SPORE-PRODUCING MEMBERS 



to it, as in certain other Pteridophytes, by successive periclinal divisions. 



The structure of the sporangium as it approaches the stage of separa- 



tion of the spore-mother-cells and of the tetrad-division is shown in 



Figs. 249 and 250 : in the latter the relation of the vascular strands to 



the sporangium is already clearly indicated, especially of those which pass 



outwards in the thickness of the septum. The tapetum appears to be 



variable ; in O. vulgatum it consists of several ill-defined layers of cells. 



This is seen in Fig. 251 A, B, which illustrates the steps leading to tetrad- 



division in that species : the cells of the tapetum first lose their identity, 



their protoplasts fusing into a continuous plasmodium surrounding the 



sporogenous cells, while the nuclei 



persist, and apparently increase in 



number by fragmentation. The 



plasmodium penetrates between 



the sporogenous cells, the whole 



mass being first broken up into 



irregular blocks (Fig. 251 A), and 



later into individual cells (Fig. 



251 B). Normally all these spore- 



mother-cells undergo tetrad-divi- 



sion, and form spores. 1 When 



ripe each sporangium bursts by 



a horizontal slit, already defined 



structurally in the tissue of the 



wall : it gapes as the tissues dry 



up, but there is no mechanical 



annulus. 



The origin of the leaf in 



as Well as that Of 





FIG. 250. 



Ophjoglossum reticulatuin, L/. Longitudinal section 

 through a sporangium before separation of the spore- 

 mother-cells ; the walls of the vascular tissue are drawn 

 the fertile Spike Which it bears, in rather more heavily. Xioo. 



has been described by Bruchmann 



for B. Lunaria! 2 - He found that the fertile spike originates in the same 

 position relative to the sterile leaf as in Ophioglossum, but much 

 nearer to its apex; indeed, at the period when they may first be 

 recognised by their respective initial cells, these are in close proximity to 



1 The statement of Rostowzew that a large number of spore-mother-cells are dis- 

 organised, and contribute with the tapetum to the nutritive plasmodium, was adopted 

 by me in Studies, ii., p. 20, as it appeared to accord with my own rather limited 

 observations. A re-examination of the question, for which a number of slides of 

 O. vitlgatum were lent by Professor Farmer, has shown that this is an error ; as had 

 been already noted for O. reticulatum by Burlinghame (Bot. Gaz., July, 1907, p. 34). 

 Occasional cells may be disorganised (Fig. 251 B), as may happen in any large sporangia; 

 but normally there appears to be no systematic disorganisation, such as is seen in 

 or the Psilotaceae. 



2 Flora, 1906, p. 213. 



