324 



MOSSES AND FERNS 



CHAP. 



rapidly thicken at the expense of the cell contents until all the 

 protoplasm is used up. The thickened bars between the pits 

 give the characteristic ladder-like appearance to the older 

 tracheid (Fig. 163, B). In cross-section these bars are nearly 

 rhomboidal, and give the familiar beaded appearance to sections 

 of the tracheid wall. 



Sieve -tubes of very characteristic form are found in the 

 bundles of all the Polypodiaceae. In O. strutJiiopteris they 



Fig. 163. — A, Vascular bundle from the rhizome of Pteris aguilina (L.) ; en, endodermis ; s, s, sieve- 

 tubes ; t, t, vessels (after Atkinson) ; B, longitudinal section of two large scalariform tracheae of 

 the same species (after Atkinson) ; C, part of a sieve-tube of Onocha strtithiopteris, x 375. 



occupy an irregular area at each end of the bundle. Their 

 differentiation begins shortly after that of the large scalariform 

 tracheids, and in some respects resembles it. The procambium 

 cells from which they arise are uniform in diameter, and have 

 squarer ends than the young tracheids. Their contents are 

 more colourless and finely granular than those of the tracheids, 

 and the nucleus not so evident. Whether the subsequent 

 division of the nucleus takes place before the thickening of the 

 wall begins was not determined. The formation of the sieve- 

 plates begins by transverse thickened bars on the lateral walls, 



