188 
A. G. Tansley. 
Dorsiventral Types of Dictyostele. 
Of course the simpler forms of dorsiventral dictyostele differ 
very slightly from a dorsiventral solenostele in which one leaf-gap 
nearly reaches to the insertion of the next trace (Fig. 52, A and B). 
When the gaps overlap for most of their length the dorsal inter- 
B 
A 
Fig. 52. Evolution of Dictyostely. l.t., leaf-trace ; d.m., dorsal mcri- 
stelc. The mid-dorsal line runs between the two leaf-traces. 
A. Nothochlana Maranta. Each leaf-gap just fails to overlap the next. 
B. Pellaea rotundifolia. Each leaf-gap is prolonged forward so as to 
overlap the next. After Gwynne-Vaughan. 
nodal portion of the solenostele becomes reduced to a strip or 
strand (meristele) running obliquely across between two successive 
leaf insertions from one margin of the large ventral portion of the 
stele to the other (d.m., Fig. 52, B). Such a structure is found in 
species of Nothochlana , Pellaa and Adiantum. In Cheilanthes 
lendigera and microphylla a form even nearer the solenostele is 
found, for here, so far as the endodermis and pericycle are 
concerned, each leaf-gap in the stele is closed up before the next 
gap above is formed, but the gap in the xylem ring remains open 
till the next leaf-gap is reached, so that a small separate xylem 
strand is formed in the thickness of the stelar wall, crossing over 
between two successive nodes from one edge of the open xylem 
cylinder to the other. 
In the more completely dictyostelic Ferns the vascular 
structure becomes considerably more complicated, but in most 
cases the manner of its origin from the solenostelic condition is 
essentially similar to that already described, though the details may 
differ considerably. For instance in Asplenium scandeus the inter¬ 
nodes are long and the course of the dorsal meristele, as it runs 
across from one margin of the ventral meristele to the other, is very 
oblique. If the dorsal meristele were to pursue a straight longi¬ 
tudinal course, and instead of coming into bodily contact with the 
ventral meristele as in Fig. 52, B, it were to make connexion with 
the ventral meristele at the same points by means of short trans- 
