Page:Proceedings of the Royal Society of London Vol 60.djvu/283

Rh and the question arises w hether th e ir occurrence is to be regarded as of morphological significance. They have been found in the proth alli of a num ber of species of ferns, and, in every case investigated, were associated w ith apogam y. In th e case of Pteris the differentiation of the tracheides in th e prothallus precedes the origin of the bud.* This is the case also w ith the single sporangia formed on the edge of the prothallus, and probably holds good for the groups of sporangia borne on th e process. B ut tracheides m ay occur in the prothalius at a distance from the place of origin of buds or sporangia. P u ttin g aside the case of the m iddle 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 num erous archegonia on th e surfaces im mediately above and below the tracheides. E longated cells, which resemble selerenchym a fibres, occur in the m idrib of certain frondose liverw orts.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 th a t such elem ents may be formed in the gam etophyte to m eet a physiological need. I t seems inadvisable, therefore, to lay stress on th e 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 o r sporangia can be recognised as a physiological advantage. Such means of procuring a sufficient w ater supply may be a necessary prelim inary to the developm ent 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 archegoniatae. Since the discovery th at in certain cases the one generation could arise directly Irom the other w ithout the intervention of the proper reproductive organs, such cases have been used in support of the view th a t the alternation in the Archegoniatae was homologous. § On the other hand, it has been maintained, both on grounds of the exceptional nature of these cases of aposporyr and apogamy, and of com parative phylogeny, that the distinction between th e two generations was a much deeper o n e; that the alternation was not homologous, but antithetic. || So far no ease has been recorded in which the proper reproductive organs of the one generation were situated upon the *§

t Goebel, ‘ Outlines,’ p. 145. + I am indebted to Professor Bower for this unpublished fact. § Pringsheim, ‘ Jahrb. f. Bot.,’ bd. 9, p. 43. II Bower, ‘ Annals of Botany,’ vol. 4, p. 347
 * Farlow, loc. cit,, p. 2G9.