696 Gwynne- Vaughan. — Observations 071 the 
meristele, but in some a small additional dorsal one will also 
be present. 
Bearing in mind the structure just described one is now in 
a position to understand the somewhat aberrant form of 
solenostely found in the dorsiventral Ferns Cheilanthes lendi- 
gera and microphylla. So far as the endodermis and pericycle 
are concerned each leaf-gap in the stele is closed up before 
the next above is formed, but the leaf-gap in the xylem-ring 
remains open until it overlaps the gap formed in the xylem 
by the departure of the leaf-trace next above. In this 
manner a small separate xylem-strand is produced within the 
stele which crosses over between each leaf-insertion from one 
side of the open xylem-ring to the other ; having precisely 
the same course and origin as the free dorsal meristele in the 
forms described above. 
In the more perfectly dictyostelic Ferns the dorsiventral 
type of vascular arrangement becomes much more com- 
plicated, but, in most cases, the manner of its origin from 
the solenostele is essentially similar to that already described, 
although it may differ considerably in detail. For instance, 
in Asplenium scandens the internodes are long, and the 
course of the dorsal meristele as it runs across from one 
side of the ventral portion of the solenostele to the other is 
a very oblique one ; moreover, since the two rows of leaves 
are on exactly opposite sides of the stem, the dorsal meri- 
stele is almost as large as the ventral. In a dissection of 
the stem, therefore, two fairly large meristeles are to be 
found, very similar to each other in form and size, and 
between each two leaf-insertions the dorsal meristele is seen 
to cross slowly over from one side of the ventral meristele to 
the other. 
If the dorsal meristele were to pursue a straight course, and 
instead of coming into bodily contact itself with the ventral 
meristele, as in Asplenium scandens and the cases mentioned 
above (Figs. 7 and 8), it were to keep up its connexion with 
it at the same points as before by means of short transverse 
strands or sutures, a structure would then result essentially 
