206 Barratt. — A Contribution to our Knowledge of the 
emphasize the proportion of parenchyma present in the xylem (PI. VI, 
Fig. 5). The explanation may be found by reference to similar elements in 
the cleared preparations. Here they are seen to be short wide tracheides 
tapering sharply at both ends, with an irregular open reticulum of thicken- 
ing, very large pits being found on the walls bordering parenchymatous 
cells ; hence when a transverse section traverses one of these pits the wall 
appears quite unthickened (PI. VI, Fig. 6). 
At this level the first indication of the presence of the branch supply 
makes its appearance (Text-fig. i, E and F). A gap is noted on one side 
of the xylem mass and the parenchymatous elements towards the centre of 
the stele come into direct communication with the central well-developed 
pith of the lateral shoot. The xylem of the latter, where it joins on to the 
main axis, is arranged in an unbroken cylinder (Text-fig. i). As the 
series of sections is traced upwards the gap in the xylem cylinder is closed 
by the appearance of elements belonging to the upper part of the branch 
stele, thus leaving no lateral gap in the wood above the branch trace. 
Gradually the primary vascular axis contracts in width and the proportion 
of parenchyma in the pith is reduced until at the level of the leaf-traces it 
has returned to the condition obtaining prior to the origin of the branch, 
and ultimately a solid protostele is found (Text-fig. i,g-m). 
The attachment of the leaf-traces is not accompanied by any disturb- 
ance in the arrangement of the vascular tissue. The number of leaf-traces 
present at this first node is variable, not always agreeing with that of sub- 
sequent nodes on the same shoot. There seems to be a general tendency 
for one at least of the three leaf-traces typically present to be either 
reduced to a very short strand or to be absent altogether. This reduced 
trace is usually that adjoining the foot, but sometimes it is the one on the 
side towards the first lateral shoot (Text-fig. 2). There is a very decided 
tendency for the three leaf-traces to join the stele at different levels and 
thus to have a spiral arrangement (PI. VI, Fig. 4). This might suggest 
that the very pronounced whorled character of the phyllotaxis of the adult 
plant may not be in itself a primitive character although it has been long 
established. 
The transition from the solid protostelic condition, just described, to 
that of the internode above is sudden, and is marked by the change from 
short wide reticulate tracheides to a few much elongated annular ones 
(Text-fig. 1, M, N, and o). 
As Vidal (6) and others have pointed out, the stem may be regarded 
as built up of a series of segments, each consisting of a node with an 
accompanying internode. This conception is certainly supported by the 
manner in which the vascular tissue develops. The internodal protoxylem 
strands are the first to differentiate, as Queva (7) has shown from his study 
of transverse sections. The strands correspond in number with the leaves 
