Tissue Patterns In Stems

Steles

Primary xylem, primary phloem, and the pith, if present, make up a central cylinder called the stele in most younger and a few older stems and roots. The simplest form of stele, called a protostele, consists of a solid core of conducting tissues in which the phloem usually surrounds the xylem. Protosteles were common in primitive seed plants that are now extinct and are also found in whisk ferns, club mosses (see Chapter 21), and other relatives of ferns. Siphonosteles, which are tubular with pith in the center, are common in ferns.

Most present-day flowering plants and conifers have eusteles in which the primary xylem and primary phloem are in discrete vascular bundles, as discussed in the section "Herbaceous Dicotyledonous Stems."

Stems

Base Coleus Stem Tip
Figure 6.2 Alongitudinal section through the tip of a Coleus stem, ca. x800. (Photomicrograph by G.S. Ellmore)

am am

Bud Gaps Plant

vascular

Figure 6.3 A portion of a stem showing leaf gaps and bud gaps in the cylinder of vascular tissue.

vascular

Figure 6.3 A portion of a stem showing leaf gaps and bud gaps in the cylinder of vascular tissue.

Flowering plants develop from seeds that have attached to their embryonic stems either one or two "seed leaves," called cotyledons (see Chapters 8 and 23). The seeds of pines and other cone-bearing trees have several (usually eight) cotyledons.

The cotyledons usually store food needed by the young seedling until its first true leaves can produce food themselves.

Flowering plants that develop from seeds having two cotyledons are called dicotyledons (usually abbreviated to dicots), while those developing from seeds with a single cotyledon are called monocotyledons (abbreviated to monocots). Dicots and monocots differ from one another in several other respects; the differences in stem structure are noted in the following sections, and a summary of these and other differences in these two classes of flowering plants is given in Table 8.1.

Herbaceous Dicotyledonous Stems

In general, plants that die after going from seed to maturity within one growing season (annuals) have green, herbaceous (nonwoody) stems. Most monocots are annuals, but many dicots (discussed next) are also annuals.

The tissues of annual dicots are largely primary, although cambia (plural of cambium) may develop some secondary tissues. Herbaceous dicot stems (Fig. 6.5) have discrete vascular bundles composed of patches of xylem and phloem. The vascular bundles are arranged in a cylinder that separates the cortex from the pith, although in a few plants (e.g., foxgloves), the xylem and the phloem are produced as continuous rings (cylinders) instead of in separate bundles.

Chapter 6

i surface of stem or root center of stem or root f

I I cell of camhium I ~l Immature xylem cell r I Immature phloem cell

Figure 6.4 An illustration of how a cell of the vascular cambium produces new secondary phloem cells to the outside and new secondary xylem cells to the inside. Note, in cross section, that the cambium gradually becomes shifted away from the center as new cells are produced. Phloem is produced before xylem in secondary growth.

Portion Dicot Stem And Parts
Figure 6.5 A. A cross section of an alfalfa (Medicago) stem, x 100. The tissue arrangement is typical of herbaceous dicot stems. B. An enlargement of a small portion of the outer part of the stem, x 500.

The procambium produces only primary xylem and phloem, but later, a vascular cambium arises between these two primary tissues and adds secondary xylem and phloem to the vascular bundles. In some plants, the cambium extends between the vascular bundles, appearing as a narrow ring, producing not only the conducting tissues within the bundles but also the parenchyma cells between them. In other plants, the cambium is not in an uninterrupted cylinder but is instead confined to the bundles, each of which has its own small band of cambium between the xylem and phloem.

Woody Dicotyledonous Stems

In the early stages of development, the primary tissues of stems of young herbaceous dicots, woody dicots, and cone-bearing trees are all arranged in similar fashion. In woody plants, however, obvious differences begin to appear as soon as the vascular cambium and the cork cambium develop. The most conspicuous differences involve the secondary xylem, or wood, as it is best known (Fig. 6.6). Some tropical trees (e.g., ebony), in which both the vascular cambium and

Stems

evtew

Plant stems, which provide support for plants, also perform many specialized functions such as food and water storage, physical protection in the form of spines, and chemical protection by tannins and oils. Stem structure and growth influence a broad range of ecological relationships, ranging from success or failure in competition with other plants for light to surviving periods of drought and protection from attack by herbivores. Since the width of stem growth rings is influenced by climate, stems also provide a temporal record of environmental conditions and can be used to date events, such as fire, that leave marks on growth rings. Much of what we know about past climates has come from studies of the growth rings on plant stems.

the cork cambium are active all year, produce an ungrained, uniform wood. The wood of most trees, however, is produced seasonally. In trees of temperate climates, virtually all growth takes place during the spring and summer and then ceases until the following spring.

