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PLANT ORGANS • 1. THE BASIC PLANT ORGANS • Plants draw resources from two very different environments: below-ground and above-ground. Plants must absorb water and minerals from below the ground and carbon dioxide in light from above the ground. This led to the development of free basic organs: roots, stems, and leaves. Roots are not photosynthetic and would starve without the organic nutrients imported from the stems and leaves. Conversely, the stems and leaves depend on the water and minerals that roots absorb from the soil.
ROOTS • The root is an organ that anchors a vascular plant, usually to the soil. It absorbs minerals and water, and often stores organic nutrients. A taproot system consists of one main vertical root which gives rise to lateral roots. The taproot often stores organic nutrients at the plant consumes wearing flowering and fruit production. For this reason, root crops such as carrots, turnips, and sugar beets are harvested before they flower. Taproot systems generally penetrate deeply into the ground.
ROOTS • In seedless vascular plants and grasses, many small roots grow from the stem in what is called a fibrous root system. No roots stand out as the main one. Roots that arise from this damn are said to be adventitious. A fibrous root system is usually shallower than a taproot system this system makes grassroots particularly useful because they hold the top soil in place, preventing erosion.
ROOTS • The entire route system helps anchor of plant, but the absorption of water and minerals occurs primarily near the root tips, where vast numbers of tiny root hairs increase the surface area of the root enormously. A root hair is an extension of a root at the dermal cell. Absorption is often enhanced by symbiotic relationships between plant roots and fungi and bacteria.
STEMS • A stem is an organ system consisting of nodes (the points at which leaves are attached), and internodes (the stem segments between nodes). In the angle formed by each leaf and the stem is an axillary bud, a structure that has the potential to form a lateral shoot, commonly called a branch. Most axillary buds of a young shoot are dormant. Thus, elongation of a young shoot is usually concentrated near the shoot apex (tip), which consists of a terminal bud with developing leaves.
STEMS • The resources of a plant are concentrated at the apex for elongation growth to increase the plant's exposure to light. But what if an animal eats the end of the shoot? Or what if light is obstructed there? Under such conditions, axillary buds began growing. A growing axillary bud gives rise to a lateral shoot with its own terminal bud, leaves, and axillary buds. Removing the terminal bud usually stimulates the growth of axillary buds resulting in more lateral shoots. That is why pruning trees and shrubs and pinching back houseplants will make them bushier.
STEMS • Modified stems with different functions have evolved in many plants as an adaptation to the environment. These modified stems, which include stolons, rhizomes, tubers, and bulbs, are often mistaken for roots. A stolon is a horizontal stem that grows along the surface of the soil. These runners enable a plant to reproduce asexually, as plantlets form at nodes along each runner. An example is found in the strawberry plant. A rhizome is a horizontal stem that grows just below the surface of the soil. An example is the edible base of a ginger plant. A tuber is an enlarged end of a rhizome that has become specialized for storing food. An example is a potato. The eyes of a potato are clusters of axillary buds that mark nodes. A bulb is a vertical, underground shoot consisting mostly of the enlarged bases of leaves that store food. An example is an onion.
LEAVES • The leaf is the main photosynthetic organ of most plants, although green stems also perform photosynthesis. Leaves generally consist of a flattened blade and a stalk (the petiole), which joins the leaf to a node of the stem. Plants differ in the arrangement of veins, which are the vascular tissue of leaves.
LEAVES • Most monocot leaves (like grass) have parallel major veins that run the length of the leaf blade. In contrast, eudicot leaves (like trees and most other plants) generally have a multi-branched network of major veins. Plants are sometimes classified according to the shape of the leaves and the pattern of the veins.
LEAVES • Most leaves are specialized for photosynthesis. However, some plant species have leaves that have become adapted for other functions, such as support, protection, storage, or reproduction. Tendrils are modified leaves which allow a pea plant to cling for support. The spines of a cactus are modified leaves which serve as protection. Succulent plants, such as the ice plant, have storage leaves for storing water. The red parts of a poinsettia plant are often mistaken for petals but are actually modified leaves called bracts that attract pollinators. Some leaves are modified for reproduction, such as those which produce tiny plantlets, which fall off the leaf and take root in the soil.
PLANT TISSUES • 2. PLANT TISSUES • Each plant organ (root, stem, or leaf) has dermal, vascular, and ground tissues. A tissue system consists of one or more tissues organized into a functional unit connecting the organs of a plant.
