Page 168 - Fundamentals of Light Microscopy and Electronic Imaging

PRINCIPLES OF ACTION OF RETARDATION PLATES 151

xylem cells are called tracheids and each is interconnected with neighboring tra-
cheids by valvelike structures called bordered pits that regulate hydrostatic pres-
sure in the xylem during fluid transport. The pits are made mostly of cell wall
materials, mainly lignin, a very dense phenolic polymer that gives wood its
strength. Pine wood is about one-third lignin, the rest cellulose.
Examine the test slides of two plant tissues at 20–40 : a radial section of
pine wood (Pinus strobus) containing bordered pits, and a transverse section of a
buttercup root (Ranunculus) containing starch grains.

1. Examine each tissue by bright-field microscopy, carefully adjusting the
microscope for Koehler illumination and positioning the condenser
diaphragm. Identify bright spherical bodies in the cortical cells of the root
and in the tracheid elements in pine wood.
2. Now examine the sections between two crossed polars by polarization
microscopy. Insert the polarizer with its transmission axis east-west and
the analyzer with its transmission axis north-south. Rotate the analyzer
until the field of view is maximally dark (extinction). Now insert the
focusing telescope eyepiece to examine the polarization cross in the back
aperture of the objective lens and close down the condenser diaphragm to
isolate the dark central portion of the cross, masking out the bright quad-
rants of light at the periphery; then replace the eyepiece to return to visual
mode. What structures stand out? Make a sketch of the polarization cross
in a bordered pit and explain the pattern of light and dark contrasts in the
polarization image.
3. Place a red-I plate on top of the specimen slide with its slow axis at a 45°
orientation to the polarizer and analyzer. Make sketches of a starch grain
and a bordered pit indicating the colors seen in each of the polarization
quadrants using the red-I plate.
4. Make drawings that show your interpretation of the molecular organiza-
tion in starch grains and bordered pits as revealed by polarized light.
Starch and lignin are crystals of long carbon chain macromolecules.
Assume that both structures exhibit positive birefringence.
5. Make sketches showing your interpretation for the orientation of cellulose
in the xylem tracheids in pine wood. What evidence supports your con-
clusion?