When the vascular cambium of a typical broadleaf tree first becomes active in the spring, it usually produces relatively large vessel elements of secondary xylem; such xylem is referred to as spring wood. As the season progresses, the vascular cambium may produce vessel elements whose diameters become progressively smaller in each succeeding series of cells produced, or there may be fewer vessel elements in proportion to tracheids produced until tracheids (and sometimes fibers) predominate.

The xylem that is produced after the spring wood, and which has smaller or fewer vessel elements and larger numbers of tracheids, is referred to as summer wood. Over a period of years, the result of this type of switch between the early spring and the summer growth is a series of alternating concentric rings of light and dark cells. One year's growth of xylem is called an annual ring. In conifers, the wood consists mostly of tracheids, with vessels and fibers being absent. Annual rings are still visible, however, since the first tracheids produced in the spring are considerably larger and lighter in color than those produced later in the growing season. Note that an annual ring normally may contain many layers of xylem cells and it is all the layers produced in one growing season that constitute an annual ring—not just the dark layers.

The vascular cambium produces more secondary xylem than it does phloem. Xylem cells also have stronger, more rigid walls than those of phloem cells and are less subject to collapse under tension. As a result, the bulk of a tree trunk consists of annual rings of wood. The annual rings not only indicate the age of the tree (since normally only one is produced each year), but they can also tell something of the climate and other conditions that occurred during the tree's lifetime (Fig. 6.7). For example, if the rainfall during a particular year is higher than normal, the annual ring for that year will be wider than usual. Sometimes, caterpillars or locusts will strip the leaves of a tree shortly after they have appeared. This usually results in two annual rings being very close together, since very little growth can take place under such conditions.

If there is a fire not resulting in the death of the tree, it may be possible to determine when the fire occurred, since the burn scar may appear next to a given ring. The most recent season's growth is directly next to the vascular cambium, and one need only count the rings back from the cambium to determine the actual year of the fire.

It is not necessary to cut down a tree to determine its age. Instead, botanists and foresters can employ an increment borer to find out how old a woody plant is. This device, which resembles a piece of pipe with a handle on one end, removes a plug of wood from the tree perpendicular to the axis, and the annual rings in the plug can then be counted. The small hole left in the tree can be treated with a disinfectant to prevent disease and covered up without harm to the tree.

A count of annual rings has produced some red faces on at least one occasion. The Hooker Oak, which was named in honor of Sir Joseph Hooker, a famous British botanist who once examined it, was located in the community of Chico, California. Until its demise in 1977, thousands of visitors from all over the world visited the huge oak, which provided enough shade for 9,000 persons on a midsummer's day. A plaque indicating the tree to be over 1,000 years old was located beneath the tree. A count of rings after its death, however, revealed the Hooker Oak to have been less than 300 years old.

When a tree trunk is examined in transverse, or cross section, fairly obvious lighter streaks or lines can be seen radiating out from the center across the annual rings (see Figs. 6.6 and 6.8). These lines, called vascular rays, consist of parenchyma cells that may remain alive for 10 or more years. Their primary function is the lateral conduction of nutrients and water from the stele, through the xylem and phloem, to the cortex, with some cells also storing food. Any part of a ray within the xylem is called a xylem ray, but its extension through the phloem is called a phloem ray. In basswood (Tilia) trees, some of the phloem rays, when observed in cross section, flare out from a width of two or three cells near the cambium to many cells wide in the part next to the cortex (see Fig. 6.6).

In radial section, rays may be from 2 or 3 cells to 50 or more cells deep, but the majority of rays in both xylem and phloem are 1 or 2 cells wide. Ray cells can be seen in cross section if a woody stem is cut or split lengthwise along a ray (Fig. 6.8). Another view of rays (in tangential section) is obtained when the stem is cut at a tangent (i.e., cut lengthwise and off center) (see Fig. 6.17).

