The Evolution of Dictyostely. 191 
the simpler forms of the dorsiventral type, it must be remembered 
that in the radial type the relative position of the two meristeles 
changes as the phyllotactic spiral is passed through, whereas in the 
dorsiventral type it is constant. Dicksonia Barometz (Fig. 55, B) 
and various species of Pteris are examples of this form of stele. 
As in the dorsiventral forms the leaf-trace generally consists, 
in the simpler types, of a single strand (though it often breaks up 
very soon into two —Fig. 55, A—or more petiolar strands), but 
Fig. 55. A. Gymnogramme calamelanos. Radial stele showing two over¬ 
lapping leaf-gaps. 
B. Dicksonia Barometz. Part of stele cut longitudinally and seen from 
inside, showing three leaf-traces with very short leaf-gaps, through one of which 
the plane of section passes. After Gwynne-Vaughan. 
sometimes it consists of two similar portions from the first ( Pteris 
tremula, Adiantum trapeziforme). In the more advanced forms, 
e.g., in the well-known case of Aspidium Filix-mas (Fig. 56), each 
leaf-trace consists of a number of similar strands arising from the 
cauline meristeles forming the sides of the leaf-gap. 
Fig. 56. Aspidium Filix-mas. A. Part of vascular system showing the 
overlapping of several leaf-gaps and the polydesmic leaf-tiacc. 
B. Transverse section of stem showing the central ring of larger cauline 
strands and numerous leaf-traces in the coitex. 
