238 THE SPORE-PRODUCING ORGANS [ch. 



Given this basal indusium, in Ferns with superficial sori, its modifications 

 so as to form the reniform type of Dryopteris (Fig. 236, C\ or the peltate 

 indusium of Polystichuni {N), are easily intelligible, and may be illustrated 

 by intermediate steps in Ferns of near affinity in other characters (von Goebel, 

 I.e. p. 1 1 50). These comparisons may be extended to include the conditions 

 seen in Didymochlaena, Athyrmiii, and perhaps even Asplenhim (Fig. 236, C). 

 But this origin of the peltate indusium of Polystichwn is probably quite 

 distinct from that of Matonia, however similar it may appear to be in form 

 and in function. 



Protection to the sorus is also afforded by overlapping of the leaf-margin, 

 which itself thins off to membranous texture. A simple case of this is seen in 

 Matteuccia intermedia, C. Chr., which may be held as representing a starting 

 point for the sequence Blechmim — Woodwardia — Scolopendriiim. The con- 

 dition o{ Matteuccia is that of a Fern with superficial, non-indusiate, basipetal 

 sori, covered over by the revolute margins of the pinnae (Fig. 226). Onoclea 

 is the same, except that a membranous basal indusium is present also 

 (Fig. 236, O). In the simplest types of Blechnum {B. discolor, tabulare, laiiceo- 

 latiim) the structure is as in M. intermedia, except that the rows of sori are 

 threaded together by vascular commissures, forming the fusion-sori already 

 described for them (Fig. 237, A). This is followed in the more advanced 

 types by an outgrowth of the upper surface of the pinna along the line of 

 greatest curvature, so as to form a flange on either side of the midrib. Steps 

 in its origin are suggested by sections of sori of various species, ending in 

 the broad-leaved type, such as B. brasiliense (Fig. 237, A—G). This flange 

 is a new formation, extended so as to add to the photo-synthetic area of 

 the pinna, and each is traversed by its own system of vascular strands 

 linked up with those of the rest of the pinna (Burck, 1874; Bower, Studies, 

 IV). Meanwhile, however, the fusion-sorus is enveloped by the original 

 leaf-margin, which is often erroneously described as an indusium. It will be 

 noted that it opens in the reverse direction from that of the Pterideae and 

 Davallieae. In them the edge of the flap is directed outwards : in Blechnum, 

 and its derivatives Woodwardia and Doodya, it is directed inwards. The fact 

 is that the so-called indusium of Blechnum is by origin a different thing from 

 the indusium oi Pteris. In the former it is a real leaf-margin which has slid 

 to a superficial position: in the latter it is a superficial indusium which has 

 in the course of evolution passed to a marginal position. 



The Acrostichoid condition, which is shown to have arisen along many 

 distinct phyletic lines, has points of special interest as regards the protection 

 of its sporangia. All the examples are ex-indusiate, though there is reason 

 to believe that many of them have sprung from an indusiate ancestry. All 

 have a mixed sorus, so that young sporangia are present after the leaf has 

 unfolded. And yet many of them grow in exposed situations. A prominent 



