THE VASCULAR CRYPTOGAMS 



47 



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. Mr. 

 Gwynne-Vaughan has recently found that the elongated 

 elements of these vessels, hitherto 

 called tracheides, are everywhere 

 in open communication with each 

 other, the closing membranes of 

 the pits being absorbed, while the 

 middle lamella of the walls also 

 disappears, leaving only the bare 

 framework of scalariform thick- 

 enings, which may thus be com- 

 pared to the bars of a towel-horse ! 



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 and 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 

 endoderrnis. 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 trachere. A, part 

 of wall in surface view. 

 Magnified 187 diameters. 

 Jj, part of wall in section, 

 showing bordered pits. 

 t, torus on closing mem- 

 brane. Magnified 375 

 (After De 



