THE NATURE OF ALTERNATION, ETC. 333 



prothallus usually occur first ; it may become thick and 

 fleshy and a cylindrical process may grow from the anterior 

 margin or the under surface. Tracheides develop in the 

 tissues of the prothallus before any other manifestation of 

 .apogamy. The apex of the cylindrical process may con- 

 tinue directly as the apex of a fern plant. In other cases 

 isolated members of the sporophyte (ramenta, roots, leaves, 

 sporangia 1 ) may be developed from the process, which 

 usually contains an axial strand of tracheides. In other 

 •cases (and sometimes in association with a cylindrical pro- 

 cess) shorter conical projections develop from or around 

 the sexual organs. One or many of these may give rise to 

 sporophytes. All these peculiarities have been observed in 

 prothalli, which would have produced normal embryos had 

 fertilisation been permitted. This induced apogamy con- 

 trasts with the cases of direct apogamy in which the 

 prothallus as soon as it has attained a certain size, and 

 apparently independently of the conditions, is able to pro- 

 duce a sporophyte apogamously. Intermediate examples 

 however occur which indicate that direct apogamy is only 

 a special case of the capability which many normal prothalli 

 show of producing the fern plant asexually. Apogamy may 

 in seems be induced in many, possibly in nearly all, ferns 

 by preventing fertilisation in prothalli growing under condi- 

 tions favourable to nutrition. The cause of direct apogamy 

 is still obscure. 



With regard to early references to apogamy, the work 

 of Wigand 2 must be mentioned. A careful study of his 

 paper and figures has led the author to the conclusion that 

 it is probable some of the prothalli he used in the course of 

 his investigations were apogamous. In this fact the ex- 

 planation of his opposition to Suminski's description of 

 the functions of antheridia and archegonia may be found. 

 The quotation from Wigand's paper given by Farlow 3 

 has, however, no reference to apogamy, but clearly 

 relates to gametophytic budding. Mercklin's observation 



1 Sporangia developed from the prothallus are as yet known only in 

 ■Scotopendrium vulgar e and Nephrodiu?n dilatatum. 



2 Wigand. 3 Farlow (1). 



