The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.
The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium see Figure 1. Figure 2. Lenticels on the bark of this cherry tree enable the woody stem to exchange gases with the surrounding atmosphere. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem.
The cells of the vascular cambium divide and form secondary xylem tracheids and vessel elements to the inside, and secondary phloem sieve elements and companion cells to the outside.
The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.
In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells bark containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss.
The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm.
The phloem of some stems also contains thick-walled, elongate fiber cells which are called bast fibers. Bast fibers in stems of the flax plant Linum usitatissimum are the source of linen textile fibers. Gymnosperms generally do not have vessels, so the wood is composed essentially of tracheids. The notable exception to this are members of the gymnosperm division Gnetophyta which do have vessels. See Article About Welwitschia P ine stems also contain bands of cells called rays and scattered resin ducts.
Rays and resin ducts are also present in flowering plants. In fact, the insidious poison oak allergen called urushiol is produced inside resin ducts. Wood rays extend outwardly in a stem cross section like the spokes of a wheel. The rays are composed of thin-walled parenchyma cells which disintegrate after the wood dries.
This is why wood with prominent rays often splits along the rays. In pines, the spring tracheids are larger than the summer tracheids. Because the summer tracheids are smaller and more dense, they appear as dark bands in a cross section of a log. Each concentric band of spring and summer tracheids is called an annual ring.
By counting the rings dark bands of summer xylem in pine wood , the age of a tree can be determined. Other data, such as fire and climatic data, can be determined by the appearance and spacing of the rings. Some of the oldest bristlecone pines Pinus longaeva in the White Mountains of eastern California have more than 4, rings. Annual rings and rays produce the characteristic grain of the wood, depending on how the boards are cut at the saw mill. Microscopic view of a 3-year-old pine stem Pinus showing resin ducts, rays and three years of xylem growth annual rings.
In ring-porous wood, such as oak and basswood, the spring vessels are much larger and more porous than the smaller, summer tracheids. This difference in cell size and density produces the conspicuous, concentric annual rings in these woods. Because of the density of the wood, angiosperms are considered hardwoods, while gymnosperms, such as pine and fir, are considered softwoods. See Article About Hardwoods See Specific Gravity Of Wood T he following illustrations and photos show American basswood Tilia americana , a typical ring-porous hardwood of the eastern United States: A cross section of the stem of basswood Tilia americana showing large pith, numerous rays, and three distinct annual rings.
The large spring xylem cells are vessels. In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth. According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings.
Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season.
There is clearly a difference between the calcium content of wood during the wet and dry seasons that compares favorably with carbon isotope measurements.
The calcium record can be determined in one afternoon at the synchrotron lab compared with four months in an isotope lab. Poussart, P. Geophysical Research Letters 3: L Anatomy Of Monocot Stems M onocot stems, such as corn, palms and bamboos, do not have a vascular cambium and do not exhibit secondary growth by the production of concentric annual rings.
They cannot increase in girth by adding lateral layers of cells as in conifers and woody dicots. Instead, they have scattered vascular bundles composed of xylem and phloem tissue. Each bundle is surrounded by a ring of cells called a bundle sheath. The structural strength and hardness of woody monocots is due to clusters of heavily lignified tracheids and fibers associated with the vascular bundles. The following illustrations and photos show scattered vascular bundles in the stem cross sections of corn Zea mays : A cross section of the stem of corn Zea mays showing parenchyma tissue and scattered vascular bundles.
The large cells in the vascular bundles are vessels. This primary growth is due to a region of actively dividing meristematic cells called the "primary thickening meristem" that surrounds the apical meristem at the tip of a stem. In woody monocots this meristematic region extends down the periphery of the stem where it is called the "secondary thickening meristem. The massive trunk of this Chilean wine palm Jubaea chilensis has grown in girth due to the production of new vascular bundles from the primary and secondary thickening meristems.
Palm Wood T he scattered vascular bundles containing large porous vessels are very conspicuous in palm wood. In fact, the vascular bundles are also preserved in petrified palm. Cross section of the trunk of the native California fan palm Washingtonia filifera showing scattered vascular bundles.
The large cells pores in the vascular bundles are vessels. The palm was washed down the steep canyon during the flash flood of September The fibrous strands are vascular bundles composed of lignified cells.
Right: Cross section of the trunk of a California fan palm Washingtonia filifera showing scattered vascular bundles that appear like dark brown dots. The dot pattern also shows up in the petrified Washingtonia palm left. The pores in the petrified palm wood are the remains of vessels. The large, circular tunnel in the palm wood right is caused by the larva of the bizarre palm-boring beetle Dinapate wrightii shown at bottom of photo.
An adult beetle is shown in the next photo. There are about 50 genera and 1, species of bamboo , all in Poaceae , the grass family. Greatest diversity occurs in China ; India has the largest reserves of bamboo. An extremely useful and beautiful plant; heavily utilized in Asia and tropical countries. The culms have a hollow pith that is solid at the nodes like all grasses.
The strong, lightweight stalks are the reason bamboo is so useful. Large stems can be used as posts and rafters in houses, split sections form side walls, long sections of stem are ideal for irrigation pipe, short sections for containers or musical instruments flutes , they re woven into baskets or screens, young shoots can be cooked and eaten …etc. Many species flower synchronously - at the same time no matter where they are suggesting an internal control mechanism.
However, major production of rubber cones from Southeast Asia intercontinental transplantation. Hevea is a genus in the Euphorbiaceae Spurge family , a family that also includes poinsettias and the manioc tapioca. In Hevea , latex is produced in a series of latex vessels laticifers of the inner bark which anastomize braid through the phloem. Laticifers occur sporadically in angioperms , but not in gymnosperms. Latex is an emulsion insoluble in water of a variety of compounds, both elastic an inelastic opium poppy.
