222 



THE SPORE-PRODUCING ORGANS 



[CH. 



Fis;. 2 



in the plane of the pinnule, and appears as though it were a minor lobe of 

 it. In some forms of H. repeiis it still curves over, and protects the sorus. 

 The lower, or abaxial, indusium is not 



always absent altogether, as in H. ni- b 



grescens. A vertical section through 

 the sorus, following the vein, shows 

 the receptacle extended and flattened 

 along the vein. At its basal limit a ves- 

 tigial lower indusium may frequently 

 be found {v.i., Fig. 217), but it is often 

 absent. When this is so, the charac- 

 teristics of the sorus, and indeed of the 

 whole Fern, are those of Dryopteris 

 {Polypodium) punctata (Thunbg.), C. 

 Chr. Many authors have remarked 

 how impossible it is to draw a line 

 between H. repens and D. pimctata. 

 But if the evolutionary sequence be 

 as described, and the lower indusium 

 is either reduced or absent, there is 

 no obligation to draw any line at 

 all between them, for they represent 

 stages in a natural progression. 



A sequence involving a " phyletic slide " of the sorus from the margin to 

 the surface of the pinnule may thus be traced, starting from a Dicksonioid- 

 Dennstaedtioid source with marginal sorus, a two-lipped indusium, conical 

 receptacle, and a gradate succession of sporangia. The sorus becomes mixed, 

 the receptacle flattened and curved downwards, the lower indusium reduced 

 and even abortive, the upper beingflattened out,and assimilated to the laminar 

 surface, while it is traversed by a vascular strand extended from the receptacle. 

 All of the Ferns involved in the series that shows these changes are rhizo- 

 matous and solenostelic, with undivided leaf-trace. They all bear hairs, not 

 scales ; and they have highly divided leaves with open venation. In fact the 

 series demonstrates, in Ferns that have always been recognised as akin by 

 habit, a transition from a marginal " Dicksonioid " sorus to a superficial 

 " Polypodioid " type. 



The case thus described at length for a series of nearly related Ferns 

 runs parallel with others distinct from them by Descent, which also show 

 the " phyletic slide " of the sorus from the margin to the surface of the leaf 

 It is well seen in the Pterids. Here, in Pteridium, the sorus originates as in 

 the Dicksonioid Ferns from the actual margin, the marginal initials forming 

 directly the receptacle (x) upon which the sporangia are borne, while the 



No\xn^^oxws,oi H. repens. « = sorus cut 

 vertically, showing the elongated receptacle; 

 «.?'. = upper indusium, traversed by a vascular 

 strand; z'.z. = vestigial lower indusium. (X15.) 

 (5 = a small part of the soral surface including 

 the vestigial indusium (v.i.), more highly 

 magnified. ( x 160.) 



