118 
A. G. Tansley. 
which conditions could arise easily from mesarch protostely by the 
disappearance of more or less of the central metaxylem. In any 
case the thickness of the xylem ring is frequently so slight that this 
difference amounts to very little. 
To return to the Hymenophyllaceac ; if we start again from such 
a form as Trichomanes reni/oniie with its two distinct hands of 
metaxylem, dorsal and ventral, Boodle shows that the type of 
Uymenophyllum scabrum, II. demissum and II. dilatatum, all species 
with comparatively stout rhizomes, only differs from that of 
T. rcnifoniie by the greater differentiation of the bands, to the 
lower of which the root-steles are attached. The leaf-trace at its 
base is again sometimes identical in structure with the rhizome stele 
(Fig. 30), but the ventral metaxylem band tends to disappear and 
the protoxylem becomes attached to the dorsal metaxylem. The 
petiolar bundle has a C-shaped xylem, with the protoxylem attached 
to the two ends of the C, and the phloem interrupted opposite the 
opening (Fig. 31). From this type may be derived the sub-collateral 
type of rhizome-stele (Fig. 32) in which the ventral band of xylem is 
absent and the protoxylem is connected with the ventral face of the 
dorsal band. The leaf-trace in such forms is scarcely distinguishable 
from the stele of the rhizome. 
The sub-collateral type is also found in species of Trichomanes, 
e.g., T. trichoideum, and it may be further reduced to the collateral 
type ( T. muscoidcs, Fig. 33), in which the phloem disappears from 
Fig. 32. Fig. 33. 
Fig. 32. Uymenophyllum tunbridgense. Stele of rhizome, sub-collateral 
type, x 380. From Boodle. 
Fig. 33. Trichomanes muscoides. 
X 480. From Boodle, 
Stele of rhizome, collateral type. 