When water evaporates, the particles fuse, producing latex-like covering. Function of latex in plants :. By-products of primary chemical processes , secreted into laticifers to keep them from interfering with normal cell functioning. Rubber was first encountered by Europeans in Mexico , where Cortes saw natives playing ball games with balls made of rubber probably from Ficus elastica.
Amazonian rainforest Indians had the curious custom of dipping their feet in sap collected from a tree, and then holding them in the smoke of a fire.
The tree was Hevea brasiliensis Euphorb. Later, the Spanish and So. Americans began to dip their hats and cloaks in latex and smoke them to make them waterproof. In , Chas. Goodyear, in the U. Normally, rubber becomes soft with heat and brittle with cold. Goodyear found that chemical combination with sulfur under heat C and pressure cures both troubles, and gives rubber toughness and resistance to wear.
The invasion of Southeast Asia by the Japanese in World War II was impelled in part by their need for rubber and by their desire to deny it to the Allies. Developed process for producing synthetic substitute for natural rubber. Synthetic rubber uses petroleum. After s, demand for natural rubber increased along with the production of radial tires. Radial tires provide better handling and run cooler than convention bias-ply tires. Relatively stable market for natural latexes.
Regeneration of phloem is fairly rapid, and if trees are tapped in alternate years, a tree may produce for 30 years. The latex drips into a cup and is brought to collecting stations where the liquid is mixed with a mild acetic acid to coagulate the rubber. Most of the rubber in the Amazon basin is gathered in a way that does not destroy the tree, so people who gather it are strongly opposed to the destruction of the rainforest.
Each year about 1 billion trees are cut down to satisfy the demand for paper and paper products, with each American directly or indirectly using lbs of paper 2 lbs of paper per day. Each Sunday edition of the New York Times consumes about acres of forest. There were cuneiform clay tablets of ancient Babylon : the ancient Sumerians are thought to have developed the first written language 5, years ago. Their pictographic messages were recorded on soft clay inscribed with angular sticks, and later baked to make the writing more permanent.
Egyptian hieroglyphic writing dates to about years after the earliest Sumerian records. The writing surface the Egyptians employed was from papyrus, Cyperus papyrus , a sedge that grows naturally in Egypt and the Jordan River Valley , by streamsides. The mat was moistened with water, pressed, and dried. Parchment - was produced by rubbing skins of sheep and goats with lime instead of tanning them into leather. Both sides of skin is scraped, smoothed, and finally rubbed with powdered pumice.
Pa p er. Paper as we know it consists of matted plant fibers. The free fibers rose to the surface, the tangle was placed on a silk mold and dried in the sun — the first silk-screen.
Upon drying the paper was peeled from the screen. Soft fibers were used initially for papermaking, not supplanted by wood pulp until about It was discovered that paper could be mass produced using woods of conifers gymnosperms. Coniferous xylem tracheids are longer than the xylem vessels of angiosperms hardwoods : about mm vs.
In , the N. Times became the first newspaper to use all wood paper. Debarked wood is chipped, heated in presence of strong chemicals, which remove lignin from the cell wall and cause the maceration of the tissues — the cells become separated. Several agents — resins, gums, starches — may be added to pulp before fibers are floated onto draining screens.
The sheet of fibers drain and rollers press and mat the fibers. Nowadays, chemical processes are used for pulping. Sulfite process — acid process. Sulfate process — alkaline process.
The liquid pulp is poured onto a screen which intermeshes the wood fiber cells as a thin layer. Water is drained off; all water is subsequently removed by a set of heavy rollers using high pressure and high temperature. The process of papermaking uses strong chemicals.
Acids are especially harmful — they cause pages to be brittle, and to disintegrate earlier than the alkali method within yrs.
Acid-free paper is required for books and documents that can last more or less indefinitely. Kinds of paper. Newspaper — unbleached conifer paper. Ledger paper — bleached. Brown wrapping paper, hand towels, grocery bags, cardboard — conifer wood in which some lignin remains. Absorbent paper towels, toilet paper - loosely woven fibers without fillers of lignin. Fine stationary and cigarette papers - made from flax. Bible paper — made from hemp.
Papermaking requires a lot of water, so mills are located along rivers. Ecological consequences. Stink of wood pulp mills - some of the chemicals used to dissolve lignin away from cellulose are sulfur compounds. If you ever travel thru So. Carolina and Georgia you can smell the stink created by the use of these sulfur compound treatments: miles and miles of rotten egg smell.
Some of these sulfur compounds are converted to sulfuric acid in the atmosphere, returning to the earth as acid rain. This alters the alkalinity-acid balance of soils with potentially serious effects on the microflora growing within the soil and the plants growing on the soil. Threatens fresh water lakes. Clear-cutting large tracts of forested lands for pulpwood and lumber.
This has created erosion of large areas, with consequent silting in of streams having severe impacts on the insulted ecosystem. Loss of Mycorrhizae, the fungi associated with roots of woody plants, and serve as a conduit for soil nutrition ot the roots. Once an area has been clear-cut and erosion begins, many of these microorganisms die, making reforestation difficult. Waste disposal - until recently mercury compounds had been used in the processing of wood.
Along with sulfur compounds, bleaches, and dyes, mercury was dumped into streams poisoning plants, animals, and people using these waters. Lignin, virtually undegradable , is routinely discharged into the water courses, accumulating in large sludge beds and reducing the oxygen needed by aquatic animals and plants.
Chlorine used in bleaching process reacts with wood pulp to form dioxin, one of the most toxic chemicals known Agent Orange of Viet Nam infamy. Dioxin accumulates in fatty tissue of fish, birds, and other animals.
Alternatives to wood pulp. Levi Strauss recently began recycling denim scraps to produce paper for company stationary.
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