xxxviii] PTEROID DERIVATIVES 57 



P. {LitobrocJiia) podophylla Swartz, where the sorus presents in section a 

 broad expanse with a flat receptacle, beneath which Hes an extended vascular 

 commissure. The numerous sporangia, with various ages intermixed and 

 numerous paraphyses, are borne upon it, but still with some indication of a 

 gradate sequence of those earliest formed (Fig. 620). 



Fig. 620. Vertical section through the mature sorus of Pteris 

 [Litobrochia] podophylla Sw., showing the greatly widened 

 receptacle bearing mixed sporangia, but with some indication 

 of a gradate sequence. ( x 85.) 



ACROSTICHUM L. 



It is but a step from this state as seen in P. podopJiylla to those few Ferns 

 which remain under the greatly reduced designation of AcrosticJium L. The 

 best known species is A. {Chrysodimit) mireuni L., which is a large, rather 

 coarse-textured, leathery-leaved Fern, peculiar as growing in brackish water 

 throughout the tropics. The stock is massive and upright, and bears large 

 scales, while smaller scales are found on the leaves which are pinnate, the 

 lower pinnae being sterile, the upper fertile. The thick fleshy roots that 

 project from the stock may be hexarch, but the thin librous roots are diarch. 

 The leaves have reticulate venation with small hexagonal areolae, and no 

 free veinlets. The fertile pinnae usually have the whole surface occupied by 

 sporangia together with numerous protective hairs ; but there is no indusium, 

 and no indication of distinct sori. Diels, while referring this Fern to the old 

 genus Acrostichnvi L., remarks that little can be made out as to the affinity 

 of this isolated type. 



