Texas Journal of Microscopy Texas Journal of Microscopy

SIGNIFICANCE OF THE “WHITE LINE” IN THE WOOD
OF ROCKY MOUNTAIN JUNIPER (Juniperus scopulorum)
FIGURES. All figures of J. scopulorum (Rocky Mountain Juniper). Fig. 1. Cross section of a stem showing heartwood with sapwoodlike
areas contained within. Fig. 2. Direct view of wood showing a typical white line; note dark wood above (outside) the sapwood-like
area. Fig. 3. Direct view of a white line showing breaks between the tracheid files and the rays at the white line. Fig. 4. “Crack” along
white line in very old stem of archeological origin. Fig.5. Two white line fissures in separate annual rings of this wood. Fig. 6. Tilted view
of fissure seen in both transverse and radial section; Note that there are no broken cells along the fissure. Fig. 7. Views of white line
fissures; note cell surfaces in the fissure. Fig 8. Similar white like fissure showing its irregularity.
10 Tex. J. Micros. 37:1, 2006
HOWARD J. ARNOTT
The Department of Biology and The Center for Electron Microscopy,
The University of Texas at Arlington, Arlington, TX, 76019
The stems of several Rocky Mountain Juniper (Juniperus
scopulorum) trees were examined by light and scanning electron
microscopy. The wood of these trees is of interest because it shows
many light colored “pie-shaped” segments within the heart wood.
The material was obtained from Rex Adams of the Laboratory for
Tree-Ring Research, University of Arizona, Tucson, AZ. The stem
sections are from trees that grew in the Zuni Mountains, Cibola
National Forest in New Mexico; like other cedars they show a clear
distinction between heartwood and sapwood. For example, in one
tree with 60 or more rings, showing frost rings in 1950 and 1952,
the first forty annual rings are a part of the deeply purple-red colored
heart wood, the remainder is a part of the light colored sap wood
which in turn is covered by the bark. These light segments are similar
in color to the sapwood; they are defined by two radial borders and
two tangential lines which surround the sapwood-colored areas.
Examination of the pie shaped segments using direct light
microscopy shows the following features: Each segment has a white
line along its most external tangential portion. In some cases the
white line can easily be seen by eye but it usually requires some
degree of magnification. The white line may be a part of a single
annual ring or may extend through 2 or more annual rings. The
two side borders of each pie segment are oriented along radii in the
same fashion as rays. Thus each pie segment has two radial borders
and two tangential borders, the inner tangential border is shorter
than the outer and the outer border consists of the “white line.”
Investigation of a serial set of cross sections, about ¾ inch thick,
of a single stem showed that the pie segments have limited vertical
extent. Individual pie segments may be present at one level and
absent one or two inches below or above; associated pie segments
show differences in their vertical extent. This pattern of variation
is currently unexplained. In this investigation a blue mold has been
seen, however, there is no direct evidence of insect or microbial
infection associated with the sapwood-like areas.
When the “white line” is viewed with SEM it appears to be a
tangential fissure which cuts through both the rays and the tracheids
breaking the wood continuity. The thickness of this fissure (or crack)
is about 15 µm, or about the same as the diameter of an early wood
tracheid. Whether passing through a field of tracheids or a ray, the
crack is relatively uniform in radial thickness. The boundary formed
by the fissure is not smooth and it does not follow a straight line;
over a short distance the white line varies as much as 100 µm in the
radial plane. This break may pass from the early wood of one ring
through the late wood of another ring. In other words the tangential
course of the fissure which causes the white line is does not seem
to follow a plan dictated by the structure of the wood. As wood
dries out this crack may expand. The fissure causing the white line
does not seem to be derived in a straight forward way from the
vascular cambium.
Why does a fissure appear white when a sanded piece of wood
is observed with a hand lens or by direct view in the light
microscope? The answer is simple; the process of sanding produces
cell wall debris, fragments of this debris are deposited in the fissure
by the sanding process. The process of sanding fills the fissure
with material having a haphazard orientation. When light is shined
on the deposited the bits and pieces what appears to be white light
is “refracted.” The diffracted light produces the white line. The
white line is an artifact resulting from sanding, however, the fissure
is a real and important “structural feature” of this wood. Exactly
what the roll the fissure plays in the physiology of heart wood
formation is not understood. One obvious explanation is that the
fissure restricts the passage of tannins and (other heartwood forming
materials) from their origin in the stem periphery.