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 go up the leaf. From a 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 pinna.'. 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 in the 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 pinna? 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. Ill) bears many 

 large oval stomata about 0.023 mm. by 0.038 mm. There are about eig - ht 

 of them per square millimeter. Thej^ are set exactly level with the 

 epidermal cells. Copeland speaks of them as follows (1902, pp. 349, 350): 



In the Polypodiacens 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 Dennsta'dtia 

 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, such as must occur in lesser degree in the case of Angiopteris. The guard- 

 cells of Dennsttctltia are thin-walled and shallow at the ends. 



