Manchester Memoirs, Vol. lix. (191 5), No. 8. 31 



referred to above as the prismatic zone. A further con- 

 sideration of the literature will serve to make these 

 differences of opinion clearer. 



The secondary zone surrounding the primary stele of 

 Isoetes was at first regarded as corresponding to ordinary 

 secondary wood although only a few of its elements might 

 become lignified. 2 This view has recently been defended 

 in detail by Miss Stokey. 3 This investigator does not 

 recognise any phloem in the stem, though she admits its 

 existence in the leaf-traces and root-bundles. She regards 

 the prismatic zone as secondary xylem composed of 

 lignified tracheides, immature tracheides and parenchyma 

 cells. Since all the secondary tissue produced internally 

 to a meristem arising just outside the primary wood is 

 regarded as xylem there is on this view nothing anomalous 

 about the secondary thickening of Isoetes. The secondary 

 xylem formed would be strictly comparable in position 

 to the corresponding tissue of the Lepidodendreae or of a 

 Gymnosperm. It will be shown later how completely 

 this interpretation is at variance with the anatomical facts. 



The view of Russow 4 on the other hand was that the 

 secondary tissue produced to the inside of the meristem 

 in Isoetes was in part at least phloem, though he recog- 

 nised that xylem might also be developed in the prismatic 

 zone. The fact that a secondary tissue consisting of both 

 xylem and phloem was produced to the inside of the 

 meristem only, and not xylem to the inside and phloem 

 to the outside, indicated that the secondary thickening in 

 Isoetes was anomalous. It has since been frequently 

 •compared with the secondary growth of arborescent 

 Liliaceae rather than with normal secondary thickening. 



- Ilofmeister. " Higher Cryptogamia," p. 361 (1S62). 

 a Jhtanical Gazette, 47, pp. 311-335 (1909). 

 4 •• Vergleichende Untcrsuchungen," 1874. 



