ASPLENIUM 



141 



though the sorus o{ Aspleniuui appears as a more difficult problem. A broad 

 outlook will have to be taken in its solution. 



Here, as elsewhere in the Ferns, the question of soral morphology is to be 

 studied in relation to the widening leaf-area. The narrow highly divided 

 Sphenopterid type of leaf, so common among Palaeozoic vegetation, was 

 probably the source from which most modern Ferns originated (see Vol. I, 

 Fig. 93, A and C: also Fig. 75). The simpler Dryopteroid and Asplenioid 

 Ferns still conform closely to that early type, as do also the Hymenophyl- 

 laceae. Primarily the Ferns appear to have been mesothermic hygrophytes, 

 as so many of those of the present day still are. The highly-divided leaf 

 presented a large proportion of surface to bulk of tissue, and this would 

 a 



P'ig. 671. rt = transverse section of the stock oi Asplenhim {Diplazitini) iiiai giiiatiDii L., a large 

 and sappy species. The dictyostele presents here three meristeles, with leaf-traces opposed to 

 each gap, each consisting of two vascular straps ( x 3). b = 2L similar section of the much smaller 

 Asplenium alattnn H.Br. Here the dictyostele and leaf-traces are similar, except that two straps 

 of each leaf-trace fuse laterally soon after their departure from the leaf-gap ( x 12). One root- 

 trace is seen in a, and two in h. 



be a successful proposition under ir.oist conditions. Rut exposure to the 

 dr)'er air of exposed sites would impose a more compact construction. An 

 increasing leaf-area, secured either by widening or by webbing of the leaf- 

 segments, would give not only an increased photo-synthetic area, but also 

 that decreased proportion of bulk to surface which exposed life demands. 

 Evidence of such widening and webbing, often accompanied by a thicker 

 texture, can be obtained by comparison of the representatives of almost any 

 large family of Ferns. The Asplenieae are no exception to this, and a striking 

 example is seen in the leaves of the huge epiphyte, A. nidus, where the 

 broad leathery leaves bear individual sori several inches in length (see 

 Vol. I, Fig. 52). So long as the leaf remains a general-purposes leaf, serving 



