OEB 59 – Plants and Human Affairs Lab 6: Wood

Part I: General wood anatomy

What is wood? Wood is secondary xylem, a highly lignified tissue. Secondary xylem in most woody plants is produced by the vascular cambium, a lateral meristem. Most dicot groups and many Gymnosperms (conifers, ginkgos and gnetum) have a vascular cambium.

The lateral meristem Vascular cambium – phloem produced to the outside, xylem to the inside. Phellogen, cork cambium, or pericambium – secondary growth; cork or outer bark produced to the outside, phelloderm (made up of living parenchyma cells) or inner bark produced to the inside. The periderm replaces the epidermis in plant parts that become woody. It prevents loss of water and acts as a protective covering for underlying tissues. The periderm consists of three layers: - Phelloderm – inside of cork cambium; living parenchyma cells - Phellogen (cork cambium) – serves as a lateral meristem to give rise to periderm - Phellem (cork) – dead at maturity; air-filled protective tissue on the outside

Cell types and structures Vessels are made up of many individual cells known as vessel elements stacked end to end. They typically have minimal or holey “end plates” (i.e. cell walls at the short end of the cell) so that water can flow easily from one vessel element to the next. When seen in cross-section, vessels are called pores. They are found only in angiosperm wood.

Tracheids are long, narrow cells that make up the xylem in conifers but are also found in angiosperm wood. They tend to give a lot of structural support, and sort of grade into fibers. Both vessels and tracheids are the water-conducting cells in the xylem. Tracheids, however, are not as efficient at transporting water as vessels because they are narrower, are not stacked end to end, and tend to have less porous cell walls at their ends compared to vessels.

Pits are features of the sidewalls in vessels and tracheids. They are areas where the cell walls are much thinner and allow for "cross-talk" between adjacent cells. They often have distinctive morphology used in wood identification.

Tyloses are ingrowths in tracheids and vessels that may partially or completely block vessels. These can be very important in choosing woods to use for fluid containment. They can also be important in wood identification.

Question: (1) Red oaks lack tyloses, unlike white oaks. Which species of oak is used in constructing wine casks? Why is this? Rays are made up of living parenchyma cells and are oriented radially rather than longitudinally (like vessels, tracheids). The rays store water, minerals, and starches, and they function to transport these horizontally across the diameter of a tree. Ray cells help resist fungi and insects that invade the wood.

Axial parenchyma is also made up of living cells, but is oriented tangentially or axially instead of radially. It is very distinct in some kinds of wood and absent in others.

Resin canals are specialized sets of cells that contain resins. They are tubular passages that exude pitch, or resin, to seal off wounds due to insect or mechanical damage.

We already talked about the difference between cross or transverse sections, radial sections, and tangential sections during lecture.

TO DO: (2) For one of the slides you observe today, draw the features that can be seen on the three different sections. Label some key features that you can distinguish.

Heartwood versus sapwood The sapwood is the outermost wood, the living, functional portion of the xylem. Remember that the conducting cells of xylem are dead anyway, but there are parenchyma cells throughout. The width of the sapwood is dependent of the species and the environment. The heartwood is the inner wood that is not conductive and not alive; it is often darker, and more susceptible to disease.

Part II: Gymnosperms versus Angiosperms

Gymnosperms have no pores (i.e. vessels). Angiosperms do. Don’t be fooled by resin canals, however! We tend to think of conifers as the main gymnosperm wood, but also Ginkgo biloba is a gymnosperm. Ring-porous woods have bigger vessels in the early season. Diffuse-porous wood has large vessels scattered relatively evenly throughout the growth ring.

TO DO: Observe the following slides: (3) Pinus spp. (4) Ginkgo biloba wood (5) Oak wood (Quercus sp.) (6) Maple wood (Acer saccharum) (7) Which one has diffuse-porous wood? Ring-porous wood?

Hardwood versus softwood Angiosperm wood is called hardwood because of the presence of fibers, which give strength. In contrast, wood of conifers, even giant 2,000-year-old sequoias, have no fibers ad are called softwoods.

QUESTION: (8) Why do some conifers have stronger wood than the fibrous wood of some angiosperms?

Density of Wood Different species vary a lot in the density of their wood (mass per volume). Compare balsa wood to ebony. Wood density is largely determined by the percentage of wood made up by vessels.

Part III: Torus-margo pits in conifers

Conifer wood is relatively simple compared to the wood of dicots (Angiosperms). Dicot wood is characterized by the presence of both tracheids and vessels, while the wood of conifers and other Gymnosperms lack vessels. Conifer wood is additionally characterized by a relatively small amount of axial parenchyma. Both groups have pits on their tracheids through which water and dissolved minerals are conducted between cells. The tracheids of conifers, however, are characterized by the presence of circular bordered pits. Pits on adjacent tracheids are aligned, forming pit-pairs. In conifers, the central portion of the pit membrane is thickened and called a torus (multiple tori). The torus is slightly larger than the opening of the pit border and can be pulled against either opening to prevent the movement of water or gases between adjacent tracheids. Research found that the evolution of the torus-margo membrane within the gymnosperm lineage from homogenous pits was equivalent to the evolution of vessels in Angiosperms (Pittermann et al. 2005, Science).

The torus is an impermeable, typically lignin-based thickening at the center of the pit membrane, surrounded by a margo, which is a ring of radial slits or pores. In conifers, gaps in the margo are around 0.1 µm in diameter, exceedingly reducing water flow resistance relative to a typical pit membrane. The torus functions to seal off the pit opening in case of air entry through a pore or crack in the cell wall, protecting the membrane from 'air seeding', during which external air is drawn into the lumen. [In the case of air seeding, the resulting bubble may expand to fill the lumen, thus embolizing the cell.]

TO DO: (9) Examine the slide of Pinus attenuata provided in the lab. Search for circular bordered pits and their associated tori. Draw a pit-pair and label the torus. Also mention what kind of section your drawing is. Part IV: Resin ducts

Resin ducts are large intercellular canals lined with resin-secreting parenchyma cells. The production of resin is thought to protect the plant from attack by decay-causing fungi and insect herbivores. Humans derive several useful products from the resins produced by trees, including turpentine, pitch and rosin.

Fossilized resins produce amber. We are currently working on the description of a new species of Columnomyces (fungus) on a new species of Proptomaphaginus (beetle) in a 25-million-year old piece of Dominican amber! The source for this amber is resin of the extinct species Hymenaea protera (Fabaceae).

Although hardwood trees (especially Fabaceae, legume family) produce resins, the best-known commercial resins are extracted from conifers. Resin oozes out when the tree is cut; when you break off a cluster off needles you can see (and smell) the sticky resin in a pine tree. Try it out (if you haven't already)!

You can broadly classify the commercially most important conifer woods on the basis of only four characteristics.

TO DO: (10) Observe the numbered unknown wood samples and prepared slides on display in the lab. Using the characteristics below, assign each to a group.

GROUP A: Resin canals present and large Pine (Pinus)

GROUP B: Resin canals present and small Douglas fir (Pseudotsuga), larch (Larix) or spruce (Picea)

GROUP C: Resin canals absent, heartwood Fir (Abies), hemlock (Tsuga), some cedars not very different in color from sapwood (Cedrus)

GROUP D: Resin canals absent, heartwood Redwood (Sequoia), bald cypress (Taxodium), different in color from sapwood yew (Taxus), some cedars (Cedrus)

Prepared Donald H. Pfister & Danny Haelewaters 2015