The growth of plants is indeterminate—that is, plants continue to growr throughout their lives. Indeterminate growth has allowed some plants to reach enormous sizes and very old ages. For example. General Sherman, a giant sequoia (Sequoiadcndron¿jigantcum) in California, is the largest living tree, with close to 1.8 million kilograms (2,000 tons) just in its aboveground parts. The oldest recorded tree, called the Eon Tree, a coastal redwood (Sequoia scmperviretis), was at least 6,200 years old when it fell in 1977. In contrast, most animals exhibit determinate growth. In other words, oncc an animal reachcs maturity', it no longer grows. Animals reach a fixed size thar is determined by their genes interacting with environmental conditions. The largest known animal was a female blue whale (Ralacnoptcra musculus), which tipped the scalcs at 170,000 kilograms (190 tons).
Plants grow and add new cclls to their tissues in regions called mcristems. Growth in length is controlled by primary mcristems located in the rips of stems and roots. The tissues derived from the primary mcristems were covered in Laboratory Topics 3 and 4. The grow th in the girth or diameter of stems and roots is controlled bv secondarv mcristems. Secondarv mcristems arc . . .
formed later in die development of a stem or root, and they occur along the entire axis of the stem or root, rather than just in the tips. In a cross section of a stem, rhc two secondary mcristems, the vascular cambium and the cork cambium, appear as circles of dividing cells parallel to the surface. In the first year of secondary growth, the vascular cambium starts as a ring between the primary xvlcm and primary phloem of die vascular bundles and the adjoining cortical cells (fig. 15.1). When the cclls of
This year's growth
Secondary xyiem -formed this year
Secondary xylem formed last year
Secondary xylem formed two years ago
This year's growth
Secondary xyiem -formed this year
Secondary xylem formed last year
Cortex and epidermis
Primary phioem Primary xylem
Cortex, epkiermts and bark
Primary phloem Provascular tissue Primary xylem
Secondary phloem i Secondary xylem
Remnant cortex and epidermis wearing away
Primary xylem Vascular cambium Periderm
Secondary phtoem Secondary xylem
Primary xylem Vascular cambium Periderm
the vascular cambium divide, secondary xvlem cells are produced to the inside and secondary phloem to the outside. Each year, a ring of new secondary xvlem and secondary phloem is produced. The cork cambium forms outside the vascular cambium and likewise produces new tissue to the inside (phelioderm) and new tissue to the outside lcork). In many trees, it is difficult to differentiate phelioderm, cork cambium, and cork, so all three tissues arc collectively called periderm. The periderm replaces the epidermis in die secondary plant body as the outer protective layer diat restricts die loss of water and controls the exchange of gases. Bark is composed of the periderm and all the layers of secondary phloem—or all the tissue outside die vascular cambium.
Materials Needed for Exercise A
Macerated tissue of hardwood
Prepared slides of 3-year-old Tilia stems, transverse section
Prepared slides of 3-year-old Pinus stems, transverse section
Prepared slides of oak i Qucrcus) wood, transverse section, radial section, and tangential section
Only trees and shrubs exhibit secondary growth and produce wood. Coniferous trees, often called softwoods, arc native to temperate regions of the world. Hardwoods arc members of the Class Dicotvledones m and reproduce with flowers instead of cones. In temperate regions, hardwoods are often deciduous trees that lose their leaves for winter. The flowering trees of the tropics arc also hardwoods, but they tend to keep their leaves all year round like the conifers. The terms "softwood" and "hardwood" imply a difference in the den-sin- of the wood. Although most coniferous woods arc "softer" than many "hardwoods," this is not always the case. Despite die overlap, it is worth examining the two ty pes of wood because dicy have anatomical differences between the wood types. 1. Cell types in wood (prepared slide). Examine a slide of macerated hardwood tissue with a microscope. To prepare this slide. w(x>d was treated to separate the cells and then stained. Look at the diversity of cells on the slide. What do you notice? What cells are similar.3 What cclls are distinct?
Many of the cclls arc long, slender, and tapered at bodi ends. Most of these arc fiber cclls. Fibers consist primarily of ccll wall. The ccll wall is composed mainly of cellulose interwoven with lignin. Fibers provide structural support for the stem and are found not only in w ood but in other plant tissues. They act much as steel rods do to reinforce concrctc. When functional, these cells are dead and have lost all their inner contents.
