XXXVIII] ACROSTICHUM 59 



stele with as many as six protoxylems, a feature related to size; for the 

 small roots of the sporeling are diarch. The stem has a complex vascular 

 system extending upwards into the highly segre- ,|i ^j^/|j|| 



gated leaf-traces (Thomas, New Phyt. IV, 1905, 

 p. 175; Frau Eva Schumann, /^Zc?r(T, 191 5, Bd. 108, 

 p. 211). The adult axis is traversed by a three- 

 angled dictyostele with a complex medullary 

 system. This has lately been brought into relation 

 with a like structure in the larger Pteroid stems 

 by the observations of Prof. J. McL. Thompson, 

 who has reconstructed the vascular skeleton up 

 to the insertion of the 12th leaf, as shown in 

 Fig. 623, and supplied the description which 

 follows. Protostelicat its base the xylem becomes 

 meduUated, and phloem may appear centrally: 

 then follows as we ascend the stem a spindle- 

 shaped island of parenchyma bounded by en- 

 dodermis : but this again fades out upwards. The 

 stele now widening more rapidly, the central 

 parenchymatous tract becomes permanently 

 established, shut off from the vascular tissue by 

 a continuous endodermis, by which the whole 

 stelar system is completely invested. The 

 medullary tract appears above the 6th leaf as 

 avascular rod traversing the cavity of the dictyo- 

 stele obliquely. Thence upwards the medullary 

 system becomes continuous, but with irregulari- 

 ties that often repeat the islands of parenchyma 

 and phloem, enclosed each by its own endo- 

 dermis. Thus the whole is a dictyostele with 

 an irregular medullary system added, cognate 

 with what is seen in Pteris (Litobrochid) podo- 

 phyllai^xg. 618). 



Each leaf-trace arises from an angle of the 

 dictyostele: it soon breaks up into distinct strands, 

 while the leaf-gap closes laterally, a medullary 

 strand actually completing the closure, as in Litobrochia podophylla: but the 

 medullary system takes no direct part in the leaf-supply. According to the 

 account of Frau Eva Schumann {I.e. p. 212) the vascular supply to the petiole 

 appears to be a very complex derivative, with high segregation, from the 

 usual horse-shoe trace, with added fusions of a type similar to those which 

 appear in the leaf of Pteris {Litobrochia) podophylla (Studies VII, p. 37, 



Fig. 623. A reconstruction from 

 sections of the vascular skeleton of 

 Acrostichuni aiireuin, from its pro- 

 tostelic base upwards: showing 

 leaf-traces and leaf-gaps, numerous 

 root-traces and a medullary system. 

 (After Prof. J. McL. Thompson.) 



