1922] BROWNE—EQUISETUM 467 
Summary 
1. The direction of differentiation of the metaxylem in the 
internode of E. giganteum is subject to slight irregularities, but is 
mainly centripetal. The outer elements are the smallest, and the 
tracheids, usually about 10-15 in number, become wider in passing 
inward. 
2. These lateral, internodal strands of xylem join on to the 
nodal wood, at which level they lose their identity. There is no 
indication in E. giganteum of their persisting as strands external 
to the nodal wood. 
3. The protoxylem is continuous through the node of E. gigan- 
teum. By the departure of the medianly situated tracheids to the 
trace the protoxylem is divided into two small groups of elements. 
These two groups diverge and enter neighboring but separate 
bundles of the internode above. Each of these small strands of 
protoxylem, diverging still farther from its sister strand, now 
situated in another bundle, fuses in the median region of the new 
bundle with an equivalent strand of protoxylem derived from the 
adjacent bundle of the internode below. The fusion is effected 
partly by a sudden increase in number of the protoxylem elements. 
4. The nodal wood of E. giganteum attains a considerable 
height and radial depth. Wide, reticulate, typically nodal tracheids 
appear considerably below and persist for some distance above the 
departure of the traces. 
5. The protoxylem elements of EZ. gigantewm are situated at 
the interior of the xylem, and pass through the inner part of the 
metaxylem of the bundle in a kind of parenchymatous sheath, two 
to four cells in thickness. A considerable number of small meta- 
xylem tracheids pass out into the trace, the metaxylem in the 
trace being usually greater in amount than the protoxylem. 
6: In young branches of E. giganteum which had not yet 
broken through the leaf sheath of the parent axis, two of the ribs 
of the ochreola contained a small vascular bundle. 
7. It is concluded that the continuity of the protoxylem of the 
internodes through the nodes, which, although not characteristic 
of all species, is not confined to E. giganteum, is a primitive char- 
acter within the genus. The question as to whether the lateral 
