220 
Lang. — Studies in the Morphology and 
will be best dealt with by returning to the structure of the stele in the basal 
region. The strand of xylem at the extreme base of the plant is very 
slender, but soon becomes a thicker though still solid strand as it is traced 
upwards (Phots. 6, 7). It is composed of a small central group of irregu- 
larly arranged tracheides surrounded by a zone of tracheides that show 
a tendency to radial arrangement. I was at first inclined to interpret this as 
a centrarch structure, regarding the central tracheides as representing the 
protoxylem. Further study has not supported this interpretation, nor does 
it seem necessary to regard the radial arrangement of the peripheral 
tracheides as evidence of their being secondary xylem. It is sufficient and 
more useful for comparative purposes to distinguish the central and 
peripheral xylem in this region of the stele. At a slightly higher level 
(Phot. 8) parenchymatous cells appear among the tracheides, and, as 
described and figured by Bower, soon form a small pith in a central 
position. The pith forms in the central xylem (Phot. 9), and elements 
of this can usually be detected at the inner edge of the tube of xylem just 
after the pith has been initiated. The appearance of the pith is related 
to the first leaf-gap in the sense that, when this forms, the pith and the 
parenchyma outside the xylem become continuous, but there seems no 
evidence for the pith being derived by intrusion either of the outer 
conjunctive parenchyma or of the cortex. 
The difficulty of arriving at a definite conclusion as to whether the 
central as well as the peripheral primary xylem is present in the expanded 
medullated stele higher up the rhizome is owing to the fact that protoxylem 
is only present in the leaf-traces entering the stele. Thus in the rest of the 
stele the protoxylem is wanting as a guide to the possible distinction of 
centripetal primary xylem from centrifugal primary xylem. It has been 
shown above that practically all the xylem of such a medullated stele as 
that represented in Phot. 1 is to be regarded as primary centrifugal xylem. 
But indirect evidence from several sources points to the existence of central 
or centripetal primary xylem also ; as the question is of some importance 
the evidence will be stated fully. 
While the inner surface of the tube of primary xylem abutting 
on the pith is often quite regular and only occasional tracheides could 
be suspected of belonging to the central xylem, this is not always the case. 
Thus in the stele represented in Phots. 16 and 17 the inner surface of the 
xylem is very irregular, and the groups of tracheides projecting into the pith 
are open to the suspicion of representing the central xylem. This is even 
more the case when, as often happens, a tracheide is met with separated 
from the xylem tube by one or several parenchymatous cells. A case 
of this kind is figured by Bower from one of the small plants reconstructed 
above, and, as stated previously, isolated tracheides developed in the centre 
of the pith of the adult region of this plant. The reconstruction in Text- 
