Stele in Two Species of Dip ter is. 769 
represented by a few scattered elements and a group of connected tracheides 
at one side. These connected tracheides form a fully differentiated leaf-trace 
(Fig. 8, PI. LVII). This departure of a leaf-trace while the rest of the 
xylem is in the meristematic condition is found in all cases, and also in 
D. conjugata . It is a common phenomenon at the apices of Fern stems. 
But the local failure to develop xylem further back in the stem does not 
seem to have been described in other Ferns. 
In an older stem a normal solenostele is found with an equally developed 
xylem-ring. The pericycle is formed by a row of 1-2 cells. The cells of 
the phloem have very thick walls. No protoxylems are apparently present 
in the xylem, which consists of scalariform tracheides with parenchymatous 
cells in between. A leaf-trace goes off with two separate xylem-groups. 
Still within the main axis it turns, ceasing to be opposite the leaf-gap. 
Fig. 7 is a section of the stele at this stage. The xylem-ring is still 
open and the internal endodermis is represented by a single cell only. 
The stele gradually increases in size and a pith is formed. The cells of the 
pith have thicker walls than those of the cortex near the stele. Near the 
growing point, when the xylem and phloem are already fully differentiated, 
the pith and cortex cells have still thin walls, and the endodermal cells 
have no contents and are not easily recognized. This shows that the xylem 
and phloem are sooner differentiated than the other tissues. 
A still older stem is represented by Fig. 17, PI. LVIII. On the whole 
it has the same characters as the one just described. The xylem consists 
of tracheides with dark brown cells in between. This brown substance may 
be a secretion of tannin. These tannin cells are represented in the younger 
plants by the colourless parenchymatous cells. No older plants of D . Lob - 
biana were available to enable one to form definite conclusions, but the 
structure does not differ very much from the one found in D. conjugata of 
approximately the same age. 
A marked difference between the two species is that the leaf-trace in 
D. Lobbiana is given off as two separate strands, one after the other on the 
same side of the xylem-gap, as shown by the diagrams (Figs. 9, 10, 11, 12, 
PL LVII). No thickening takes place at the margin of the xylem leaf-gap 
in this species. On the whole, however, there is a great resemblance in the 
development of the stelar structure of the two species. 
Petiole. The first-formed petioles have here also a simple structure. 
In a petiole 1 cm. long, the lamina of which divides twice dichotomously, 
the vascular strand is composed of a few xylem-elements forming a slightly 
curved arch and surrounded by phloem. Probably protoxylem is present. 
The pericycle consists of one layer of cells. The stele divides into two at 
the base of the lamina. 
A petiole 3-5 cm. long had a lamina which showed three successive 
dichotomous branchings. At the base of the petiole the stele is formed by 