As a tree ages, the protoplasts of some of the parenchyma cells that surround the vessels and tracheids grow through the pits in the walls of these conducting cells

Chapter 6

Chapter 6

Laticifers Rubber Tree Wood
Figure 6.6 Across section of a portion of a young linden (Tilia) stem, ca. x300.

and balloon out into the cavities. As the protoplasm continues to expand, much of the cavity of the vessel or tracheid becomes filled. Such protrusions, called tyloses (singular: tylosis), prevent further conduction of water and dissolved substances. When this occurs, resins, gums, and tannins begin to accumulate, along with pigments that darken the color of the wood.

This older, darker wood at the center is called heartwood, while the lighter, still-functioning xylem closest to the cambium is called sapwood (Fig. 6.9). Except for giving strength and support, the heartwood is not of much use to the tree since it can no longer conduct materials. A tree may live and function perfectly well after the heartwood has rotted away and left the interior hollow. It is even possible to remove part of the sapwood and other tissues and apparently not affect the tree very much, as has been done with giant trees, such as the coastal redwoods of California, where holes big enough to drive a car through have been cut out without killing the trees (Fig. 6.10).

Sapwood forms at roughly the same rate as heartwood develops, so there is always enough "plumbing" for the vital conducting functions. The relative widths of the two types of wood, however, vary considerably from species to species. For example, in the golden chain tree (a native of Europe and a member of the Legume Family), the sapwood is usually only one or two rings wide, while in several North American trees (e.g., maple, ash, and beech), the sapwood may be many rings wide.

Pines and other cone-bearing trees have xylem that consists primarily of tracheids; no fibers or vessel elements are produced. Since it has no fibers, the wood tends to be softer than

Stems cork cork cambium phelloderm cortex primary phloem broad phloem ray narrow phloem ray vascular cambium annual ring of xylem—

broad xylem ray narrow xylem rays primary xylem

Figure 6.6

pith

Continued

Stems annual ring of xylem—

broad xylem ray narrow xylem rays primary xylem pith

Narrow Xylem Narrow Phloem

primary phloem secondary phloem

primary phloem secondary phloem

that of trees with them and is commonly referred to as softwood, while the wood of woody dicot trees is called hardwood.

In many cone-bearing trees, resin canals are scattered not only through the xylem but throughout other tissues as well. These canals are tubelike and may or may not be branched; they are lined with specialized cells that secrete resin (discussed in Chapter 22) into their cavities (Fig. 6.11). Although resin canals are commonly associated with cone-bearing trees, they are not confined to them. Tropical flowering plants, such as olibanum and myrrh trees, have resin ducts in the bark that produce the soft resins frankincense and myrrh of biblical note.

While the vascular cambium is producing secondary xylem to the inside, it is also producing secondary phloem to the outside. The term bark is usually applied to all the tissues outside the cambium, including the phloem. Some scientists distinguish between the inner bark, consisting of primary and secondary phloem, and the outer bark (periderm), consisting of cork tissue and cork cambium. Despite the presence of fibers, the thin-walled conducting cells of the phloem are not usually able to withstand for many seasons the pressure of thousands of new cells added to their interior, and the older layers become crushed and functionless.

The parenchyma cells of the cortex to the outside of the phloem also function only briefly because they too become crushed or sloughed off. Before they disappear, however, the cork cambium begins its production of cork, and since new xylem and phloem tissues produced by the vascular cambium arise to the inside of the older phloem, the mature bark may consist of alternating layers of crushed phloem and cork.

The younger layers of phloem nearest to the cambium transport, via their sieve tubes, sugars and other substances in solution from the leaves where they are made to various

Chapter 6

Laticifers Rubber Tree Wood
Figure 6.7 Climatic history illustrated by a cross section of a 62-year-old tree. (Courtesy St. Regis Paper Company)

Stems 97

Para Rubber Cross Section Vascular

parts of the plant, where they are either stored or used in the process of respiration (discussed in Chapter 10). This sugar content of the phloem was in the past recognized by native Americans. Some stripped the young phloem and cambium from Douglas fir trees and used the dried strips as food for winter and in emergencies.

Specialized cells or ducts called laticifers are found in about 20 families of herbaceous and woody flowering plants. These cells are most common in the phloem but occur throughout all parts of the plants. The laticifers, which resemble vessels, form extensive branched networks of latex-secreting cells originating from rows of meristematic cells. Unlike vessels, however, the cells remain living and may have many nuclei.

