Development of Sporangia upon Fern Prothalli. 257 



and the question arises whether their occurrence is to be regarded as 

 of morphological significance. They have been found in the pro- 

 thalli of a number of species of ferns, and, in every case investi- 

 gated, were associated with apogamy. In the case of Pteris cretica 

 the differentiation of the tracheides in the prothallus precedes the 

 origin of the bud.* This is the case also with the single sporangia 

 formed on the edge of the prothallus, and probably holds good for 

 the groups of sporangia borne on the process. But tracheides may 

 occur in the prothallus at a distance from the place of origin of 

 buds or sporangia. Putting aside the case of the middle lobe, the 

 prothallial nature of which is open to doubt, a large bundle of tra- 

 cheides was found in the substance of a fleshy prothallus of a variety 

 of Scolopendrium vulgare, which bore numerous archegonia on the 

 sarfaces immediately above and below the traeheides. Elongated 

 cells, which resemble sclerenchyma fibres, occur in the midrib of cer- 

 tain frondose liverworts. f A still more instructive example is 

 afforded by the presence of tracheides in the massive endosperm of 

 certain cycads. J This latter case shows clearly that such elements 

 may be formed in the gametophyte to meet a physiological need. Ib 

 seems inadvisable, therefore, to lay stress on the presence of tra- 

 cheides as a means of distinguishing between the two generations, 

 and the more so since their occurrence in a portion of the prothallus 

 which is about to bear a bud or sporangia can be recognised as a 

 physiological advantage. Such means of procuring a sufficient water 

 supply maybe a necessary preliminary to the development of a young 

 sporophyte or a group of sporangia. 



Lastly, it remains to consider the view to be taken of the presence 

 of the characteristic reproductive organs of the asexual generation 

 upon the gametophyte, and to consider its bearing upon the nature 

 of alternation of generations in the archegoniatse. Since the dis- 

 covery that in certain cases the one generation could arise directly 

 from the other without the intervention of the proper reproductive 

 organs, such cases have been used in support of the view that the 

 alternation in the Archegoniatse was homologous. On the other 

 hand, it has been maintained, both on grounds of the exceptional 

 nature of these cases of apospory and apogamy, and of comparative 

 phylogeny, that the distinction between the two generations was a 

 much deeper one ; that the alternation was not homologous, but anti- 

 thetic. || So far no case has been recorded in which the proper 

 reproductive organs of the one generation were situated upon the 



* Farlow, loc. cit., p. 269. 



f Goebel, Outlines,' p. 145. 



J I am indebted to Professor Bower for this unpublished fact. 



Pringsheim, ' Jahrb. f . Bot.,' bd. 9, p. 43. 



|| Bower, ' Annals of Botany,' vol. 4, p. 347. 



