Wood (secondary xylem) formed by the vascular combium ...



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 thick-walled 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?

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