The Structure and Function of Stems in Woody Plants
Biology of Woody Plants
Wood (secondary xylem) is formed by the
vascular cambium during the moist days of
spring (i.e., early in the growing season is
called spring or early wood and consists of
large, thin-walled cells (fig. 1). Later in the
season, the drier days of summer gradually
slow the activity of the vascular cambium
and cause it to produce summer or late wood
made of smaller cells with thicker walls.
This environmentally induced difference
between spring and summer wood is
influenced strongly by auxins: auxins stimulate the formation of spring wood, and
applying auxin-transport inhibitors stimulates the formation of summer wood.
Differences between spring and summer wood are abrupt in trees such as pine and are
visible in most temperate trees as growth rings. Each growth increment increases the
diameter of the tree trunk; once formed, each increment remains unchanged in size or
position during the life of the tree. Secondary xylem, or wood, is the inner derivative of
the vascular cambium and comprises about 90% of a typical tree.
Several terms and concepts are important to know
Softwood vs. Hardwood: Softwoods generally refer to the conifers, particularly pine,
spruce, larch, fir and redwood. Their wood is relatively homogeneous because it is about
90% tracheids and lacks vessels. Hardwoods are dicots native to temperate and tropical
regions and include oak, maple, ash, walnut and hickory. Unlike softwoods, which
contain only tracheids and ray parenchyma cells, hardwoods contain fibers and vessels.
Fibers make most hardwoods stronger and denser than softwoods.
Figure 2. Secondary phloem tissue
Fig 3. Two year old woody stem of Tilia
Xylem: Vascular plant tissue. Comprised of many cells types: tracheids, ray
parenchyma, fibers and vessels. Xylem serves to conduct water from the roots to other
parts of the plant, vertically and laterally. It is the primary constituent of wood.
Phloem: Vascular plant tissue. The outer derivative of the vascular cambium. It
includes sieve elements and parenchyma cells that often alternate with bands of thickwalled fibers and prevent splitting of the inner bark. Only the inner centimeter or so of
secondary phloem contains functional sieve elements; sieve elements in the outer parts
of secondary phloem are dead and nonfunctional and help protect the inner tissues.
Parenchyma. Tissue, cell type. A largely undifferentiated cell type that serves several
functions: structure, storage, photosynthesis, etc.
Tracheids: Cell type in xylem tissue of conifers.
Vessel elements: Cell type in xylem tissue of angiosperms.
Sieve elements: Cell type in phloem. For translocation of sugars, secondary products
and water.
Periderm. Tissue generated in part by the phloem and cork cambium. Consists of
several layers and serves to form the bark of the plant.
Cross section, tangential section and radial section: Your instructor will discuss these
different sections which describe the microscope slides, models and pictures in lab.
Materials:
You will have at your disposal a variety of microscope slides, models and wood samples
with which to study secondary wood growth. Some important slides to look at are:
First, second and third year Tilia (basswood) stem cross-sections. Identify the xylem,
phloem, vascular cambium, and periderm.
Figure 4. Macerated xylem tissue
Figure 5. Close-up of vascular cambium
Macerated wood. This is xylem tissue that has been treated and teased apart. Can you
identify the different cell types? Look for tracheids, fibers and vessels.
Slides of herbaceous dicot and monocot stem cross-sections. This makes a nice
comparison with the woody dicot stems to show you the essential differences and
similarities between the three types. Identify the xylem, phloem, and parenchyma tissues,
the epidermis and possibly the pith (which is really a region, not a cell type or tissue).
Various wood samples. We will have assorted blocks of wood and veneers available.
Examine them both for their colors and grains and with a dissecting scope to see the
variation in trachieds and seasonal wood. How does the way the wood is cut affect its
appearance?
Figure 6. Assorted wood samples. What causes the difference in patterns and colors?