THE VASCULAR CRYPTOGAMS 



47 



The wood consists of tracheides and parenchyma ; the 

 tracheides of Ferns are generally of the kind called 

 scalariform, or ladder-like, from the peculiar structure 

 of their walls, shown in Fig. 23. This structure 

 depends on the form of the pits, 

 which are slightly bordered and 

 much elongated in the transverse 

 direction, so that the thickened 

 ridges between them resemble the 

 rungs of a ladder. The tracheides 

 are of great length, with pointed 

 ends. True vessels, arising from 

 the fusion of distinct cells, are rare 

 in Ferns. 



In the stele figured (see Fig. 

 22) there is only one group of 

 protoxylem (px) lying on one side 

 of the wood. In the larger steles 

 of the stem there are usually two 

 or three such groups. Spiral 

 tracheides occur at these points, 

 but usually become destroyed very early as the stem 

 grows in length. Surrounding the wood is a layer 

 of parenchyma containing starch, and then we come to 

 the phloem-zone, consisting of sieve-tubes arid paren- 

 chyma. The former have their sieve-plates on the 

 lateral as well as on the oblique terminal walls. They 

 are not unlike those which we observed in Pinus. 



The phloem again is surrounded by a belt of paren- 

 chyma very rich in starch, beyond which we come to the 

 endodermis. The endodermis is really two cells thick, 

 but its inner layer cannot be distinguished from the 

 pericycle except by the fact that its cells fit on exactly 



FIG. 23. Portions of scala- 

 riform tracheae. A, part 

 of wall in surface view. 

 Magnified 187 diameters. 

 B, part of wall in section, 

 showing bordered pits. 

 t, torus on closing mem- 

 brane. Magnified 375 

 diameters. (After De 

 Baiy.) 



