Page:Proceedings of the Royal Society of London Vol 60.djvu/278

252 neck closed and branched. The central cell of the abnormal antheridia is arrested at a more or less early stage of development, while the cells of the wall and the base take on active growth.

The sporangia are either isolated or associated together in groups, which bear a striking resem blance to sori. They are borne upon the process or close behind it upon the true middle lobe, and are rarely found upon prothalli which have not produced a cylindrical process. W hen this is the case, they are always isolated and situated on the edge of a th in continuation of the prothallus arising from the apical depression.

Single sporangia occur frequently on the edge of the prothallus, which, as described above, crosses the base of the process. In a num ber of examples a single sporangium occupied a median posh tion, and, from earlier stages observed, it is probable th at it is to be traced back to the original grow ing point of th e prothallus. In other cases several sporangia were formed in this region. Isolated sporangia are also found on the process, but more frequently groups are met with. They occupy the upper or lateral faces of the process, and whenever sporangia in early stages of development are found, they are situated on its apex. I t is probable th a t the groups of older sporangia had become displaced from this position by the fu rth er grow th of the process. The groups were at a considerable distance from each other.

The relative positions of sporangia and sexual organs is a point of some interest, and was readily determined. Archegonia were present close to the sporangia, and at th e same level on the process. W hen the process, after producing sporangia, had continued its grow th, archegonia and antheridia were present on the portion beyond the sporangia, as well as on the older part, and, in cases in which more th an one group of sporangia had developed, the intervening region bore sexual organs. Rhizoids are also produced abundantly from the shaded side of the process, and, so far as external appearance is concerned, there is no reason to doubt the prothallial nature of the region on which the sporangia are situated.

The tissue underlying the sporangia, however, presents peculiarities in structure which may modify this conclusion to some extent. Beneath the single sporangia developed on the edge of the prothallus a few tracheides, w hich agree in every respect with those present in apogamous prothalli, were always to be found. Sim ilar elements were always present in the tissue beneath the groups situated on the process. It is possible th at here, as in the case of the sporangia upon the prothallus edge, the first tracheides are developed before the young sporangium can be recognised. All th at can be stated with certainty is th at they are already present beneath very young sporangia. The tracheides may become connected together into a