There is an apical meristem (tissue in which cells actively divide) at the tip of each stem, and it is this meristem that contributes to an increase in the length of the stem. The apical meristem is dormant before the growing season begins. It is protected by bud scales of the bud in which it is located and also to a certain extent by leaf primordia (singular:
primordium), the tiny embryonic leaves that will develop into mature leaves after the bud scales drop off and growth begins. The apical meristem in the embryonic stem of a seed is also dormant until the seed begins to germinate.
When a bud begins to expand or a seed germinates, the cells of the apical meristem undergo mitosis, and soon three primary meristems develop from it (see Fig. 4.1). The outermost of these primary meristems, the protoderm, gives rise to the epidermis. Although there are exceptions, the epidermis is typically one cell thick and usually becomes coated with a thin, waxy, protective layer, the cuticle. A cylinder of strands constituting the procambium appears to the interior of the protoderm. (The procambium produces water-conducting primary xylem cells and food-conducting primary phloem cells.)
The remainder of the meristematic tissue, called ground meristem, produces two tissues composed of parenchyma cells. The parenchyma tissue in the center of the stem is the pith. Pith cells tend to be very large and may break down shortly after they are formed, leaving a cylindrical hollow area. Even if they do not break down early, they may eventually be crushed as new tissues produced by other meristems add to the girth of the stem, particularly in woody plants. The other tissue produced by the ground meristem is the cortex. The cortex may become more extensive than the pith, but in woody plants, it, too, eventually will be crushed and replaced by new tissues produced from within. The parenchyma of both the pith and the cortex function in storing food or sometimes, if chloroplasts are present, in manufacturing it.
Stern-Jansky-Bidlack: I 6. Stems I Text I I © The McGraw-Hill
Introductory Plant Biology, Companies, 2003
Standing in Fields of Sto ne wareness
The combination of high altitude; dry, cool air; low rainfall; high winds; and poor soil has provided sustenance for the oldest known living species on the earth! These ancient warriors, whose great age was unknown until 1953, are the bristlecone pines that flourish atop the arid mountains of the Great Basin from Colorado to California. The oldest— determined to be almost 5,000 years old—is located in the Ancient Bristlecone Pine Forest high in the White Mountains of California. Some of the trees standing today were seedlings when the great pyramids were built, were middle-aged trees during the time of Christ, and today are hoary patriarchs standing in fields of stone.
There are two species of bristlecones, one living in the western-most regions (Pinus longaeva) and the other inhabiting the eastern regions (Pinus aristata). The trees do not grow very tall, with none over 60 feet and many much shorter. Typical of their girth is the bristlecone named the "Patriarch" that is just over 36.5 feet around, but it is a relative youngster at 1,500 years. With the short summer growing season high in these mountains, bristle-cones typically grow 1/100th of an inch or less in diameter in any given year. The trees stand in isolation, each appearing as a sentinel, overlooking an otherwise barren rock-strewn landscape. Their needles are remarkable. While occurring five per bundle and about 1 to 1.5 inches in length, they can live for 20 to 30 years before being cast off. This extraordinary leaf longevity gives the trees a stable photosynthetic output and sustains the tree during years of unusual stress when producing new leaf tissue would be difficult. The trees must generate new leaves and cones as well as produce enough reserves for the long winter months, all on scant annual precipitation of about 10 inches.
Ecologists have long noted the peculiar distribution and growth habits of certain plants. Why do plants grow where they do? What adaptations permit them to survive in life-threatening environments? The bristlecones' age and habitat offer several insights. These trees grow in places on earth where no other plant can grow. One answer appears to be the type of soil in which they are anchored. Stands of bristlecone pine grow on outcrops of dolomite, an alkaline limestone substrate of low nutrient but higher moisture content than the surrounding sandstone. The granite and sandstone formations surrounding the dolomite outcroppings support sagebrush and Limber pine, but not bristlecones. At these altitudes, the radiant sunlight is extreme. Dolomite reflects more sunlight than other rocks and thus keeps the root zones cooler and moisture-laden during the important growing season.
Box Figure 6.1 An ancient bristlecone pine (Pinus aristata) from the White Mountains of California.
Another survival tactic is revealed by taking small core samples of the wood. These trees have large amounts of die-back (deadwood that is no longer functional), reducing the amount of tissue that the leaves need to supply with food. For example, one bristlecone over 4,000 years old is nearly 4 feet in diameter, but its functional conducting tissue is only 10 inches wide.
