192 
A. G. Tansley. 
In a good many of the more advanced dictyosteles described, 
both of dorsiventral and radial type, and in some solenosteles also, 
there are of course gaps in the cylinder which are not leaf-gaps. To 
all such steles I should propose to apply the term perforated .' This 
perforation may perhaps be regarded in a general way as a reduction- 
phenomenon, the failure to develop vascular tissue where it is not 
wanted leading to the formation of a gap in the cylinder-wall, 
through which the ground-tissue of the pith is continuous with that 
of the cortex. But of the conditions of such gap-formation in any 
given case we know practically nothing. The splitting up of the 
original curved leaf-trace into two or more separate strands is also 
no doubt somewhat the same phenomenon. To this we shall have 
to return in a subsequent lecture. 
Reduced Solenosteles and Dictyosteles. 
Some of the higher Ferns show aberrant types of solenostele 
and of simple dictyostele, which are regarded both by Jeffrey and 
Gwynne-Vaughan as reduced from the typical structures. Antro- 
phyum semicostatum (and, according to Poirault, other species also) 
has a solenostele in which the phloem is present in a normal 
B 
A 
F'ig. 57. A. Antrophyum semicostatum. Diagram of transverse section 
through leaf-gap. The stele and the two leaf-trace strands have no phloem 
- on the inner side of the xylem. 
B. Antrophyum plantagineum. Diagram of transverse section through 
internode. Internal phloem is absent except a little on the ventral side of the 
stele. 
condition on the outer surface and passes round the edge of the 
leaf-gap, but quickly disappears on the internal surface of the 
vascular ring (Fig. 57, A). In A. reticulatum , also, the internal 
phloem is absent except on the margins of the leaf-gaps. Jeffrey 
1 When the parts of the stele (meristeles) left between the per¬ 
forations are reduced to thin strands (Fig. 54) the steles may 
be said to be “ dissected.” 
