 

 archegonia, appearing later, form a row on each side of the ridge (Fig. 

 224B). In Helminthostachys the prothallium is somewhat similar to that 

 of Botrychium, but is more irregular and has a lobed basal portion that 



