Holloway. — The Prothallus and Young Plant of Tmesipteris. 13 



marked features of the growing head of the prothallus. Developing 

 antheridia in surface view are shown in fig. 14. There is a single 

 opercular cell at the apex of the protuberance, whose walls early become 

 brown in colour, thus defining the cell very clearly. This browning 

 soon extends to the walls and contents of all the outer cells on the free 

 portion of the antheridium. In the ripe antheridium the interior mass 

 of spermatocytes can clearly be seen in surface view. The antheridium 

 is emptied through the breaking-down of the opercular cell, the aperture 

 thus formed becoming enlarged in still older individuals by the breaking- 

 away also of those cells which adjoin the opercular cell. Thus the cha- 

 racteristic appearance of old antheridia all over the main prothallus-body 

 is that of brown cup-shaped structures projecting from the prothallus- 

 surface (fig. 14, &c). 



Fig. 14. — One of the large heads of prothallus shown in fig. 6, with antheridia in 



various views. X 52. 

 Fig. 15. — Small head of a prothallus, showing archegonia in various stages of 

 development, x 66. 



The young archegonium is first visible in surface view from the 

 division into four of its outej: cell and their arrangement quadrantwise. 

 At first, near the apex of the prothallus, this group of four cells is 

 colourless, but in older organs the cell-wails and the aperture of the 

 neck-canal between them becomes brown in colour, and the archegonia 

 are thus clearly defined in surface view (fig. 15). The neck of the arche- 

 gonium early projects from the surrounding epidermal cells, and is straight 

 rather than curved. Generally speaking, in older parts of the prothallus 

 the neck has broken short off, so that the characteristic appearance of 

 the group of four cells which surround the aperture of the archegonium 



