SPORE-PRODUCING MEMBERS 415 



to be noted that the origin of the tissue of the sterile septum, which 

 separates the sporangia, seems to be similar to that of the sporogenous 

 masses themselves. 



As the development proceeds, it is still difficult to recognise with 

 certainty the exact limits of the sporogenous masses : this is probably due 

 to the fact that there is no very clearly denned tapetum, nor is the whole 

 of the sporogenous mass used up in the actual formation of spores ; but a 

 considerable proportion of the cells- composing it, acting as a diffused 

 tapetum, become broken down, and disappear in a manner similar to that 

 to be described more in detail in Psilotum (Fig. 231). 



Finally, a strand of vascular tissue, of which the origin may be traced 

 in Figs. 230 A, B, is formed, extending upwards into the sporophyll ; 

 on entering the sporangiophore, it passes up to the base of the septum, 

 and there branches right and left, the two branch-bundles traversing the 

 margins of the septum. 



When mature, the wall of the synangium consists of a superficial layer 

 of deep cells, with thick cell-walls, a band of thinner-walled compressed 

 cells, three to four layers thick, supporting the superficial layer internally 

 (Fig. 230 c). These cells have pitted walls, and are not definitely 

 limited internally, but irregular tatters of cell-wall project into the cavities 

 of the sporangia, showing thus that there is no clear limit between the 

 wall of the sporangium and the tapetum. 



The septum shows in the main a structure similar to this inner band 

 of the wall, with which it is continuous; it consists of a firm plate of 

 narrow tabular cells, four to six layers in thickness, with profusely pitted, 

 woody walls. The septum is also coated by the remains of thinner-walled 

 disorganised cells. As already noted, the branches of the vascular bundle 

 which enters the sporangiophore pass right and left up the margin of the 

 septum ; these bundles are seen as bands of tracheides in transverse sections 

 through the lower part of the septum (Fig. 230 c.) ; the bundles are not 

 sharply differentiated from the surrounding tissues, and they appear to consist 

 only of xylem. A number of tracheides, continuous with the bundle, 

 extend along the central part of the septum ; and from the position of the 

 bundle, it appears to belong to the septum, rather than the external wall 

 of the synangium. The results obtained thus from radial sections have 

 been verified by comparison also of sections in other directions. 1 



An examination of the imperfect synangial sporangiophores which 

 occur at the limits of the fertile region shows them to be frequently 

 unilocular : this may be in some cases due to the abortion of one or 

 other of the two loculi, but in other cases it is clearly by imperfect 

 development of the septum between them, as vestigial remains of it may 

 often be found. It has been demonstrated that tissue which would nor- 

 mally develop as septum may in these cases develop as tapetum, or 

 even as sporogenous tissue (Fig. 231); and thus the unilocular condition 



.S7 //<//<., i.. p. 543. 



