XXI] GAMETOPHYTE 145 



The drawings not lettered in Fig. 433 are added to show the differences of 

 thickness and of segmentation seen in the sporangial stalks. 



The origin and structure of the young sporangium in the Osmundaceae 

 is thus seen to be more robust than in ordinary Leptosporangiate Ferns. This 

 may be connected with the greater nurhber of the spore-mother-cells pro- 

 duced, and consequently the greater potential 

 output of the spores. In Osniunda Russow long 

 ago estimated the number of spores in a single 

 sporangium as over 500, and assumed therefore 

 the number of spore-mother-cells to be 128 

 {V ergleicliende U liter sucliungen^ p. 87, 1872). An 

 estimate of the number of spores may be based on 

 the number of spore-mother-cells seen in a median 

 section (Fig. 434), which is 30 to 32. As the 

 sporogenous mass is approximately spherical and 

 the diameter of each cell about one-sixth that of 

 the whole sphere, the total number of spore- Fig.434. Sporangiumofaf;«//«r/<z 



^ ;v^<?//jcontainingalargesporo- 



mother-Cells would approach I 28. Actual count- genous tissue surrounded by a 



ingsofsporesshowed for ^.;;///W« figures midway ZTi.^lTS'iJ^J^^ 

 between 256 and 512, distinctly below Russow's von Goebel.) 

 estimate. In Todea Barbara the numbers are often nearer to 256: but in 

 T. superba and hymenophylloides, which are "filmy" in habit, the output is 

 still lower, approximating in the last-named species to 128. The bearing of 

 these facts will be discussed later: the main results readily accord with the 

 relatively robust structure and the variability of the sporangial development. 



The Gametophytei 

 The spores of the Osmundaceae contain chlorophyll, and soon lose their 

 power of germination, the first stages of which are variable. An apical cell 

 is soon established, as in most Leptosporangiate Ferns, but it is replaced 

 as a rule later by a series of marginal cells. A downward-projecting midrib 

 becomes conspicuous in the maturing thallus. The growth thus established 

 may be continued for years, and the thallus may attain a length of four 

 centimetres (Goebel). Forking occurs sometimes, and adventitious buds are 

 often found, as in the Marattiaceae. The prothalli, though of the cordate type, 

 are large, fleshy, and dark green in colour, resembling certain Liverworts. 

 Altogether they are more massive than those of ordinary Leptosporangiate 

 Ferns. The gametangia are essentially of the same type as these, but the 

 antheridia produce a larger number of spermatocytes in rough parallelism 

 with the large number of spores in the sporangium (see Vol. I, p. 292). The 

 archegonia are borne on the sides of the projecting midrib, and Campbell 



1 See Campbell, Mosses and Ferns, 3rd edn., pp. 346-356, 1918. 

 B. II 10 



