40 LANG, Stele of the Shoot of Fsoetes lacustris. 



a fairly broad zone of procambium with radially-arranged 

 cells, and this zone surrounds the central region where 

 the cells exhibit no such regular arrangement. No ligni- 

 fication has yet taken place, but the leaf-traces can be 

 seen to enter and form part of the outer procambial zone 

 but not to penetrate the central column of tissue. This 

 relation is more clearly seen in the slightly lower section 

 {Photo n), where the first tracheides of the entering leaf- 

 traces {px.) are lignified. The differentiation has not yet 

 affected the procambial tissue, whether outer or inner, 

 though the two regions can be distinguished, as in the 

 preceding photograph. The first tracheides of the base 

 of the leaf-trace xylem to be lignified are the most central 

 ones, and these are the only elements in the stele that 

 may reasonably be identified as protoxylem (px. in 

 Photos 8, II, 12), 



In Photo 12, lignification of the tracheides has com- 

 menced both in the inner and outer regions of the pro- 

 cambium. The slight obliquity of the section leads to 

 this being evident only on the left-hand side, where the 

 plane is lower. It will be clear that no direction of 

 lignification is to be recognised, that tracheides are 

 differentiated both in the inner and outer xylem, and 

 that it is the arrangement and shape of the outer pro- 

 cambial cells that determine the different appearance of 

 the elements of the outer xylem. At least one layer of 

 parenchymatous cells is left between the outermost 

 tracheides and the phloem ; this constitutes the xylem- 

 sheath. 



The developmental stages followed in successively 

 lower sections in the preceding photographs explain the 

 resulting mature structure of, the stele shown in Photo 13. 

 In this the inner xylem (a), into contact with which the 

 tracheides of the leaf-traces (b x ) can be followed, can be 



