764 de Bruyn .— The Ontogenetic Development of the 
plant. The xylem cylinder is mostly uniform laterally and ventrally, but 
in some places the ventral side is still thicker than the dorsal one. The 
xylem consists of tracheides with rows of parenchymatous cells. Some of 
these have brown contents; probably this secretion is tannin, since in the 
mature plant the presence of tannin sacs is a constant feature. Mesarch 
protoxylems are perhaps present, but they are not very distinct. In the 
series described here the leaf-traces follow each other very quickly, and 
before a gap is closed the next leaf-trace is already prepared for by the 
part of the xylem-ring destined for the next trace becoming thinner and 
projecting outwards. The result is that a complete uniform xylem-ring is 
never present. The gap is, however, always closed before the projecting arc 
of xylem separates from the rest, so that the stele is still a true solenostele. 
In older stems the distance between two leaves is much greater, and there 
are long internodal stretches with an equally developed xylem-ring. An 
interesting fact is that the xylem at the margins of both sides of the gaps 
of this plant is distinctly thickened. The endodermis does not closely 
follow this thickening. In the mature plant, as described by Seward and 
Dale, 1 this thickening of the margins always occurs and is still more 
developed. Here all the other tissues have the same shape. In an old 
plant examined by me the closing of the leaf-gap was effected by these 
thickened gap-margins approaching each other on both sides till they fused. 
The stele had then the form of a ring whose xylem is much thicker on the 
dorsal than on the ventral side. Gradually this dorsal part becomes thinner 
till the tissue, of the whole xylem-ring has the same thickness. This 
behaviour does not agree with the statements of Gwynne-Vaughan and 
Miss Kershaw. Thus Gwynne-Vaughan 2 writes of D. conjugata : ‘The 
free xylem-strand has almost separated off from the solenostele altogether, 
being connected with it only at two points between which a tongue of 
ground tissue has inserted itself.’ This statement may perhaps refer to the 
parenchymatous tissue which is formed in the abaxial part of the thickening, 
but this part belongs to the arc which goes off as the leaf-trace. The 
parenchyma separates the xylem of the limb of the horseshoe from its 
curved end. There is no sign whatever of a free xylem-strand nor of any 
thickening in the internodes. The figure of Seward and Dale 3 of the 
mature stem shows an equally developed solenostele. Miss Kershaw 4 
writes : ‘In D. conjugata the thickening of the margin of the leaf-gap 
extends through the internodes as well as the nodes, and this portion of 
the xylem has become almost completely separated off from the solenostele.’ 
1 Seward and Dale, loc. cit., p. 499. 
2 Gwynne-Vaughan : Observations on the Anatomy of Solenostelic Ferns, Part II. Ann. of Bot., 
vol. xvii, 1903, p. 700. 
3 Seward and Dale, loc. cit., Fig. 8. 
4 Kershaw, loc. cit., p. 687. 
