36 Lang, Stele of the Shoot of Isoetes lacustris. 



arrangement of the outer tracheides of the xylem thus 

 resulting is not to be taken as indicating their origin from 

 a cambium ; nor is a like inference to be drawn from the 

 radial seriation of the cortical cells. That this has 

 nothing to do with the activity of the meristem usually 

 established just outside the stele is shown by the fact 

 that the stele is fully differentiated at a level above which 

 the anomalous secondary growth begins (Photo 3), and 

 still more strikingly by the fact that the stelar differen- 

 tiation and cortical growth described above take place 

 in stems in which no anomalous secondary growth is 

 present {Photos 2, 16). When this secondary growth 

 takes place it adds a thicker or thinner zone of secondary 

 prismatic tissue immediately outside the primary phloem 

 {Photo 1). It has further to be added to this account of 

 the structure as seen in longitudinal sections that the leaf 

 traces are differentiated along the lines of the radiating 

 zone around the central procambial region.' They thus 

 extend inwards through the cortex and into the outer 

 zone of the primary stele (cf. especially Photo 16). It 

 will be shown later that all the tissues of the trace (xylem, 

 parenchyma and phloem) are continuous with correspond- 

 ing tissues of the stele. 



The various regions distinguished above and their 

 general relations to the apical meristem are expressed 

 diagrammatically in Text-fig. 7, A, on p. 44, which will also 

 serve as a key to the interpretation of the photographs of 

 such sections. The lettering of the figure will be explained 

 in relation to the corresponding diagram of a transverse 

 section. 



Having thus obtained a general idea of the method of 

 apical growth and of the relation to this of the regions of 

 the stele and cortex as seen in longitudinal section, a 

 more accurate study of the construction of the stele may 



