462 Boodle.—Anatomy of the Hymenophyllaceae . 
typical for species with a comparatively stout rhizome, and 
the sub-collateral type of H. tunhridgense for species with 
very delicate rhizome. 
The rhizomes of large and small specimens of the same 
species were examined in order to see what differences in 
structure they would show. It seemed likely that some signs 
of transition between the two-banded and the sub-collateral 
types might occur. And this was found to be the case. The 
size of the stele in the rhizome varies considerably within the 
limits of a species. As one would expect, a robust plant 
usually differs from a meagre plant in having a stouter stele, 
comprising a larger number of tracheides, &c.; the tracheides 
tending to larger diameter in the larger plant. This difference 
in several species did not in any way affect the type of struc¬ 
ture ; but the following case is interesting as showing the 
way in which reduction may sometimes take place, and may 
have occurred in a phylogenetic series. Fig. 18 is a drawing 
of the stele (xylem and parenchyma only included) of a large 
plant of Hymenophyllum cruentum , Cav. If this is compared 
with Fig. 19, which represents the stele of a smaller plant, 
the following points will be noticed :—the xylem and paren¬ 
chyma form a much larger mass in the larger plant (Fig. 18) 1 , 
the tracheides are slightly more numerous in this plant, and 
are decidedly larger. But the important feature is that in 
the larger plant there are two groups of tracheides (/, /), three 
on one side and four on the other, belonging to the lower 
xylem-band, while in the other plant (Fig. 19) either the 
lower band is entirely absent, or perhaps the two lowest 
tracheides on one side and one on the other may represent 
a remnant of the lower band. In Fig. 18 the formation 
of about four more tracheides would complete the lower 
band, and the structure would then be of the same type as 
in H. scabrum . In Fig. 19, on the other hand, the xylem 
arch is more flattened, and a further flattening, or a massing 
of the metaxylem-elements, would produce the sub-collateral 
type. This elucidates the manner in which the sub-collateral 
1 The two figures have the same magnification. 
