1922] BROW NE—EQUISETUM 459 
is only about 1 mm. higher up that the lateral metaxylem consists 
of a number of tracheids more or less equivalent to the number 
found in a mature internodal bundle. The serial sections were 
14 pw in thickness, and thirty-two bundles of a section often showed 
different stages of the differentiation of the laterally situated 
tracheids. It was possible, therefore, to examine very numerous 
examples of incompletely differentiated metaxylem. Some irregu- 
larities were observed, but no doubt was possible that in the very 
great majority of cases the small outer tracheids are the first to 
become lignified. Thus in this species, as GWYNNE-VAUGHAN sus- 
pected, the metaxylem develops centripetally. Not infrequently, 
after the lignification of two or three small elements at the periphery 
of the wood, the next element to be lignified is larger and much more 
deeply seated. Commonly, however, the lignification proceeds 
more or less regularly from without inward. 
VI. General considerations 
Conflicting statements have appeared as to whether the proto- 
xylem of Eguisetum persisted through the nodes or disappeared 
at this level. JEFFREY (9), in 1899, wrote that in the nodal region 
the vascular tissue was massive and entirely devoid of typical 
protoxylem elements, and in comparing the node of Eguisetum 
with the description given by WILLIAMSON and Scorrt of that of a 
Calamite, he states that in the recent genus the protoxylem comes 
to an end below the node, and that it is absent from the inside 
of the nodal wood. Lupwics (10) also appears to regard the 
protoxylem as disappearing at the nodes. He writes as follows: 
At the node the protoxylem passes out as a bundle into the leaf, the 
groups of metaxylem approach one another and, uniting with the xylem of 
the next internode, completely fill up the carinal canal. In the position of the 
latter we find a large number of vessels with reticulately thickened walls, 
whereas the elements of the protoxylem show annular thickenings. 
More recently Miss BARRratt (1) has asserted that the proto- 
xylem does not traverse the node. In 1901, however, GWYNNE- 
VAUGHAN described the forking of the leaf trace protoxylem of 
Equisetum at the node and the fusion of each fork with the adjacent 
fork of protoxylem of a neighboring bundle. In 1908 Bower (2) 
