38 Lang, Stele of the Shoot of Isoetes lacustris. 



complete and secondary growth is just commencing. In 

 this figure, and still more clearly in Photo 7, where the 

 primary structure is almost complete while there is no 

 trace of a secondary meristem, the primary phloem forms 

 the outer limit of the stele and is succeeded by the cortex. 

 The sieve-tubes of the primary phloem are characterised 

 by their small dense-looking nuclei, and can be traced 

 continuously from the stem to the leaf-traces. Within 

 the phloem of both stem and leaf-trace come parenchy- 

 matous cells with large and normal-looking nuclei belong- 

 ing to the xylem-sheath. The xylem of the entering 

 traces can be traced into relation to the central region 

 which here forms the main bulk of the primary xylem of 

 the stem stele, but comparison of Photos 6 and 7 will 

 show that other tracheides have been developed at the 

 periphery of the xylem in the intervals between the inner 

 ends of the leaf-trace xylem -strands. These form the 

 outer primary xylem, and although this is not strongly 

 developed in this plant it will be evident as it bulges out 

 the outline of the stele between the leaf-traces in Photo 6. 

 In Photo 7 its differentiation is just commencing, and at 

 places the radial seriation of cells from the region of 

 outer xylem, through the xylem sheath, phloem and into 

 the cortex can be made out. 



In Photo 8 a section of the stele at a still higher level 

 is shown. The phloem is just beginning to be differentiated 

 and serves to define the outline of the stele from the 

 cortex. The tissue of the stele is not yet differentiated 

 from the procambial condition. The only lignified elements 

 are the first formed tracheides of the base of one leaf- 

 trace (px), which serve to show the depth to which the 

 leaf-traces extend. Within this is the central region of 

 the procambium, exhibiting no regular direction of cell 

 division. Around this central region, and including the 



