BIOG 1105-1106 | Cornell Introductory Biology, Individualized Instruction

Slide 1: a longitudinal section of xylem showing vessels. Notice that the cells are elongated. The secondary walls of vessel elements are often strengthened by the formation of rings or spirals, as you see in the cells in the center. The cells to the right show numerous pits. Are these cells living or dead at functional maturity?

Slide 2: a longitudinal section of pine wood showing a tracheid with bordered pits. The secondary cell walls of tracheids are interrupted by pits. They may occur anywhere on the cell wall, but are particularly numerous on the tapered ends of the cell, where it abuts the next cell beyond it. Most tracheids have bordered bits in which the secondary walls of two adjacent cells are interrupted and their edges overhand the pit chamber, forming the pit borders. The primary walls and middle lamella are continuous through the pit and form the pit membrane, which is generally very thin and highly permeable to water and dissolved substances. Water and dissolved substances move from tracheid to tracheid through the pits.

Slide 3: Cross section of woody stems showing formation of a lenticel. Lenticels represent areas where the cork cambium is more active than elsewhere, resulting in the formation of tissue with numerous intercellular spaces. Gases can move freely through these areas of loosely arranged cells and therefore can reach the living cells in the inner layers of the stem.

Slide 4: a longitudinal section of phloem. Several sieve tube members are shown here. You can identify them by the slime plug (brown), an artifact of fixation. Notice the face view of a sieve plate at the center top. Sieve plates are usually found at the end of the elements, but also may be found on the side walls. Remember that sieve tube members are living cells with cytoplasm; albeit modified.

Slide 5: cross section of a pine stem. How old is this stem? (Answer: 4 years old.) Note: The large round canals in the stem are resin ducts.

Slide 6: cross section of a woody stem showing periderm. The red cells are the non-living cork cells. They are produced by a layer of meristematic tissue called the cork cambium that in this slide is represented by a layer of thin cells lying under the red-stained periderm cells. The cork cambium also produces parenchyma cells to the inside. Cork cells, cork cambium, and parenchyma cells (inner derivitives of cork cambium) together constitute the periderm.

In most woody stems, the first periderm appears during the first year of growth, usually originating in a layer of cortical cells immediately below the epidermis, or, less commonly, in the epidermis itself. The tissues outside the newly formed periderm will die, and eventually will slough off.
The first-formed periderm may keep up with the increase in girth of the stem for several years, with the cork cambium exhibiting periods of activity and inactivity. In most woody stems, additional periderms are formed as the stem increases in circumference. After the first periderm, the subsequently formed periderms originate deeper and deeper in the bark from parenchyma cells of the phloem that become meristematic. The way in which new periderms form influence the appearance of the outer bark. If the newly formed periderms arise as more or less continuous rings around the stem, relatively smooth bark results. In most stems, however, the newly formed periderms arise as discontinuous overlapping layers, resulting in the formation of a scaly or furrowed bark.

The next few slides show surface views of different types of bark.

Slide 7: Birch bark. Notice the horizontally elongated lenticels characteristic of birch.

Slide 8: Sequoia bark. The bark is furrowed because here periderms do not always occur in a continuous cylinder, but as overlapping layers.

Slide 9: Yellow birch bark. The old layers of periderm peel off as new ones form.

Slide 10: Sycamore bark. On this slide you can easily see that periderm forms in discontinuous, overlapping layers.