58 



PTEROID FERNS 



[CH. 



The key to the puzzle is found in the less common species, A. praestantis- 

 simmn Bory, native in the Antilles, figured in Hooker's Garden Ferns, 

 1862, PI. 58. Its habit is rather 

 coarse, with a thick upright stock, 

 simplypinnate leaves, and reticulate 

 venation oi\}[\&Litobrochia-\.y^&. The 

 leaves are dimorphic, but in many 

 specimens the sorus of the fertile 

 pinnae extends only part way from 

 the margin to the midrib. This and 

 the presence of an upper indusium 

 comparable with that of Pieris sug- 

 gested to Sir W. Hooker a com- 

 parison which has recently been 

 revived by Frau Eva Schumann 

 {Flora, 191 5, pp. 220, 243). She 

 indicates specially the relation to 

 P. splendens, and has added many 

 facts bearing on the comparison. 



Structure linking the fully Acro- 

 stichoid state with the P/^r/i--typeof 

 sorus may be found by examining 

 pinnae of .^. praestantissimnni from 

 the apex downwards. At the distal 

 end a purely P/^rz> structure appears 

 with well-marked indusial flap and 

 a narrow receptacle (Fig. 621, a): 

 lower down the receptacle widens, 

 and the sporangia are at first re- 

 stricted to it {U) ; but lower down .still 

 they encroach upon the free surface 

 of the pinna {c). Thus the fertile area 

 is not due merely to a widening of 

 the receptacle, as in P.podophylla, but 

 to an actual spread of the sporangia 

 on to the leaf-surface (Fig. 622). 



Such a comparison is insufficient 

 to determine affinity without struc- 

 tural evidence. 



Structurally Acrostichnin aiircuni is a remarkable plant in all its parts: 

 this may probably be related to a gross habit of growth in its brackish 

 habitat. The thick strut-roots have a bulky lacunar cortex surrounding a 



Fig. 621. a — f = successive sections of a pinna'ot 

 Ac7-ostichum praestantissinium Bory, from the apex 

 downwards, showing the gradual widening of the 

 Pteroid sorus inwards from the margin. ( x 20.) 



->^,-RecepL 



Fig. 622. Margin of a fertile ^^mnTiol Acrosticlnan 

 praestantissir)ium, showing in surface view the 

 indusium, receptacular commissure, and soral 

 area. ( x 10.) 



