54 Lang, Stele of the Shoot of Isoetes lacustris 



PLATE III. 



Photo g. Transverse section just below the apex of another 

 plant, showing the central and outer regions of the 

 menstematic tissue. ( x 130.) 



Photo to. Somewhat lower section of the same plant. The 

 stele is delimited by the differentiation of the phloem on 

 the left-hand side. The xylem is still unlignified but 

 the outer and inner procambial regions are distinguish- 

 able. ( x 130.) 



Photo 11. Slightly lower section of the same plant, showing 

 that the first apparent lignified elements belong to the 

 bases of leaf-traces in the outer zone of xylem. ( x 130.) 



Photo 12. Still lower section of same plant, showing the pro- 

 gress of differentiation in both the central and outer 

 regions of the procambium. ( x 130.) 



Photo 13. Fully differentiated stele of same plant, showing all 

 the primary tissues. ( x 130.) 



