IX 



FILICINE^ LEPTOSPORANGIAT^ 



331 



tracheid (Fig. 184, B). In cross-section these bars are nearly 

 rhomboidal, and give the famihar 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. striithiopteris they 

 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. The formation of the sieve- 



en 



Fig. 183. — Woodwardia radicans. A, Part of a transverse section of a vascular bundle 

 of the rhizome, X400 (about); B, transverse section of a root, X70; t, tracheids; 

 s, sieve-tubes; en, endodermis. 



plates begins by transverse thickened bars on the lateral walls, 

 less regular than in the tracheids, and the bars more or less 

 anastomosing so as to enclose thin areas, the sieve-plates (Fig. 

 184, D, E). These occur all over the lateral walls, as well as 

 the transverse ones. While it could not be positively shown, it 

 is extremely probable that the pores, afterwards formed, pene- 

 trate completely the thin membrane of the sieve-plates, and 

 throw the adjacent sieve-tubes into communication. 



While it is usually supposed that there are no nuclei in the 

 adult sieve-tubes, in several instances, evidences of the presence 

 of a number of small nuclei were met with. A further inves- 

 tigation of this point is desirable. 



With the tracheary tissue is mingled more or less wood- 



