Manchester Memoirs, Vol. Ivi. (191 2), No. \% 9 



lower down. \Fig- 5 ) As Bower has shown tracheides 

 commonly occur at the margin of the pith close to the 

 inner side of the xylem ring in Botrychmin. His account 

 does not take the position of the protoxylem into con- 

 sideration, but in the development of tracheides through- 

 out the pith in B. lunaria it is clear that they are internal 

 to the protoxylem. They thus correspond in position 

 to the centripetal xylem of HelniintJiostacJiys. The primary 

 xylem would in these cases be mesarch. This presum- 

 ably holds also for the injured stem of B. ternatiim with a 

 mixed pith figured and described by Bower. Whether 



Fig. 5. Transverse section of stele of a young plant of 

 Botrychium lunaria, which has borne a branch at the node 

 below. Tracheides have developed scattered through the 

 piih ; these constitute a centripetal xylem. ( x 60.) 



any of the tracheides or groups of tracheides that are 

 sometimes found close to the inside of the cylinder of 

 xylem in normal stems are centripetal is more doubtful ; 

 there is no positive evidence of mesarch structure in a 

 normal stem of BotrycJiuim. 



In the account in the preceding paragraph the first 

 developed centrifugal elements of the xylem in B. lunaria 

 are regarded as primary even though they may show a 

 tendency to radial arrangement. It is not impossible, 

 though difficult, to draw a line of division between the 

 centrifugal primary xylem and the secondary xylem in 



