60 Apogamy and Development of Sporangia upon Fern Prothalli. 



possessed by the gametophyte. Their possible importance in relation 

 to the theory of homologous alternation appears to the writer to be 

 of this nature. If that theory be true, the sporophyte and gameto- 

 phyte are modifications of a similar form. The gametophyte, espe- 

 cially the simple free-living prothallus of the Ferns, has departed 

 less widely from that form. Such an organism as a fern prothallus 

 would therefore appear to be suitable for experimental work, in the 

 hope that its behaviour under altered conditions would afford hints 

 as to the sort of changes which, in the original algal form, led to 

 the evolution of the sporophyte. The altered conditions in this 

 series of experiments are of a similar kind to those which are 

 assumed by Professor Bower to have occurred on the spread of 

 algal forms to the land, and to have conduced to antithetic alter- 

 nation. 



The results may now be used in picturing the manner in which 

 alternation of generations might have come about by the modifica- 

 tion of originally similar individuals into gametophyte and sporo- 

 phyte. It is assumed for this purpose that the sporophyte of the 

 vascular cryptogams did not arise by the elaboration of a structure 

 resembling a bryophytic sporogonium. It is recognised that the 

 theory of antithetic alternation, as elaborated by Professor Bower, 

 affords a consistent and satisfactory explanation, if the assumptions 

 necessitated by the theory are granted. The p'resent theory, which 

 is put forward merely as a provisional hypothesis, is founded on 

 another class of facts. 



With the spread of algal organisms to the land, where in the 

 absence of any vegetation affording shade, some at least would be 

 exposed to more intense illumination, the flattened form would 

 probably be assumed. Prolonged drought and the influence of direct 

 sunlight, inducing directly a change of form into a cylindrical 

 body, might be accompanied by the substitution of a reproductive 

 organ forming dry reproductive cells (spores) for those adapted to 

 an aquatic existence. The acquisition of more highly developed 

 absorbent organs (primitive roots) would further the existence and 

 growth of this modified gametophyte. This spore-producing stage 

 would at first follow the sexual stage in any individual exposed to 

 dry conditions. It is possible to imagine, however, how the asso- 

 ciation of the asexual with the sexual individual might come about. 

 Absence of fluid water would prevent the liberation of motile spores 

 from the zygote. The latter would be obliged to germinate in situ, 

 and the fact that it did so under dry conditions would tend to the 

 shortening of the sexual stage, and the speedy assumption of the 

 sporophytic form and mode of reproduction. From the spore, which 

 would always separate from the parent, a sexual individual would 

 arise, since germination could only take place in a damp spot. AS 



