1922| BROW NE—EQUISETUM 465 
smaller size of the peripheral tracheids this does not seem to be 
ikely. Perhaps the strongest argument against the primitiveness 
of the centripetal development of the lateral metaxylem lies in 
the position of this tissue outside the endarch protoxylem, which 
makes it difficult to regard it as a vestige of the central centripetal 
‘xylem of a protostele. For the present, therefore, it seems pre- 
mature to draw any definite conclusions from the exarchy of the 
internodal metaxylem in the axis of E. giganteum. 
GWYNNE-VAUGHAN also stated that in E. hiemale, and better 
_ still in EZ. giganteum, the lateral internodal strands of xylem could 
be traced, after joining on to the nodal wood, as externally pro- 
jecting ridges into the internode above.* He claimed that these 
lateral strands diverged while passing through the node, so that 
in the internode above they were situated on adjacent sides of 
separate bundles; that at the next node these strands again 
approached one another, and that in the internode above this 
node the strands were again included in the same bundle. In 
fact, according to this author, the course of the metaxylem strands 
at the node is exactly that described in this paper as characteristic 
of the protoxylem of E. giganteum in this region. In this species 
the lateral strands of metaxylem are composed of numerous 
elements, and if they persisted through the node they would be 
very conspicuous. From the serial sections of the nodes at my 
disposal, however, it was clear that the identity of the lateral 
strands was completely lost in the relatively large tracts of nodal 
xylem. As we approach the nodes the tracheids of the lateral 
strands gradually assume the appearance of typical nodal tracheids. 
QvEva has noted this gradual passing of the lateral metaxylem 
into the nodal wood in E. maximum. In this species, in which 
according to him the lateral metaxylem consists of tracheids with 
spiral or annular thickening, elements with a type of ornamentation 
intermediate between these and reticulate thickenings occur, he 
says, below the nodal wood and in continuity with the internodal 
metaxylem. Elements with this internodal type of thickening 
* I regret that in a recent paper (5) I alluded to Gwynne-VAUGHAN’s claim of the 
persistence of the lateral strands “inside and over” the ring of reticulate tracheids. 
e passage should read “‘outside and over the ring of reticulate tracheids.” 
