Page:Proceedings of the Royal Society of London Vol 60.djvu/284

258 other w ithout the intervention of the vegetative organs. A t first sight such appears to be the case in the prothalli of the two species described; sporangia were present in close proxim ity to the sexual organs, the vegetative Organs of the sporophyte being, at most, represented. by a mass of cells underlying the group of sporangia, and even this distinction may not be recognisable beneath the single sporangia on the edge of the prothallus.

Several reasons may be adduced, however, against regarding these phenomena as evidence th a t the alternation of generations found in th e ferns is not antithetic. In the first place, it is to be noted th at the two form s in which sporangia have been observed upon the gametophyte are highly variable species, and th a t the varieties studied were well-m arked crested forms. F u rth er, the conditions under which the prothalli existed were in several respects unnatural. Among them ' the fact th at fertilisation was prevented by not w atering the cultures from above, and th at a prolonged grow th of the unfertilised prothalli was thereby induced, is of special interest, for it appears th at apogamy is liable to occur under such conditions in ferns which, as a rule, reproduce sexually. W hile these considerations do not of themselves preclude deductions being made from these peculiar forms of reproduction, they necessitate especial caution in their use in the discussion of broad morphological questions.

Further, a num ber of reasons exist for considering the production of sporangia on the prothallus as a special case of apogamy. In Sculopendrium vulgarea sporophyte may develope from the tip of the cylindrical process. This may happen after a group of sporangia has been developed. In one case two ram enta were present, one on each side of a group of sporangia; they were in every respect sim ilar to the ram enta which develope on the tip of the process when it is being transform ed into the apex of a bud. W henever a group of very young sporangia was seen it was situated upon the apex of the lobe, and the sporangia were in a more advanced stage of developm ent the farther th e group to which they belonged was removed from the apex. This has been most clearly seen in the case of Lastrcea dilatin which no buds w ith vegetative organs have as yet been seen, although in one case ram enta were associated w ith the sporangia, but it also holds for Scolopendrium. The explanation of these facts, w hich appears most probable, is th at each group of sporangia had occupied the apex of the process when very young, and had become farth er removed from this position as the process continued to increase in length. It is uncertain whether this growth is by direct continuation of the original growing point of the process, or w hether the development of a group of sporangia at the apex necessitates the formation of a new growing point; possibly both forms occur. If the latter be the case a process on which several