As noted in laboratory Topic 3, most of the cells in paper pulp arc fibers. You may have noticed some of the cells were broken or frayed at the edges. When we recycle paper, we reuse the fibers. In the processing, many of the fibers break and become shorter, so there is a practical limit to how many times paper can be recycled. L'su-ally some new virgin pulp is added with recycled fibers to make the final product.
Another long, narrow plant cell is a trachcid. These arc similar to fibers but not quite as long, and they usually do not break in the maceration process. The trachcids have pits in the side walls that allow water to pass from one ccll to the next. Like fibers, they are dead when functional and essentially composed entirely of ccll wall.
Vessel elements are shorter, wider, and shaped like a barrel. The top and the bottom have large openings so when vessel elements are connected end to end in the stem, they form long tubes called vessels. These cclls are the primary water-conducting cclls in flowering plants. In contrast, conifers have only trachcids and no vessel elements.
The final cell type you should find is a thin walled, short cell. This is a parenchyma ccll of a xylcm ray. Usu-alls vou can observe contents within the ccll since when
functional these cells arc alive and retain their organelles and cytoplasm.
Identify each type of cell and sketch the cclls in the following spaces.
2. Transverse section of basswood (Tilia). With a microscope, examine a prepared slide of a transverse section of a 3-vear-old stem of Tilia and compare it with figure 15.2. In the very center of the stem, you
FIGURE I 5.2 CROSS SECTION OF A WOODY. 3-YEAR-OLD BASSWOOD (TILIA) STEM.
will see some of the remnants of the primary tissue. The thin-walled, large cells in the centcr arc parenchyma of the pith. The pink-staining cells clustered near the outer boundary of the pith arc part of the primary xvlcm. Three or more conccntric layers of secondary xylem lie just outside the pith and primary xylem. Secondary xylem is wood. In temperate regions such as the United States, one ring of tissue is formed each year. Which ring do you think is the oldest—the innermost or the outermost? The cells in the annual rings have different diameters at different seasons. In spring, the cells are relatively large bccause they have plenty of nutrients and arc growing fast. The tissue in this region of the ring is callcd earlywood or springwood. Toward the end of the growing season, late summer or fall, the cells arc not growing as fast, and more material is deposited in the cell walls. The cells have smaller lumens, or openings. The resulting tissue is denser and callcd latewood or summcrwood. In Tilia, you can easily see when all ccll division stopped for one year and started again the following spring; it is a dear, circular ring. In many types of wood, including 7/7/7*, rays of parenchyma cells form lines along the radii of the stem. These xylem rays appear as stripes running perpendicular to the concentric growth rings. The cells in the rays arc thin-walled parenchyma cells. Since they arc alive, carbohvdrates and minerals must be transferred later-
ally from the phloem. The nutrients arc transferred from cell to cell. In older wood, the innermost parenchyma cells of rays balloon into adjacent vessel elements, forming structures called tyloses. The tyloses help block the vessels of the older, inner growth rings and thus prevent many fungi from easily growing up through the open vessels. As the cells become tvloscs, thev release many
• » * j phenolic compounds. The phcnolics have antifungal properties that can provide another means of containing and controlling fungal infections.
At the outer edge of the secondary xylem (wood) lies the vascular cambium, a ring one to two cells in thickness. The vascular cambium is the mcristcm that divides to produce secondary xylem to the inside and secondary phloem to the outside, lhe cells of the vascular cambium are usually thin-walled and relatively small in diameter
in cross section. Sometimes they are hard to see. but look for them at the outer edge of the xylem.
Immediately outside the vascular cambium is the secondary phloem. This tissue conducts carbohydrates and minerals throughout a tree. In summer and fall, the carbohydrates arc moving from the pho-tosynthesizing leaves to all the growing portions of the plant, including the roots. In spring, some of the stored carbohydrates in the roots move up to the growing buds of the stems.
The secondary phloem is identified by the alternating triangular-shaped sections of darker staining ccUs. The triangles that point toward the center of the stem arc the phloem rays. Like the rays in the xylem, the phloem rays consist primarily of parenchyma cells. Alternating with these rays are wedges pointing outward. This region of the phloem contains the sieve-tube elements, which arc the primary carbohydrate-conducting cells.
