Centripetal Xylem in Equisetnm. 589 
In structure these bundles are unusual : the xylem of each is in three 
parts, a median inner and two lateral external portions. Among these 
groups of elements lies the phloem. The innermost xylem-group consists 
of the protoxylem, the first-formed tracheides, spiral or loosely ringed in 
sculpture. In a mature internode the position of this wood is occupied by 
a cavity, the so-called ‘ carinal canal 5 , formed by the breaking-up and 
tearing-apart of these early-formed tracheides during elongation of the 
internode, and by the enlargement of the so-formed lacuna during radial 
and tangential growth of the surrounding cells. The more or less scattered 
and disarranged remnants of the disrupted cells are found in this opening 
in the tissues. The two other xylem-groups lie farther out, on the flanks 
of the canal, separated therefrom by one or more parenchyma cells. They 
consist of rows, or in some species of groups, of a few elements which 
develop considerably later than the protoxylem. In general these tracheides 
are scalariform, reticulate, or pitted, but occasionally annular and even 
spiral thickenings are found. These form the metaxylem of the bundle. 
Over their order of development, and the significance of the complexity of 
this bundle, considerable controversy has arisen. Owing to this and to the 
intimate relation of the leaf-trace to this bundle, the writer has made some 
study of the order in which the tracheides arise and the relationship of 
the different parts of the bundle to the whole. 
In the nodal region the primary wood of Equisetnm is much increased. 
The internodal strands alternate in the well-known manner. At the node 
they fork ; each metaxylem group passes to the bundle nearest in the 
internode above, and, after passing the next node above, returns to the 
same bundle. The carinal canal disappears as the bundle approaches 
the node. The protoxylem, there filling the position of this lacuna, 
behaves in much the same manner as the metaxylem. The forking and 
fusion of these portions, with some enlargement or additional growth, forms 
a solid ring of wood just above the node. This band, the supranodal wood, 
projects inwardly somewhat, and externally to a considerable extent, the 
extension in the latter direction becoming greater when secondary growth 
occurs. 
In structure the primary bundle of the Calamitean stem was collateral. 
It possessed near its inner margin a carinal canal, which, according to 
Williamson and Scott , 1 is identical in nature, origin, and position with that 
in Equisetnm . Extending outwardly in a wedge-shaped mass from the 
external periphery of this lacuna is the metaxylem. As seen in radial 
sections its development was centrifugal. Beyond this lay the phloem, or 
in large or mature stems, secondary wood. The supranodal primary wood 
of these old plants also much resembled that of Equisetnm, Its develop- 
1 Williamson, W. C., and Scott, D. H. : The Organization of Fossil Plants of the Coal Measures. 
Phil. Trans, of the Royal Soc. of London, vol. clxxxvB; 1894. 
