570 MOSSES AND FERNS chap. 



but more recently has been advocated by Scott (3), Coulter 

 (i), and others. This view maintains that the sporophyte 

 arose as a modification of the gametophyte, and not as an essen- 

 tially new structural type. The homologous theory of alterna- 

 tion is based largely upon the phenomena of apospory and 

 apogamy, and also, to a lesser extent, upon experiments in 

 regeneration. Pringsheim showed that the protonema of a 

 Moss might arise from the cut end of the seta, as well as from 

 the tissues of the gametophyte, a case of apospory, but as yet 

 there are no instances known of the converse, i. e., the origin 

 of the sporophyte in the Mosses by apogamy. Pringsheim 

 believed that the protonema is not essentially different from the 

 vegetative tissues of the sporophyte from which it might be 

 made to develop, and that thereforfe no line can be drawn 

 between strictly gametophytic and sporophytic structures. It 

 must be remembered, however, that the protonema normally 

 develops from certain sporophytic cells (spores), and its devel- 

 opment under abnormal conditions from other sporophytic tis- 

 sue is not inexplicable. It is, moreover, a significant fact that 

 the cells of the seta, from which the protonemal filaments arise, 

 a fact which Pringsheim himself recognises, correspond in posi- 

 tion to the sporogenous tissue of the capsule, and are probably 

 homologous with them. The phenomenon of apospory in cer- 

 tain Ferns is comparable to that in the Mosses, and recently Lang 

 (4) has been able to induce in Anthoceros a development of 

 structures which seem to be rudimentary gametophytes. The 

 origin of these in all cases was not clear, but they seemed usually 

 to arise from the outer tissues of the sporophyte, and not from 

 the sporogenous layer. Stahl ( i ) also found that protonema- 

 formation might arise from the parietal region of the capsule 

 in Ceratodon. 



The strongest argument in favor of homologous alterna- 

 tion is the phenomenon of apogamy, or the origin of the sporo- 

 phyte as a vegetative bud upon the gametophyte, and apospory, 

 or the origin of the gametophyte by budding from the sporo- 

 phyte. Apogamy has been observed in a number of species 

 of Ferns belonging to the Polypodiacese, Hymenophyllacese, 

 and Osmundaceas. How far apogamy may be considered a 

 natural phenomenon, and how far it is a pathological condition 

 induced by artificial means, needs further elucidation. It 

 undoubtedly in some species like Pteris cretica entirely supe^- 



