I40 



ASPLENIOID FERNS 



ASPLENIUM Linn. 

 The typical genus around which the Asplenieae are habitually grouped 

 is the Linnaean genus Asplenmm. But here, as frequently elsewhere, the 

 type that is nominally central raises rather than solves the problem of 

 phyletic origin. The distinctive features of the Ferns of this large genus 

 are that the sori arise as a rule singly on the fertile veins: they are linear or 

 linear-oblong in form, with the indusium opening so as to face the mid-rib 

 of the fertile segment (Fig. 669). Sometimes the sori may be hooked at the 

 end, as in Athyrium: or double, back to back, as they are typically in 

 Diplaziuin: or the indusium may occasionally open towards the margin of 

 the segment instead of inwards. All of these facts bear upon the problem 



Fig. 670. Sporangia of Aspleniuni Tricho- 

 manes L., after C. Miiller, showing the stalk 

 consisting of a single row of cells. ( x 140.) 



of their morphology. The sporangia are small, with a long stalk formed of 

 a single row of cells (Fig. 670). The spores are bilateral, and have a 

 perispore. These Ferns are herbaceous or firm in their texture, with creeping, 

 ascending or upright stock. The leaves are alternate or spiral. Lattice- 

 thickened scales are present. The axis is dictyostelic, with leaf-traces 

 sometimes of a single meristele, but consisting of two straps which usuall)' 

 fuse upwards (Fig. 671). The leaves may be simple, but they are usualh' 

 pinnatifid, and their branching may run to 4 or even 5 partitions. The 

 venation is usually free, but in some there may be intramarginal fusions 

 {Thamnoptetns Presl), or even reticulation {Asplenidictynvi J. Sm.). This large 

 genus comprises 429 species, and is cosmopolitan. The habit and vascular 

 structure readily accord with what is seen in the simpler types of Dryopteris, 



