DIPTEROID FERNS 



[CH. 



finally settling down into a network of venatio anaxeti, which as such 

 supplies the vegetative expanse (see Studies V, Text-fig. lo). But in the 

 fertile or soral patches an additional vascular supply arises in the form of 

 branches which, running parallel to the lower surface, supply the sori. This 

 has long been known, and has been described by Mettenius {Fil. Hart. Lips. 

 1856, p. 26) and by Hofmeister {Higher Cryptogamia, 1862, p. 252), as well 

 as by others. Examination of the fertile patches shows, with varying dis- 

 tinctness in different species, that the sporangia are restricted to more or 

 less definite parallel lines. This is seen most clearly where the area is less 

 prolific, and the leaf has the appearance of being only half fertile. There the 



Fig. 719. VTiX'i oi ^\^x(\\-n2.o{ Platy cerium aethio- 

 picttm which is only half fertile, i.e. with the sori 

 isolated and small. The diplodesmic state is less 

 pronounced than in Fig. 715, (5. ( x 3.) 



linear sori may appear isolated, and relatively short: and these give the 

 clue to the real constitution of the fertile areas of Platycerium. But it is 

 obvious even in the fully developed fertile patches of P. aethiopiciim. 



If such a semi-fertile area be cleared and stained, the course of both 

 vascular systems can be followed (Fig. 719). The substantive system is 

 of the Dipteris-type, but the free twigs are elongated in the form of re- 

 ceptacular tracheides, which bore downwards so that their distal ends 

 approach the lower surface, along which they extend. Often each remains 

 restricted to its own areola: but in not a few cases the receptacular strand 

 extends past the vein limiting its own areola, thus crossing in a lower 

 plane to the next. This is what has been seen occasionally in Cheiropleuria 



