190 A. G. Tanslcy. 
parts of the original solenostele are lost and almost the only 
recognisable regions are the exits of the groups of leaf-traces. The 
dorsal meristele, however, usually retains its individuality, remaining 
stouter than the other vascular strands (Fig. 54, A, B). Sometimes 
it pursues a perfectly straight course, but often it bends from side 
to side in front of the successive leaf-gaps (Fig. 54, B) as in the 
more primitive forms. 
A 
B 
Fig. 54. Diagrams of highly dissected dorsiventral dictyostelcs. The 
vascular system is supposed to be divided longitudinally on the ventral side 
and spread out flat. The dorsal meristele (d.m.) and leaf-traces arc the only 
parts of the original simple dictyostele, which retain their individuality. From 
Mettenius. A. Acrostichum axillare. B. Polypodium Paradises. 
Radial Types of Dictyostely. 
In Ferns which have their leaves arranged in several 
orthostichies round a prostrate or erect stem, three or more leaf- 
gaps usually overlap at a given level, and the solenostele is broken 
up into as many more or less equivalent meristeles arranged in a 
ring round the axis. If the structure is still but little removed from 
solenostely it may happen that two leaf-gaps only overlap at any 
one level, and then the vascular arrangement, seen in transverse 
section, generally consists of a large gutter-shaped meristele with 
another small one lying across its opening (Fig. 55, A). Although 
his structure is very similar in appearance to that presented by 
