340 MESSES. CYEIL WEST AND H. TAXED A ON 



Possibly tliis layer of parencliyraa, wliicli retains its meristematic activity for a 

 considerable period, was mistaken for a cambium by Scott and Hill (28, PI. 23. fig. 8). 



The primary cortex is formed by tlie active division of the cells of the apical nieristem 

 which abut on the plerome. These cells ( X X in PL 39. figs. 67-69) divide most frequently 

 by oblique anticlinal walls and give rise to parenchymatous tissue which is not sharply 

 marked off from that of the secondary cortex and of the leaf-bases. 



Structure op the Primary Tissues of the Stem. 



1. Frimary Xylem. 



Stokey (32, p. 314) maintains that "in the short and compact stem there is no 

 indication of a procambial strand, and as tliere are never any tracheids present above 

 the last leaf-traces the evidences of a cauline portion are not satisfactory." However^ 

 neither of these objections will stand the test of a critical examination. Let us consider 

 the first statement that " there is no indication of a procambial strand." In all four 

 species examined by us, a regular arrangement of the meristematic cells lying 

 immediately above the mass of primary xylem tissue was noticed ; tliese meristematic 

 cells form a procambial strand, which is less distinct than that of Lycopodium, owing 

 to the fact that in Isoetes the cells are not vertically elongated ; this difference may be 

 correlated with the very slow growth of the stem of Isoetes as compared with that 

 of Lycopodium, With reference to the second objection, we need only mention the 

 fact that both transverse and longitudiaal sections through the apical region of the 

 stem clearly show that the tracheids lying immediately below the procambial strand are 

 fully differentiated before those of any leaf-traces which would connect up with the stem 

 stele at that level (PI. 34. fig. VI), 



Hofmeister (16, p. 356), referring to Isoetes lacustris, states that '*many other cells 

 besides the primary cells of the vascular bundles take part in the wood-formation." 



Lastly, as further evidence for regarding the stem-stele of the adult Isoetes as a 

 cauline structure, we may mention the fact that the tracheids of the leaf-trace are 

 markedly narrower than, and altogether different in appearance from, thos3 of the stem- 

 stele (PL 34. figs. 11, 12 ; PL 35. fig. 21). 



Hence it follows that at least part of the central xylem mass of the adult plant 

 represents the xylem belonging to the ytem itself. In other words, the adult plant of 

 Isoetes japonica possesses a true cauline stele [cf. Scott & Hill (28), p. 426]. The 

 primary xylem-cylinder of the stem of I. japonica is composed of numerous large, oval, 

 reticulate tracheids intermixed with abundant xylem parenchyma, the cells of which 

 retain their meristematic condition (PL 34. figs. 11, 12; PL 4Q. figs. 75, 78). 



The tissues of the primary xylem-cylinder are obviously subjected to a very 

 considerable strain in the horizontal direction, which results in the almost complete 

 disorganization of the" tracheidal elements. The cells of the xylem parenchyma, which 

 are capable of further division, are able to accommodate themselves to this puUin 

 strain, and appear healthy even in quite old plants (PL 35. fig. 14), 





