Manchester Memoirs, Vol. lix. (191 5), No. 8. 45 



The procambial tissue formed immediately behind 

 the growing point (a) gives rise to the central column of 

 primary xylem (a) only. The other primary tissues of 

 the stele are differentiated in the inner portion of the 

 radiating rows of meristematic cells (/3) surrounding the 

 central column at the apex ; the cortex is derived from 

 the outer portion of this radiating tissue. There is thus 

 a radial sedation of the elements from the cortex to the 

 outer xylem quite independent of any secondary growth. 

 A zone of prismatic tissue may or may not be intercalated 

 as an anomalous secondary growth produced on the inner 

 side of a meristem arising just outside the primary phloem. 



This analysis of the stele of Isoetes is in no way theo- 

 retical, but is based on the facts of development and 

 structure recorded in the photographic illustrations to 

 this paper. It can be recognised in every well grown 

 plant, though the various regions may be more or less 

 fully represented. It will perhaps be of assistance to the 

 reader in the study of the photographs to represent the 

 facts diagrammatically for a transverse and a longitudinal 

 section of the stele. In the accompanying figures {Text- 

 fig. 7, A B) the various tissues are denoted by the letters 

 used in the table above, and the corresponding letters 

 are used in the plates. 



The description here given of the structure of the 

 stem of Isoetes is consistent with the facts recorded by 

 other investigators, especially by Russow, Hegelmaier, 

 Bruchmann and Scott and Hill, though it gives greater 

 precision to the distinction of the various regions. All 

 these investigators recognised the anomalous position as 

 well as nature of the prismatic secondary zone. On the 

 other hand, the interpretation of the stele, with its 

 secondary tissue given by Miss Stokey, is directly con- 

 tradicted by the results of the present investigation, for 



