OSMUND ACEAE 535 



is believed to be truly comparable to the " central cell " of the Marattiaceous 

 sporangium. 



The archesporial cell thus surrounded by the tissue which will form 

 the sporangial wall undergoes segmentation to form the tapetum (Fig. 296 K 

 and F), and the irregularity seen in other segmentations is fully maintained 

 in this also. From a comparison of Figs. 296 F and o it will be seen that 

 sometimes the segmentation of the tapetum is almost according to the 

 ordinary Leptosporangiate type (Fig. 2960); the most interesting cases are, 

 however, those in which the archesporium is truncate at the base ; in 

 these it appears that the cell or cells below it contribute to the completion 

 of the tapetum (Fig. 296 F) ; this is a point of some special interest, for one 

 of the most constant differences between the Leptosporangiate and 

 Eusporangiate Ferns has been in the origin of the tapetum : but Todca 

 occasionally shows an intermediate condition. The division of the tapetum 

 into two or partially three layers follows 

 (Figs. 296 F to i), together with the sub- 

 division to form the sporogenous group ; 

 beyond this point it will be unnecessary 

 to trace the development of Todea barbara. 

 In conclusion the drawings not lettered in 

 the centre of the group are added, as 

 showing differences of thickness and seg- 

 mentation of the sporangial stalk ; these 

 are both from sporangia, of the age of 

 Fig. 296 H. 



An abnormality, which has been ob- FlG - 2Q6 ''' 



served occasionally in Todea barbara, has """"trif^ction. s> x^ g . iuni> '" 

 been found not unfrequently on certain 



fronds of Osmunda regalis, vi/. the occurrence of synanagia : one of these 

 is represented in Fig. 296 bis : each half shows, except on the side where 

 it is coherent with the other, the ordinary sporangial structure. Such 

 developments are no very surprising consequence of the origin of two 

 sporangia close side by side, with a deeply seated central cell ; normally, 

 each develops separately, but the obliteration of the groove between them 

 would give a synangial structure, and clearly this is not a very great 

 modification. But what interpretation is to be put upon such cases, 

 whether they may be counted as retrogressions or the reverse, must for 

 the present be left open. 



The more robust and massive origin and structure of the sporangia 

 of the Osmundaceae than of ordinary Leptosporangiate Ferns is to be 

 connected with the greater number of spore-mother-cells produced, and 

 consequently the greater potential output of spores. In Osmunda Russow 

 long ago estimated the number of spores in the single sporangium as over 

 500, and assumes therefore the number of spore-mother-cells to be I28. 1 



1 Vcrgl. UnUrs., p. 87. 