Latex is a thick fluid that is white, yellow, orange, or red in color and consists of gums, proteins, sugars, oils, salts, alkaloidal drugs, enzymes, and other substances. Its function in the plant is not clear, although some believe it aids in closing wounds. Some forms of latex have considerable commercial value (see the discussion under "The Spurge Family" in Chapter 24). Of these, rubber is the most important. Amazon Indians utilized rubber for making balls and containers hundreds of years before Para rubber trees were cultivated for their latex. The chicle tree produces a latex used in the making of chewing gum. Several poppies, notably the opium poppy, produce a latex containing important drugs, such as morphine and heroin. Other well-known latex producers include milkweeds, dogbanes, and dandelions.

Monocotyledonous Stems

Most monocots (e.g., grasses, lilies) are herbaceous plants that do not attain great size. The stems have neither a vascular cambium nor a cork cambium and thus produce no secondary vascular tissues or cork. As in herbaceous dicots, the surfaces of the stems are covered by an epidermis, but the xylem and phloem tissues produced by the procambium appear in cross section as discrete vascular bundles scattered throughout the stem instead of being arranged in a ring (Fig. 6.12).

Each bundle, regardless of its specific location, is oriented so that its xylem is closer to the center of the stem and its phloem is closer to the surface. In a typical monocot such as corn, a bundle's xylem usually contains two large vessels with several small vessels between them (Fig. 6.13). The first-formed xylem cells usually stretch and collapse under the stresses of early growth and leave an irregularly shaped air space toward the base of the bundle; the remnants of a vessel are often present in this air space. The phloem consists entirely of sieve tubes and companion cells, and the entire bundle is surrounded by a sheath of thicker-walled scle-renchyma cells. The parenchyma tissue between the vascular bundles is not separated into cortex and pith in monocots, although its function and appearance are the same as those of the parenchyma cells in cortex and pith.

In a corn stem, there are more bundles just beneath the surface than there are toward the center. Also, a band of scle-renchyma cells, usually two or three cells thick, develops

Chapter 6

transverse surface bark <

transverse surface bark <

Oil Palm Vascular Bundle Tangential
tangential surface

fiber |

sieve-tube member | phloe"^ / cambium ray radial surface xylem

Figure 6.8 A three-dimensional, magnified view of a block of a woody dicot.

Coastal Redwood Seed
Figure 6.10 This coastal redwood is thriving despite the removal of its lower heartwood and a little of its sapwood.

Stems

Tannins And Resin Ducts

resin canals

Figure 6.11 Resin canals in a portion of a pine (Pinus) stem, ca. x400.

resin canals

Figure 6.11 Resin canals in a portion of a pine (Pinus) stem, ca. x400.

immediately beneath the epidermis, and parenchyma cells in the area develop thicker walls as the stem matures. The concentration of bundles, combined with the band of scle-renchyma cells beneath the epidermis and the thicker-walled parenchyma cells, all contribute to giving the stem the capacity to withstand stresses resulting from summer storms and the weight of the leaves and the ears of corn as they mature.

In wheat, rice, barley, oats, rye, and other grasses, there is an intercalary meristem (discussed in Chapter 4) at the base of each internode; like the apical meristem, it contributes to increasing stem length. Although the stems of such plants elongate rapidly during the growing season, growth is columnar (i.e., there is little difference in diameter between the top and the bottom) because there is no vascular cambium producing tissues that would add to the girth of the stems.

Palm trees, which differ from most monocots in that they often grow quite large, do so primarily as a result of their parenchyma cells continuing to divide and enlarge without a true cambium developing. Several popular house plants (e.g., ti plants, Dracaena, Sansevieria) are monocots in which a secondary meristem develops as a cylinder that extends throughout the stem. Unlike the vascular cambium of dicots and conifers, this secondary meristem produces only parenchyma cells to the outside and secondary vascular bundles to the inside.

Singular Detailed Meristem Cell

bundle sheath cell sieve-tube member companion cell vessel element air space ground tissue

(parenchyma)

Figure 6.12 A portion of a cross section of a monocot (corn—Zea mays) stem, ca. x 100. (Photomicrograph by G.S. Ellmore)

bundle sheath cell sieve-tube member companion cell vessel element air space ground tissue

(parenchyma)

Figure 6.12 A portion of a cross section of a monocot (corn—Zea mays) stem, ca. x 100. (Photomicrograph by G.S. Ellmore)

Figure 6.13 A single vascular bundle of corn (Zea mays) enlarged, ca. x 800.

