AS SEEN IN PTERIDOPHYTA 



93 



margin. Fig. 47 illustrates how the series of cells of the archesporium 

 is continuous both upwards and downwards beyond the limit of its fertility : 

 this indicates a probability that its marginal limitation has been due to 

 sterilisation, a view which is strengthened by comparison with Sj>hagtn/ni 

 (Fig. 48) ; for there the archesporium is in the form of a complete dome : 



KK,. 49. 



Danaea elliptica, Smith. Drawings illustrating partial septations of the sporangium. 

 A, tangential section through three sori, showing the loculi in ground plan : the septa are 

 often thin, so that pairs of loculi are in close juxtaposition ; the loculi marked (.r) 

 are large, and show one or more partial septa. X 20. B, C-, D, E, show such loculi with 

 partial septa in greater detail : in D and E it is difficult to decide whether the i ells 

 marked (?) will develop as tapetum or as spore-mother-cells, x 150. 



a sterile condition of the cells at its apex would give a barrel-shaped 

 archesporium, as is seen in the Bryineae. Thus it will appear that any 

 evidence of sterilisation in the Musci is less direct than that in the 

 Hepaticae. 



Evidence of sterilisation of potential sporogenous cells is common in 

 the homosporous Pteridophyta : and, as in the Bryophytes, the function 



