THE SPORANGIUM 639 



diagnostic feature in Ferns, it requires special notice, and will be taken 

 first. In the synangial Marattiaceae and Pecopterids the opening mechanism 

 is very simple : a slit-like rupture is formed, and it gapes by drying up of 

 the adjoining cells, while the firmer region of the wall stands rigid. This 

 may be complicated by changes of form of the whole sorus, as in Marattia. 

 But in those early forms of Ferns in which the sporangia were separate, 

 there is commonly a band of mechanical tissue, composed of several rows 

 of indurated cells : this band varies in extent and in position in different 

 sporangial types. It has been stated by Scott, 1 referring to the annulus 

 of certain primitive Ferns which is more than one cell wide, that 

 " this was perhaps a general character of the annulate Fern-sporangia of 

 Palaeozoic age : at least no clear case of a uniseriate annulus has yet been 

 demonstrated." In fact, it seems that in the Primary rocks the distinctive 

 Leptosporangiate annulus was at least rare, if indeed it existed at all. 



It may be a question whether the more complex mechanism thus seen 

 in Eusporangiate Ferns is really the true correlative of that in the Lepto- 

 sporangiate type. A comparison of the indurated tissues in the sporangia 

 of Angiopteris and Gleichenia shows certain points of essential similarity, 

 though the details do not correspond. The firm resistant arch of indurated 

 tissue described in Angiopteris consists of cells of a similar nature to 

 those which form the annulus of Gleichenia ; its position is in the main 

 the same, though it does not stretch as a continuous hoop round the 

 back of the stalk, but stops short on either side of its base. When we 

 consider the similarity *of the sorus in these two genera, and of their 

 sporangia with the same orientation and dehiscence, the comparison of 

 these two bands seems inevitable, notwithstanding that the one is usually, 

 though not always, a single row, and the other a broad and ill-defined 

 band. But a further important fact is that among early Ferns of reputed 

 Leptosporangiate affinity the annulus is not always a single row of cells ; 

 this is seen in the Schizaeaceae, for Senftenbergia has an annulus of several 

 rows : Zeiller has shown that living species of Lygodium may have a 

 double-rowed annulus, which is an intermediate step to the type with 

 a single row. 2 Again, in living species of Gleichenia occasional cells of 

 the annulus have been found to be divided, showing thus a reminiscence 

 of a pluriseriate state. Such evidence favours the opinion that the simple 

 annulus is the correlative of the pluriseriate, and that a simplification of 

 its structure has accompanied the reduction in size and spore-output of 

 the sporangium in the course of descent. In the Leptosporangiate Ferns 

 the homogeny of the annulus seems the only view which is in accord 

 with the constancy of its occurrence in plants which are so clearly related 

 to one another. 



The position of the annulus and of the point of dehiscence appears to 

 have undergone change in the progressive evolutionary course. In the 



1 Progress us Rei Botanicae, i., p. 184. Compare Kidston, Phil, Trans. , Ser. B, 

 vol. 198, p. 188. -Bull. Soc. Bot. de France, xliv., p. 214. 



