

32 LANG, Stele of the Shoot of Isoetes lacustris. 



Russow, to whom the name " prismatic layer " is due, 

 appears to regard it as for the most part of the nature of 

 phloem. He regarded the innermost portion of this 

 tissue as corresponding to primary phloem, and clearly 

 states that the phloem of the leaf-trace is continuous with 

 this, just as the xylem of the trace is continuous with that 

 of the stele. This view is thus sharply opposed to that 

 considered above ; it regards the stele as having xylem 

 and phloem with which the xylem and phloem of the 

 leaf-traces are continuous, while the secondary prismatic 

 zone is of the nature of phloem with xylem elements (or 

 cauline bundles) developed in it and is derived from a 

 cambium of anomalous position. This cambium would 

 originate just outside the primary phloem, i.e., in a 

 position corresponding to the pericycle. 



Without entering into the work of other investigators 

 it will be sufficient to consider the description of the stele 

 of Isoetes Jiystrix by Scott and Hill, 5 which is the most 

 critical investigation of the stele of Isoetes yet made. 

 These investigators agree with Russow in regarding the 

 secondary thickening which gives rise to the prismatic 

 zone as anomalous, and the tissue produced centripetally 

 from it is shown to consist histologically of parenchyma, 

 phloem, and tracheides. The whole of the phloem in the 

 stem is regarded as secondary. In another part of the 

 paper, however, they point out clearly that the tissue 

 immediately outside the stelar xylem is parenchyma, and 

 that, in certain cases, the phloem immediately outside of 

 this may be reasonably regarded as primary. In any 

 case, however, they agree with Russow as to the con- 

 tinuity of the phloem of the leaf-trace with the innermost 

 elements of the prismatic zone, which have the histological 

 characters of phloem. 



b Annals of Botany, XIV. (1900), pp. 413-454. 



