VII PTERIDOPHYTA—FIUCINEAI—OPHIOGLOSSACEJE 265 



The phloem is composed also of the same elements, large 

 sieve-tubes, arranged in a pretty definite zone next the xylem, 

 and smaller cells of similar appearance, but not showing the 

 multinucleate character or perforated transverse walls of the 

 latter. The sieve-tubes are large (Fig. 145), and in longi- 

 tudinal section are seen to consist of rows of wide cells with 

 either horizontal or oblique division walls. The transverse 

 walls separating two members of a sieve-tube are somewhat 

 swollen and show small perforations, which are not always 



Fig. 145. — Part ot a vascular bundle from the petiole of B. Virginianum, X245; xy, 

 xylem; ph, phloem; s, s, sieve-tubes; B, two sieve-tubes in longitudinal section, 

 X490; sp, sieve-plates; n, nuclei. 



easily demonstrated. According to Janczewski (4) these pits 

 do not penetrate the membrane between the cells, but Russow's 

 (5) assumption that there is direct communication between the 

 cells is correct, although difficult to prove. Russow also states 

 that callus is present in the sieve-plates of Botrychmm, although 

 poorly developed. According to Janczewski the pores are not 

 confined to the transverse walls, but may also occur, but much 

 less frequently, in the longitudinal walls. The contents of the 



