2 I 8 
Lang*— Studies in the Morphology and 
occurring in the pericycle will be recorded, and the resulting pericyclic 
wood described for both the stem and leaf-trace. 
The stelar structure in the various regions of the rhizome is most 
readily appreciated from transverse sections. The chief features of interest 
concern the xylem, and it will be sufficient to state at the outset that con- 
junctive parenchyma, sieve-tubes, pericycle, and endodermis are found in 
the order named outside this. The staining and photographic processes 
have been directed to demonstrating the xylem as clearly as possible, but 
the other tissues of the stele are well seen in PI. XX, Phots. 1 6, 17, as they 
occur in a stele of fair size without marked secondary thickening. It will 
be convenient to commence with the structure of the xylem in the definitely 
medullated stele, deferring the consideration of the transition from a solid to 
a medullated xylem till afterwards. 
The stele represented in cross-section in PI. XX, Phot. 1, is typical of 
a more or less extensive region of the rhizome after medullation has taken 
place. The wide pith is surrounded by a continuous tube of xylem two or 
three tracheides in depth. This I regard as primary xylem. No proto- 
xylem is present, except when a leaf-trace begins to be recognizable, and 
then the endarch xylem of the trace corresponds to an arc of the primary 
xylem. Judging by the relative position of the protoxylem of the leaf- 
trace the primary xylem of the stem can be described as centrifugal ; the 
question whether centripetal primary xylem can be recognized will be con- 
sidered below. There is typically no cambium or secondary xylem, and the 
occasional radial arrangement of the tracheides can be accounted for by the 
direction of the divisions in the procambium. The stele shown in Phot. 1 
has no internal endodermis, but it is quite characteristic of the intermediate 
region of the rhizome, where an internal endodermis may occur. The more 
bulky steles in Phots. 16 and 17 are similar in that the xylem is practically 
all primary, but the zone of primary xylem is thicker. 
The transition to the thicker adult region of the rhizome may be 
gradual, but often takes place rather suddenly, especially when a marked 
intermediate region has been present. It is then associated with the begin- 
ning of secondary thickening of the xylem, though not due to this. Phot. 2 
shows the structure in the transition region. The ring of primary xylem, 
which was alone present in the stele represented in Phot. 1, is seen around 
the pith, but active divisions have evidently gone on in the region imme- 
diately outside the primary xylem and an irregular zone of secondary wood 
has been developed. The incompleteness of the secondary xylem brings 
out the distinction between it and the primary wood. 
In the light of these sections the structure of the stele in the fully 
adult region will readily be understood. This is represented in Phot. 3, 
and small portions of the vascular ring from two distinct plants are shown 
in Phots. 4 and 5 ; the latter figures are to the same scale of magnification 
