SPOROPHYTE. 27 
The rachis of the leaf repeats in general the structure of the petiole. 
It is deeply channeled above, rounded beneath (fig. 69). The epidermis 
is very thick-walled, and is supported by a layer of sclerenchyma. This 
layer is thinner than it is in the petiole, especially on the back and sides of 
the rachis. In the ridge on either side of the groove it remains strong. 
About midway of each side of the rachis there is a line of spongy paren- 
chyma with chloroplasts. Here the sclerotic sheath is interrupted and 
stomata are found in the epidermis (figs. 69, 114). The vascular bundle 
becomes narrower as we goup the leaf. Froma very short, thick, V-shaped 
cross section it finally becomes oval. Now, the xylem is a band with 
hooked ends. The protoxylems are only two, lying inside the hooks. 
Cavity parenchyma still accompanies them (fig. 114). 
The oval form of bundle just described is also found in the base of the 
rib of the larger pinne. These ribs, indeed, fully repeat the outlines and 
structure of the upper part of the rachis. The vascular bundle of the rib 
springs from that of the rachis in the following manner (¢/. figs. 88-92). 
The hook of the xylem at one end of the U-shaped petiolar bundle grad- 
ually becomes twice as deep as before, and a bridge of xylem is formed 
across the middle of the hook. A constriction now cuts through the bridge, 
separating from the bend of the hook a small ring of xylem filled with 
phloem. The constriction later affects the endodermis, and the new bundle 
is completely separated. As it bends out into the pinna, the xylem ring 
grows thicker below and thins out to nothing above, until only a single 
transverse band of tracheids remains inthe middle of the bundle. A strand 
of protoxylem lies on the upper side of this band, and is continuous with 
that of the rachis. 
The veins and veinlets of the leaf are collateral in structure, cylindrical, 
with distinct endodermis. Their xylem consists wholly of spiral tracheids. 
The ribs of the uppermost pinne are also collateral throughout, with xylem 
above and phloem beneath. 
The structure of the lamina is very simple. An unbroken epidermis of 
wavy-margined cells (fig. 112) covers the upper surface. If any cuticle 
is present, it is extremely thin. Over the veins the cells are elongated and 
approximately rectangular. The lower epidermis (fig. 111) bears many 
large oval stomata about 0.023 mm. by 0.038 mm. There are about eight 
of them per square millimeter. They are set exactly level with the 
epidermal cells. Copeland speaks of them as follows (1902, pp. 349, 350): 
In the Polypodiacez I have examined, there is an approach to what Haberlandt 
calls the type [of stoma] of swimming plants, in that the ridge of entrance is well 
developed, while the ridge of exit is inconspicuous or not present. In Denunstedtia 
punctilobula Bernh. (figs. 40, 41) [cf our fig. 104] this thickening of the ridge of entrance 
has gone far enough to give the stoma a rigid appearance, but it is really motile. 
Opening seems to be effected by a movement of the ridge of entrance outward as well 
as backward, suchas must occur in lesser degree in the case of Angrofteris. The guard- 
cells of Dennstedtia are thin-walled and shallow at the ends. 
