XXXVIII] PTERIDIUM 43 



in Vol. I, p. 4 (Fig. 3)'. It may be regarded as consisting of a highly perforated 

 solenostele, represented by the outer ring of meristeles, and a medullary 

 system consisting of a variable number of tracts, usually two, disposed as an 

 inner ring. The connections of these at the departure of a leaf-trace are 

 shown somewhat diagrammatically in Fig. 607, from which it is seen that both 

 the inner and the outer systems contribute to the leaf-trace, of which the 

 constituent strands are disposed in the usual horse-shoe, while a compensation 

 strand (c.s.) connects the inner system with the outer above the leaf-insertion. 

 This is clearly a complex and apparently an advanced type of construction. 

 It may be interpreted as consequent on relatively large size, a double 

 solenostele being affected by numerous and large perforations. Size will not 

 explain this altogether, but the fact that the dimensions are large is probably 

 a real factor in the comparative problem. Thus the external features seem 



Fig. 608. PleridhDii aqtnlinum. Part of the margin of 

 a fertile segment of the second order, greatly enlarged. 

 r= receptacle, after removal of the sporangia, whose 

 positions ai-e indicated by scars; /rt = outer, «= inner 

 indusium, the cellular structure of each being indicated 

 to the left of the drawing. (After Luerssen.) 



to mark the Bracken as relatively primitive: the vascular anatomy 

 suggests advance. We shall naturally examine the soral characters with 

 enhanced interest, since these bear a special importance as evidence in 

 such questions. 



Of all the Pteroids the Bracken has the most elaborate sorus in that there 

 are two well-formed indusial lips between which lies the receptacle (Fig. 608). 

 The sorus is continuous along the margin of the pinnules, often for consider- 

 able distances; but its continuity is apt to be broken irregularly. Detailed 

 examination shows that the receptacle is traversed by a vascular commissure, 

 which runs beneath the insertion of the sporangia, and links together the 

 endings of the otherwise free veins. In all of these structural facts Pteridmm 

 corresponds to Li7idsaya. Speaking of the Section Paesia St Hilaire, in 

 which he included Pteridium, Sir W. Hooker remarks that "According to 



1 Unfortunately this figure was inverted in the setting: the lower side of the rhizome being placed 

 upwards on the page. 



