A NEW MEDULLOSEAN STEM. 



95 



The lack of structure in the tissue immediately external to the xylem has already been 

 commented upon in dealing with the separate steles. Only here and there are indi- 

 cations of radial rows of thin-walled cells, presumably representing the phloem. 



The longitudinal sections add very little to the facts already obtained from the trans- 

 verse sections. The evidence of the latter with regard to the loose parenchymatous 

 nature of the ground-tissue is confirmed; gum-canals or secretory sacs cannot be 

 distinguished, but patches of sclerotic cells are fairly abundant, especially near the steles. 

 The masses of xylem in longitudinal section show little evidence of branching or anasto- 

 mosing {cf. steles 7 and 8 on cut surface, PL 10. fig. 13). In some cases small branches 

 or traces, such as occur in the transverse sections, are seen separating. The brown 

 tissue outlining the xylem is crushed and structureless, as seen in longitudinal 



sections. 



The structure of the xylem may be determined only occasionally in the longitudinal 

 sections. The tracheides have large bordered pits on their radial walls, typically in two 



p 



alternating series (PI. 10. fig. VI a\ PL 11. fig. 5), although here and there only one 

 series, and in one or two cases three series, are present (PL 10. fig. 17 h). The pits are 

 always in contact and are usually flattened, resembling those of the Cordaitese and 

 Araucariese. Where the wood is cut tangentially sections of the bordered pits are seen, 

 but the middle lamella is not definitely distinguishable in any of the cases noted. The 

 medullary rays, as seen in tangential section, are fairly high, consisting of three or four 

 to fifteen cells in a vertical series. Their uniseriate nature is clearly shown (PL 10. 



fig. 18). 



In the main masses of vascular tissue there are no indications of protoxylem in 



longitudinal section, but in the ground-mass between two of the steles is a strand of 

 xylem consisting almost entirely of spiral elements. 



(c) Summary of Structure. 



(1) The stem-fragment under consideration shows parts of two series of vascular 

 structures embedded in a parenchymatous ground-mass, which also contains a central 

 arched stele and many isolated patches of vascular elements ; sclerotic nests and bands of 

 periderm occur irregularly in the ground-tissue. 



(2) Each of the " steles "of the inner series consists of two parts, the outer small and 

 normally orientated, the inner larger and inversely orientated. The two parts are 

 almost in contact, the tissue between being more or less obliterated. 



(3) The members of the second series consist of normally orientated xylem only ; 



they are therefore termed " partial steles." 



(4) The steles pursue a fairly straight course through the short length of stem, 

 apparently with little branching or anastomosing. Here and there traces are seen 

 separating from the inverse portions of the inner series ; while the lateral portions of the 

 partial steles are much broken and irregular. 



(5) The xylem is compact and coniferous in type, having the protoxylem elements at 



the inner marsrin of the masses. 



SECOND SERIES. — BOTANY, VOL. Till. 



P 



