50 Lang. Stele of the Shoot of Isoetes lacustris. 



EXPLANATION OF PLATE I. 



All the figures are from untouched photographs and, with 

 the exception of Photo 19, are of Isoetes lacustris. a, central 

 region of procambium ; /3, radiating rows of meristem around 

 central region ; a, central primary xylem : b 1 , base of xylem 

 of leaf-trace ; P, outer xylem ; px, protoxylem ; <r, xylem sheath ; 

 d, primary phloem ; e, secondary prismatic tissue ; /, meristem ; 

 g, cortical tissue. 



Photo i. Median longitudinal section of the stem of a well- 

 grown stock, in which the primary stele is surrounded 

 by a zone of anomalous secondary thickening. ( x 67.) 



Photo 2. Median longitudinal section of the stem of an equally 

 large plant, in which, however, no anomalous secondary 

 thickening has taken place. The continuity of the 

 phloem, conjunctive parenchyma and xylem of the leaf- 

 trace with the corresponding tissues of the stele can be 

 followed, and this specimen demonstrates the existence 

 of primary phloem in the stele of Isoetes. ( x 67.) 



Photo 3. Median longitudinal section of the apical region of 

 another stem, to show the general relation of the 

 differentiation behind the apex to the mature structure. 

 (x 130.) 



