Lab 5: Wood and Bark 1 Name: ______Date/Lab time: ______

Lab 5:Plant Anatomy II: Secondary Plant Body (Wood and Bark)

Supplies: Slide of branch roots Cross-section (prepared slides) of Quercus, Tillia, and pine woods Assortment of woody plants Wood longitudinal sections? Wood samples Palm tree (plant showing primary thickening) Get bamboo and Palm cross-sections?

Vocabulary to know: Bark, Cork, Cork cambium, Early wood, Fibers, Hard wood, Late wood, Phloem rays, Primary thickening growth, Rays, Shoot apex, Soft wood, Vascular cambium, Vessels , Wood, Xylem rays

LAB SYNOPSIS: We will examine the products of secondary growth in plants (AKA wood and bark).

Introduction:

Primary growth - the result of growth produced by apical meristems on the shoot and root.

Primary growth produces the plant’s primary tissues (ground tissue, vascular tissue & epidermis). These tissues form the plant’s primary body (stems, leaves and root). Primary growth results in plants growing taller and deeper as seen in herbaceous plants like poppies, most mints, ferns and grasses.

Secondary growth - the result of growth produced by lateral meristems within the stem and root.

Secondary growth results in plants growing wider. Two lateral meristems develop; the vascular cambium and the cork cambium. These will produce the plant secondary body, wood and bark as seen in shrubs and trees.

Vascular cambium- a lateral meristem that produces cells in two directions. Those cells produced towards the inside produce secondary xylem (wood). Those cells produced towards the outside produce the secondary phloem.

Cork cambium- a lateral meristem that produces cells in one direction. Cells are produced towards the outside. Those cells will produce cork.

PROCEDURE- Observations of hardwoods and softwoods.

1. Examine the large blocks of wood samples that are present in class. Note the following features: Pith - recall this is the original pith as produced during primary growth. Annual growth rings - result for different sized tracheary elements produced in spring vs. summer.

Lab 5: Wood and Bark 2 Heartwood - older secondary xylem. No longer functions in water conduction. Darker in color due to deposited tannins and other components that harden wood and inhibit growth of pathogens. Sapwood - younger secondary xylem. Still function in water conduction. Rays - run radially through wood. Contain still living cells that store nutrients and aid in lateral conduction. Vascular Cambium - produces secondary xylem towards the inside, and secondary phloem towards the outside. Bark - outside the vascular cambium. Contains functional secondary phloem, older phloem, the cork cambium and cork. Protects the living cambium layers. Lenticels - slightly raised regions of thin walled cells that extend out through the bark. These ruptures of the bark allow for gas exchange.

PROCEDURE- Wood Anatomy (microscope slides) Note: We will be looking at cross sections of Tilia (linden/basswood) and Pinus (pine). It might be easiest to set-up multiple microscopes within your group with one or the other slide. Otherwise you will need to switch slides several times.

I. SECONDARY XYLEM (WOOD)

Hardwood - any wood from a dicot (linden, oak, hickory, ash, rhododendron, etc.)

1. Examine a prepared slide of a cross-section of the hardwood from Tilia (linden tree). Note the following: pith, wood and bark. Within the wood note:  Three major types of sclerenchyma are seen (vessels, tracheids and fibers). Vessels are very variable and have a wider diameter then do tracheids or fibers. Parenchyma is also present.  Rays run radially through the wood. Rays are made up mostly of parenchyma and aid in lateral movement of materials in wood.  Annual growth rings. Note what features form the growth rings.

Hardwood- Using higher magnification, draw one growth ring showing the interface between the early wood (spring wood) and the late wood (summer wood). Label the early wood (spring wood) and the late wood (summer wood). Early wood has wide tracheary elements while the tracheary elements in late wood are narrower. Indicate with an arrow the direction to the outside of the stem.

Lab 5: Wood and Bark 3 Note below: How do the diameters of the tracheary elements relate to the trees environment throughout the year? i.e. throughout the year, some are narrower some are wider. Why?

------Softwood - any wood from a conifer (lacks vessel elements and fibers).

2. Examine a prepared slide of a cross-section of the softwood from Pinus (pine). Note the following: pith, wood and bark. Within the wood note:  Only one type of sclerenchyma is seen (tracheids). Parenchyma is also present.  Rays run radially through the wood. Rays are made up mostly of parenchyma and aid in lateral movement of materials in wood. Water and nutrients in sapwood, and phenolics and terpenes in heartwood.  Annual growth rings. Note what features form the growth rings.

