8 Lang, Vascular Atiatomy of the Opliioglossiccce. 



of the leaf-trace from it in rhizomes of Hebninihostachys 

 of various age and size offer many points of comparison 

 with various Coenopterideae. The mesarch structure of 

 the xylem of the stele is comparable to what is described 

 for Zygopteris Grayi and Z. coriugata, the centripetal 

 xylem in HelniintliostacJiys corresponding to the central 

 xylem of the Zygopteridea;. The completion of the ring 

 of centrifugal xylem in the trace and the clepsydroid stage 

 of the trace are also comparable features. The structure 

 of the stele and the relation of the leaf-trace to it in some 

 }'Oung centrach rhizomes of HelmiiitJiostachys suggest 

 comparison with Botryopteris, especially with B. cylindrica. 

 The structure of stems with secondary xylem may perhaps 

 be compared with Bottycliioxylon. The general relations 

 of the tissues in the stele of Hclminthos/achys affords 

 probably the closest parallel in existing plants to the 

 stele of Zygopteris. 



The stelar structure of BotrycJiiuiii has recently been 

 discussed by Bower.^ While confirming and adding to 

 the facts which he records, his interpretation of the 

 structure may be slightly modified and brought into line 

 with that of the stele of HclunntJiostachys given above. 

 In BotrycJiimn lunar ia the stele of the plant passes 

 from a stage with a solid centrarch xylem to a condition 

 in which a cylinder of endarch xylem surrounds a pith. 

 This structure as a rule persists for a considerable length 

 of the stem of the plant without any active secondary 

 thickening taking place, and may be compared with a 

 stage passed through rapidly in HchniuthostacJiys. In 

 B. lujiaria tracheides may be developed in the pith. This 

 was seen most strikingly in the case of a stem the apex 

 of which had been destroyed while a branch had arisen 



^ On the riimary Xylem and the Origin of Medullation in the 

 Ophioglos^acfit. Atmah of Botany, Vol. XXV. (191 1), p. 537- 



