46 Lang, Stele of the Shoot of Isoetes lacuslris. 



she regards the whole prismatic tissue as imperfectly- 

 developed xylem, and does not appear to regard its 

 position or that of its meristem as other than ordinary. 

 Indeed, on the view this investigator takes of the nature 

 of the prismatic tissue (and bearing in mind that she does 

 not recognise any primary phloem in the stem continuous 

 with that of the leaf-trace), the secondary tissue of Isoetes 

 would correspond in position to the ordinary secondary 

 wood of a Gymnosperm or one of the Lepidodendrese. 

 The facts, when fully examined, seem, however, quite 

 inconsistent with such an interpretation. A similar mis- 

 conception vitiates the description of the stele of Isoetes 

 by C. E. Bertrand, Cornaille and Hovelacque, 12 a descrip- 

 tion that is of special interest since it was expressly made 

 for comparison with the Lepidodendreae. 



I do not propose to enter at present into the com- 

 parative bearings of this somewhat detailed consideration 

 of the stelar structure of Isoetes. The fact that the differ- 

 entiation of the stele proceeds with practically no longi- 

 tudinal extension is an advantage in relating the mature 

 structure to the procambial stage and to the stem apex. 

 The result of the analysis of the stele of Isoetes is to 

 facilitate its comparison with other plants. Two aspects 

 of this may be merely indicated in conclusion. 



It would appear that the correlative of the centripetal 

 primary xylem of the Lepidodendrese in the stele of 

 Isoetes is not the whole xylem, but the central column on 

 which the bases of the leaf-traces abut. This central 

 xylem is centripetal in that the only elements to which 

 the name protoxylem can be applied are the central 

 elements of the mesarch leaf-traces where these form part 

 of the stele. The outer xylem of Isoetes, between the 

 poles of the entering leaf-traces may in a sense correspond 



1 '-' Assoc. Franc, pour l'avancement des Sciences. St. Etienne, 1897, P.4S3. 



