163 
Apogainy in Ferns. 
We propose to give a detailed account of each of the different plants 
separately, reserving a general discussion of the results for the conclusion 
of the paper. 
i. A thyrium Filix-foemina var. clarissima , Jones. 
The fronds of this Fern, as has been known since the investigations 
of Mr. Druery and of Prof. Bower, produce aposporous prothallia in 
connexion with the sporangia, which latter never in this plant reach 
maturity, but their peripheral cells grow out directly into prothallial fila- 
ments. 
Two principal types of prothallium may be distinguished, firstly the 
filamentous or flattened form, which we shall speak of as the ‘ expanded 5 
form, and secondly a more tuberous or fleshy growth, which we shall call 
the ‘bulbous 5 prothallium (Fig. 17). The expanded prothallia were of far 
commoner occurrence in the conditions under which our cultures were 
carried on. They are delicate plants resembling ordinary prothallia with 
fimbriated edges, and bear antheridia and archegonia freely. But they 
nearly always fail to produce sporophytes, though we have succeeded 
in keeping them growing for several years. 
The bulbous prothallia, on the other hand, are far more prolific in 
sporophytes, and nearly all of those we have succeeded in raising have 
originated from prothallia of this type. 
Although the two types of prothallia are connected by intermediate 
forms, it will be convenient to treat their origin and structure separately. 
A. The Expanded type. Professor Bower has given an account of the 
development in his memoir of 1887. 1 Our observations confirm his 
statements and in some slight degree serve to extend them. The sporangia 
generally begin to show signs of abortion soon after the archesporium 
is delimited, but frequent exceptions were encountered in which the arrest 
of development was delayed until after several archesporial divisions 
had occurred. Any peripheral cell of the sporangium, and sometimes 
of the stalk, may grow out as a prothallial filament, but those near the 
sporangial tip were most commonly the seat of the new growth. 
The course of development is subject to some variation, but a fila- 
mentous structure is nearly always produced at first (Figs. 2, 3). 
We have followed the development of these earlier stages with special 
care, as it was expected that it might be possible to trace in them nuclear 
changes which would indicate a transition from the sporophyte to the 
gametophyte. Every stage of the nuclear division has been seen, both 
in the developing sporangia and in the prothallial outgrowth up to 
the four-celled stage of the latter, and the result has been to confirm 
1 Bower, F. O., On Apospory and Allied Phenomena. Trans. Lin. Soc., new ser., vol. ii, 1887. 
