48 Lang, Stele of the Shoot of Tsoetes lacustris* 



primary xylem and secondary xylem, 15 so that this double 

 comparison is not unjustified. 



The leaf-traces, as they join the central column of the 

 stele, seem to have a determining influence, not only on 

 the development of the outer xylem, but on the differentia- 

 tion of the surrounding soft tissues. The xylem-sheath 

 and phloem are both continuous from the leaf-traces to 

 the periphery of the stele. We seem here to be dealing 

 with fundamental facts of stelar construction, since parallels 

 can be drawn, not only with the more primitive steles of 

 the sporophyte, but with the stele-like central strand of 

 the Polytrichaceae. 



It is impossible to discuss here the general considera- 

 tions thus raised, but it will be evident that a more 

 accurate analysis of the stele of Isoetes not only affords 

 points for comparison with the Lepidodendreae, but 

 promises to be of interest from the standpoint of general 

 stelar morphology. 



Lang. Ann. Bo/., XXVIII., p. 23Q. 