Outside the phloem lie the remaining tissues. Although there are several tissues in this outer region, it is difficult to distinguish them, especially in Tilia. Collectively, we can refer to all these tissues as the periderm. The bark is everything outside the vascular cambium, so ii includes both the periderm and the secondary phloem.
Find all die tissues in the slide, and label diem on figure 15.2.
3. Transverse section of pine (Pinus). Examine a prepared slide of a transverse section of a 3-year-old stem of Pi mis. Before we go into depth, what do you notice that is different from the Tilia?
Pinus is typical of many conifers. If you look closely, Pinus docs not have anv vessel elements, onlv . * * * .
tracheitis in the secondary xylem (wood). You should see some large cavitics scattered within the secondary xylem. These cavitics are resin ducts, or resin canals. The resin ducts arc lined with parenchyma cells that secrete an olcorcsin. We make turpentine and resin
FIGURE 15.3 CROSS SECTION OF A WOODY. 3-YEAR-OLD PINE (PINUS) STEM.
from this olcoresin. For the tree, it probably works as an insect repellent or antifungal agent. You can see the difference between earlywood and latewood by comparing the average diameters of the tra chcids. In Pin us, the gradation between earlywood and latewood is more gradual throughout the growing sea son. Annual growth rings bccomc evident, however, when growth stops. Although we typically see only one growth ring per year, occasionally a double ring may occur. Many trees of the tropics that grow all year long show little or no growth-ring pattern.
Pinus also produces rays in the xylem and phloem. IIow is Pinussimilar to and different from Tilia:
Like Tilia, Pinus has a vascular cambium, secondary phloem, and periderm, and may still have a visible pith. Find all these tissues in the slide, label them on figure 15.3.
4. Three-dimensional structure of oak (Qiiereus).
Examine a prepared slide of oak (Querent) wood showing three faces of a three-dimensional block of the wood: transverse section, radial scction, and tangential section fig. 15.4). The transverse section is the same plane you observed in Tilia and Pinus. If the tree were standing, it would correspond to a horizontal cut. The radial surface would be a vertical cut parallel to the radius of the tree. The tangential surface would also be vertical, yet perpendicular to the radius and almost parallel to die outer surface of the stem. All three thin sections were made within the secondary xylem, so you arc only looking at cclls w ithin wood usually no bark or pith). It'vou stan with die transverse section, vou should rcc-ognize similarities with both Tilia and Pinus. You should see at least a portion of a growth ring and be able to distinguish earlywood from latewood. In Qticreus% the vessels arc more abundant in the earlywood, and the latewood is primarily all fibers. The high number of fibers helps make oak a relatively dense or "harder7' wood. The vessels give the wood a porous appearance. When vessels are arranged as .in oak (i.e., all the vessels clustered in die earlywood), the wood is referred to as ring porous. When vessels are scattered across the entire growth ring, both earlywood and latewood, the wood is referred to as diffuse porous. Do you think Tilia is classified as ring porous or diftiisc porous? Why?
In the radial section, the vessels appear wide and open. The stem, and thus the vessels, arc cut lengthwise. The cut was made down the radius of die stem, so you should see both die earlywcxxl and the latewood of at least one grow th ring. What arc the long, narrow cells tapered at both ends? Where arc the vessels more abundant? Where arc the fiber» more abundant?
While still looking at the radial view, what do you suppose are the bands of cells running across the vessels and fibers? How can you be more certain: What is the shape of these cclls?
The tangential section is also cut lengthwise, so the vessels and fibers look similar to those seen in the radial view. The xylem rays are cut perpendicular, so they appear
202 Applied Botany
FIGURE 15.4 THREE DIMENSIONS OF WHITE OAK (QUERCUS) WOOD. (A) THREE-DIMENSIONAL VIEW. (B) TRANSVERSE SECTION (C) RADIAL SECTION. (D) TANGENTIAL SECTION.
as a band of ceils diat collectively have a cigar shape. Some rays may be several cclls thick, while others are narrow. Xylera rays arc one of the diagnostic features used to distinguish types of wood.
When you can recognize vessels, fibers and parenchyma ray cells, you can use your knowledge to distinguish different types of wood as explained in the next exercise.
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