Chapter 6

Several commercially important cordage fibers (e.g., broomcorn, Mauritius and Manila hemps, sisal) come from the stems and leaves of monocots, but the individual cells are not separated from one another by retting (a process that utilizes the rotting power of microorganisms thriving under moist conditions to break down thin-walled cells) as they are when fibers from dicots are obtained. Instead, during commercial preparation, entire vascular bundles are scraped free of the surrounding parenchyma cells by hand; the individual bundles then serve as unit "fibers." If such fibers are treated with chemicals or bleached, the cementing middle lamella between the cells breaks down. Monocot fibers are not as strong or as durable as most dicot fibers.

the runners and develop into new strawberry plants, which can be separated and grown independently. In saxifrages and some other house plants, runners may produce new plants at intervals as they grow out and hang over the edge of the pot.

Stolons are similar to runners but are produced beneath the surface of the ground and tend to grow in different directions but usually not horizontally. In Irish potato plants, tubers are produced at the tips of stolons.

Some botanists consider stolons and runners to be variations of each other and prefer not to make a distinction between them.

Tub ers

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Responses

  • TRANQUILLA
    Why corkcambium absent in coastal plant?
    8 years ago
  • sophie
    Where is the ground tissue arranged in a dicot root?
    8 years ago
  • eliana
    How are the three tissues arranged in a herbaceous dicot stem?
    8 years ago
  • wiktoria
    How to preparation Rubber tree?
    8 years ago
  • ABEL
    What are pigments appear in parenchyma cells of coleus stem?
    8 years ago
  • will
    Where is the siphonostele on a plant?
    8 years ago
  • julia
    What function do Fibres in xylem have?
    8 years ago
  • Carmen Burpee
    What are patches of vascular tissue in herbaceous dicots called?
    8 years ago
  • jared
    What tissue surrounds the xylem?
    8 years ago
  • jonne
    Where does parenchyma fail and secondary xylem win as a support tissue?
    8 years ago
  • stephanie
    Where in a plant do the vascular bundle(phloem xylem)occur?
    8 years ago
  • enrica
    Are resin canals found in dicots?
    8 years ago
  • rahel ambessa
    What term is applied to stem parenchyma tissue that is not separated into cortex and pith?
    8 years ago
  • colette pinckney
    Is xylem or phloem closer to the surface?
    8 years ago
  • ruth
    Is para rubber plant a dicot?
    8 years ago
  • manuela freytag
    What type of stele is present in herbaceous dicot stem?
    8 years ago
  • damiana
    Do herbaceous monocot and dicot stems have eusteles or protosteles?
    8 years ago
  • natascia
    What tissue is found in the centre of an herbaceous stem?
    8 years ago
  • vanna
    How develop plant from stem?
    8 years ago
  • veijo
    What is vascular tissue in a plant?
    7 years ago
  • tanta proudfoot
    Why does the woody stem looks different than the zea and medicago stems?
    7 years ago
  • AALIYAH RITCHIE
    Why do plants have there vascular tissue arranged in a ring near the periphery of the stem?
    7 years ago
  • tiblets
    Is heartwood primary xylem?
    7 years ago
  • Yohannes
    Why does the pith in a woody stem seem relatively smaller than a nonwoody stem?
    7 years ago
  • NEREA COCCI
    Why does a three year old woody stem look so different than the zea and medicago stems?
    6 years ago
  • bruna castiglione
    Are the vascular tissues in one year old tillia arrangedindescrete bundles or continous ring?
    6 years ago
  • elisa
    Why monocot leaves prefer in paper making?
    4 years ago
  • cooper macleod
    Which primary tissue is in a stem of a 100 year old oak tree?
    4 years ago
  • emanuele
    What is the pattern of tissue of stem?
    3 years ago
  • ANITA
    What breaks down pith of older woody stems?
    2 years ago
  • dawn
    What is the woody dicot stems of a plants?
    2 years ago
  • Belisarius Lothran
    Why tree wood removeing their protoplast?
    2 years ago
  • Robert Fuerst
    What are the transverse section of conifer and dicot wood?
    1 year ago
  • melba diggle
    How are the tissues of a Coleus stem arranged?
    6 months ago

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