42 Lang, Stele of the Shoot of Isoetes lacustris. 



The continuity of the phloem between leaf-trace and 

 stele, and the existence of primary phloem in the latter, 

 is most unequivocally shown in those shoots in which no 

 anomalous secondary growth takes place and only the 

 primary components of the stele are present. The phloem 

 of stele and leaf-traces can be recognised even in the 

 low power photograph of such a specimen in Photo 2, 

 the continuity of \\\c phloem from leaf-trace to stele is 

 more clearly shown for the older leaf-trace, in which the 

 callus formation makes the sieve-tubes prominent, in 

 Photo 15. It is shown for the upper functional leaf-trace 

 in Photo 16, which also gives a more detailed view of 

 the apical region of this stock, the transition to the 

 differentiated region of the stele and the absence of any 

 anomalous secondary growth. 



In Photo 16 also the xylem of the leaf- trace can be 

 followed for some depth into the outer zone of xylem of 

 the stele. The radial seriation of the cells which will 

 give rise to the outer xylem can also be seen between 

 the functional leaf-trace and the meristematic one above 

 it. The relations between the entering leaf-traces and the 

 outer xylem is still better seen in Photo 17, where the 

 attachment of three leaf-traces to another stele is repre- 

 sented. Above the uppermost leaf- trace the correspond- • 

 ing regions can be traced in procambial tissue. In this 

 stem secondary growth is about to follow directly on the 

 establishment of the primary structure. Lastly, in Photo 

 18, where the plane of section passes tangentially through 

 the outer xylem of another stele, the groups of tracheides 

 forming the xylem of the entering leaf-traces (a 1 ) are 

 embedded among the tracheides of the outer xylem (a""). 



Confusion between the primary phloem of the stem 

 stele and the secondary prismatic tissue is possible, where, 

 as is usual, the anomalous meristem proceeds to form 



