VIII THE PTERIDOPHYTA—OPHIOGLOSSACEJ£ 247 



also have the phloem more strongly developed on the outer 

 side. The tracheary tissue is much like that of the stem, but 

 the tracheids are smaller and the walls thinner. 



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. 126), and in longitudinal 

 section are seen to consist of rows of wide cells with either hori- 

 zontal 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 easily demonstrated. 

 According to Janczewski ^ these pits do not penetrate the 

 membrane between the cells, but Russow's ^ 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 BotrycJiium, 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 

 sieve -tubes consist of a thin parietal layer of protoplasm in 

 which numerous nuclei are imbedded. Little glistening 

 globules are also found, especially close to the openings of the 

 pores of the sieve-plates. 



The lamina of the sterile segment of the leaf is composed 

 of a spongy green mesophyll, more compact on the upper 

 surface. The epidermal cells show the wavy outlines char- 

 acteristic of the broad leaves of other Ferns, and develop 

 stomata only upon the lower side of the leaf. 



TJie Root 



The roots arise singly at the bases of the leaves, and in 

 older plants branch monopodially. Like those of OpJiioglossinn 

 they have no root-hairs, but the smooth surface of the younger 

 roots becomes often strongly wrinkled in the older ones. 

 Sections either transverse or longitudinal, through the root tip, 

 when compared with those of Ophioglossum, show a very 

 much greater regularity in the disposition of the cells. This 

 is less marked in B. ternatum, and probably an examination 



^ Janczewski (4). - Russow (5). ^ Janczewski (4), p. 69. 