Compare this pine wood to the wood of the linden tree wood observed above. Notice how this wood is quite uniform and how the cells are smaller in diameter. The xylem conducting tissue of gymnosperms is made up of only tracheids no wide diameter vessel elements and no fibers. Because conifers lack fibers it is called softwood . Angiosperm wood, which has lignified vessel element and strengthening fibers, is called hardwood .

Softwood- Using higher magnification, draw one growth ring showing the interface between the early wood (spring wood) and the late wood (summer wood). Label the early wood (spring wood) and the late wood (summer wood). Early wood has wide tracheary elements while the tracheary elements in late wood are narrower. Indicate with an arrow the direction to the outside of the stem. Note that many softwoods contain resin ducts, which are only rarely seen in hardwoods.

Notice that tracheary elements of softwoods (conifers) have the same early wood/late wood pattern in the diameter as do hardwoods (flowering plants). However, is softwood more or less uniform in appearance as compared to hardwoods?

3. Now look at both the Tilia and Pinus wood in longitudinal-section (do we have these? ). Identify tracheids in Pinus . Notice how tracheids are long tapering cells. Now look at the Tilia wood. Note it too has tracheids but additionally has long narrow fibers and shorter wider vessels.

Lab 5: Wood and Bark 4 II. SECONDARY PHLOEM 1. Again look at the Tilia wood cross-section. Identify the phloem tissue to the outside of the xylem. Notice the layer(s) between the xylem and phloem which consist of small meristematic cells. This is the vascular cambium that divides producing cells to either side that will develop into secondary xylem (inside) or secondary phloem (outside)

2. Look closely at the phloem tissue. Phloem in Dicots is composed of sieve tube members and companion cells. It is usually difficult to see individual phloem conducting cells but they are the relatively larger cells in the phloem. Identify phloem rays , which are parenchyma cells. The secondary phloem makes up the inner most layers of the bark.

3. Now identify the cork. It is the protective covering that develops in plants with lots of secondary growth. It replaces the epidermis, which is ruptured by longitudinal growth. The cork is produced by another lateral cambium, the cork cambium .

Bark- all layers outside the vascular cambium. A waterproof protective layer in woody dicots. Made up of secondary phloem, cork cambium and cork.

III. CORK CAMBIUM The cork cambium is a meristematic area outside of the vascular tissue (it originally arises in the stem cortex). The cork cambium divides producing cell in only one direction. This direction is towards the outside producing cork cells.

4. The bark is made up of the entire regions outside of the vascular cambium. It includes the periderm, cortex (if any ground tissue still remains) and phloem tissue. As bark is shed from the tree, the cork cambium needs to be regenerated within the secondary phloem.

Lenticels - slightly raised regions of thin walled cells that extend out through the bark. These ruptures of the bark allow for gas exchange.

5. Label the cross-section of Tilia wood (next page). Label: Region that makes up bark, Early wood, Late wood, Pith , Secondary phloem, Secondary xylem (wood), Vascular cambium

Lab 5: Wood and Bark 5

Label: Region that makes up bark, Early wood, Late wood, Pith, Secondary phloem, Secondary xylem (wood), Vascular cambium, Lenticel

Lab 5: Wood and Bark 6 PRIMARY THICKENING GROWTH

Monocots lack secondary growth. How then are palm trees able to grow so tall? Some Monocots exhibit primary thickening growth where the thickening of the stem is due to the broadening of the shoot apex through time. Plants that exhibit this type of thickening growth are thus often larger at their apex and narrower at their base. This thickening occurs during primary growth and is similar to regular primary growth. However, it is a very different pattern of growth than seen in secondary growth. The inner cells of palms are highly lignified but lack growth rings and are not true wood.

1. Observe the plants and plant organs in lab that demonstrate primary thickening growth.

For example, most cactus grow wider by a primary thickening meristem.

Questions

1. Based on your observations of secondary growth, does secondary xylem and/or secondary phloem provide strong support that holds up trees of dicots?

2. Based on your observations of secondary growth, does secondary xylem and/or secondary phloem provide strong support that holds up palm trees?