Lab 8 -- Secondary xylem

These photos were taken from sections made by students in the lab on secondary xylem, where we looked at woody stems of young pine and several dicot flowering plants.  The woodiness of the tissue made it harder to get thin sections, but many of them showed the various cell types quite nicely.

In this first set of images, we see transverse sections of young pine stems.  Most of the stem cross-section was made up of secondary xylem, containing almost exclusively tracheids.  The first image below shows three growth rings of secondary xylem.  Notice how the tracheids have thinner walls early in the growth ring (towards the center of the stem) and the walls are thicker in the later part of the growth ring.


Here is a single growth ring of the pine.  The changes in tracheid wall thickness described above are more apparent.  Also notice how the cells get narrower (in the radial direction) in the latewood part of the growth ring (towards the right).  The rays are uniseriate.  Notice the resin duct near the top center.


Here's a closer view of the tracheids in pine secondary xylem.  Notice how the pits are located exclusively on the radial walls (the ray at the lower right should tell you which way the tissue is oriented).  There is a tracheid near the center where the torus in the pit membrane is visible as a thin dark disk.


These are images from young sycamore stems.  You can still see parts of the old vascular bundles where a vascular cambium developed and then began producing secondary xylem and phloem.  Note the wide rays of these secondary tissues.








This is an image from a one-year-old sycamore stem (one growth ring).  The xylem appears to be diffuse porous - vessel diameter is roughly constant across the growth ring and they are distributed evenly.  Notice the fairly wide rays.


In this closer view of sycamore xylem, you can see the multiseriate rays (lots of starch).  Most of the cells around the vessels have fairly thick walls and might be fibers, although a longitudinal section would be useful for checking the wall pitting.


The next five images show transverse sections from young cottonwood stems.  The xylem appears to be somewhere between diffuse porous and ring porous, and so the term semi-ring porous is sometimes used in a case like this - the vessels gradually decrease in diameter going out from the start of the growth ring.


The next two images are of a three-year-old cottonwood stem.



Here's a closer view of cottonwood vessels surrounded by mostly thick walled cells that might be fibers.


In this view of cottonwood xylem at the transition between two growth rings, you can see xylem parenchyma (with starch grains) associated with the margin of the growth ring (running vertically near the center of the image).


These last three images are transverse views from live oak stems (Quercus virginiana).  The first image below is from a 1-year-old stem.


These last two live oak views are from 3-year-old stems, but unlike most deciduous oaks with ring porous wood, this species is evergreen and has very indistinct growth rings.  There are many long tangential bands of xylem parenchyma present in the secondary xylem.