Other characteristics provide bristlecones with survival advantages. Because their dense resin-filled wood renders them inhospitable sites for colonization by pathogenic fungi or bacteria, they are relatively free from these attacks. These trees are often struck by lightning, but with the absence of ground cover and decaying leaf litter, fire rarely spreads from tree to tree.
The age of these living trees is determined, like others, by an instrument called an increment borer, a type of drill that is inserted into the tree trunk at its widest girth. Using a hollow drill bit, a linear core of wood is obtained that can be "read" for the number of annual growth rings. In this way, the age of a tree can be obtained without cutting the tree down to examine the annual rings revealed in the cross section of the stump. Each year, a tree will add a layer of wood to its trunk, and these become the annual rings that can be observed in a cross section of the trunk. During spring growth of wood, large-diametered water-conducting cells are formed; later in the summer, the water-conducting
Awareness Box Continued cells produced have a small diameter. This difference in appearance between early (spring) wood and late (summer) wood is sufficient to make each growth increment distinctive, and thus counting of the rings possible.
In 1957, "Methuselah" was discovered and determined to be 4,723 years old. Methuselah remains today as the world's oldest living organism. But what about tomorrow? After surviving nearly five millennia, Methuselah is being protected against a more insidious enemy. Standing in its field of stone without a marker because of fear from vandalism, Methuselah serves as a reminder that the human species can be just as destructive as any microbe.
All five of the tissues produced by this apical meristem complex (epidermis, primary xylem, primary phloem, pith, and cortex) arise while the stem is increasing in length and are called primary tissues. As these primary tissues are produced, the leaf primordia and the bud primordia (embryonic buds in the axils of the leaf primordia) develop into mature leaves and buds (Fig. 6.2). As each leaf and bud develops, a strand of xylem and phloem, called a trace, branches off from the cylinder of xylem and phloem extending up and down the stem and enters the leaf or the bud. As the traces branch from the main cylinder of xylem and phloem, each trace leaves a little thumbnail-shaped gap in the cylinder of vascular tissue. These gaps, called leaf gaps and bud gaps, are filled with parenchyma tissue (Fig. 6.3).
A narrow band of cells between the primary xylem and the primary phloem may retain its meristematic nature and become the vascular cambium, one of the two lateral meristems. The vascular cambium is often referred to simply as the cambium. The cells of the cambium continue to divide indefinitely, with the divisions taking place mostly in a plane parallel to the surface of the plant. The secondary tissues produced by the vascular cambium add to the girth of the stem instead of to its length (Fig. 6.4).
Cells produced by the vascular cambium become tra-cheids, vessel elements, fibers, or other components of secondary xylem (inside of the meristem, toward the center), or they become sieve tube members, companion cells, or other components of secondary phloem (outside of the meristem, toward the surface). The functions of these secondary tissues are the same as those of their primary counterparts— secondary xylem conducts water and soluble nutrients, while secondary phloem conducts, in soluble form, food manufactured by photosynthesis throughout the plant.
In many plants, especially woody species, a second cambium arises within the cortex or, in some instances, develops from the epidermis or phloem. This is called the cork cambium, or phellogen. The cork cambium produces boxlike cork cells, which become impregnated with suberin, a waxy substance that makes the cells impervious to moisture. The cork cells, which are produced annually in cylindrical layers, die shortly after they are formed. The cork cambium may also produce parenchyma-like phelloderm cells to the inside. Cork tissue makes up the outer bark of woody plants;
it functions in reducing water loss and in protecting the stem against mechanical injury (see the discussion of periderm in Chapter 4).
Cork tissue cuts off water and food supplies to the epidermis, which soon dies and is sloughed off. In fact, if the cork were to be formed as a solid cylinder covering the entire stem, vital gas exchange with the interior of the stem would not be possible. In young stems, such gas exchange takes place through the stomata, located in the epidermis (see Figs. 7.6 and 9.13). As woody stems age, lenticels (see Fig. 4.14) develop beneath the stomata. As cork is produced, the parenchyma cells of the lenticels remain so that exchange of gases (e.g., oxygen, carbon dioxide) can continue through spaces between the cells. Lenticels occur in the fissures of the bark of older trees and often appear as small bumps on younger bark. In birch and cherry trees, the lenticels form conspicuous horizontal lines.
Differences between the activities of the apical meristem and those of the cambium and cork cambium become apparent if one drives a nail into the side of a tree and observes it over a period of years. The nail may eventually become embedded as the stem increases in girth, but it will always remain at the same height above the ground, as the cells that increase the length of a stem are produced only at the tips.
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