DERMAL TISSUE SYSTEM • The dermal tissue system is the outer protective covering of a plant. Like our skin, it forms the first line of defense against physical damage and pathogenic (disease causing) organisms. In non-woody plants, the dermal tissue usually consists of a single layer of tightly packed cells called the epidermis. In woody plants, protective tissues known as periderm replace the epidermis in older regions of the stems and roots. In addition to protecting the plant from water loss and disease, the epidermis has special characteristics in each organ. For example, at the tip of roots, the epidermis has extensions called root hairs which absorb water and minerals. In the epidermis of leaves and most stems, a waxy coating called the cuticle prevents water loss.
VASCULAR TISSUE SYSTEM • The vascular tissue system carries out long distance transport of materials between roots and shoots. The two vascular tissues are xylem and phloem. Xylem conveys water and dissolved minerals upward from roots in to be shoots. Phloem transports nutrients such as sugars from where they are made (usually the leaves) to where they are needed (usually the roots, developing leaves, and fruits). The vascular tissue of a root or stem is collectively called the stele.
GROUND TISSUE SYSTEM • Tissues that are neither dermal nor vascular are part of the ground tissues system. Ground tissue that is internal to the vascular tissue is called pith, and ground tissue that is external to the vascular tissue is called cortex. The ground tissues system includes various cells specialized for functions such as storage, photosynthesis, and support.
TYPES OF GROWTH • Unlike most animals, plant growth occurs throughout the life of the plant. Except for periods of dormancy, most plants grow continuously. Eventually of course, plants die. Based on the length of their lifecycle, flowering plants can be categorized as annuals, biennials, or perennials.
Annuals • Annuals complete their lifecycle (from germination to flowering to seed production to death) in a single year or less. Many wildflowers are annuals, as are the most important food crops, including the cereal grains and legumes.
Biennials • Biennials generally live two years, often including a cold period (winter) between vegetative growth (first spring/summer) and flowering (second sprain/summer). Beets and carrots are biennials but are rarely left in the ground long enough to flower.
Perennials • Perennials live many years and include trees, shrubs, and some grasses. Some buffalo grass of the North American plains is believed to have been growing for 10,000 years from seeds that sprouted at the close of the last ice age. When a perennial dies, it is usually not from old age, but from an infection or some environmental trauma, such as fire or severe drought.
Meristems • Plants have embryonic tissues called meristems that allow the plant to grow indefinitely. Apical meristems, located at the tips of roots and in the buds of shoots, enable a plant to grow in length, a process known as primary growth. Lateral meristems allow for growth in thickness, known as secondary growth. In woody plants, the lateral meristems are called the vascular cambium and the cork cambium. The vascular cambium adds layers of secondary xylem (wood) and secondary phloem. The cork cambium replaces the epidermis with periderm which is thicker and tougher.
PRIMARY GROWTH • Primary growth lengthens roots and shoots. The new growth produced by apical meristems affects the entire plant if it is herbaceous. In woody plants, it only affects the youngest parts which have not yet become woody. Although apical meristems lengthen both roots and shoots, there are differences in the primary growth of these two systems.
PRIMARY GROWTH OF ROOTS • The root tip is covered by a root cap, which protects the delicate apical meristem as the root pushes through the abrasive soil during primary growth. Growth occurs just behind the root tip, in three zones of cells at successive stages of primary growth. Moving away from the root tip, they are the zones of cell division, elongation, and maturation.
PRIMARY GROWTH OF ROOTS • The primary growth of roots produces the epidermis, ground tissue, and vascular tissue. Water and minerals absorb from the soil must enter through the epidermis. Root hairs enhance this process by greatly increasing the surface area of epidermal cells. In most roots, the stele is a vascular cylinder, a solid core of xylem and phloem. However, in many roots, the vascular tissue consists of a central core of parenchyma cells surrounded by alternating rings of xylem and phloem.
PRIMARY GROWTH OF SHOOTS • The apical meristem of a shoot is a dome-shaped mass of dividing cells at the tip of the terminal bud. Leaves arise as leaf primordia, which are finger-like projections along both sides of the apical meristem. Axillary buds can form lateral shoots as well. Within a bud, leaf primordia grow in length due to both cell division and cell elongation.
SECONDARY GROWTH • Secondary growth adds girth to stems and roots in woody plants. Secondary growth is produced by lateral meristems. The vascular cambium adds secondary xylem and secondary phloem. Cork cambium produces a tough, thick covering consisting mainly of cork cells. Primary and secondary growth occurs simultaneously like in different regions. While and apical meristem elongates a stem or root, secondary growth commences where a primary growth has stopped.
SECONDARY GROWTH • The vascular cambium is a cylinder of meristematic cells one layer thick. It increases in circumference and also lays down successive layers of secondary xylem to its interior and secondary phloem to its exterior. In this way, it is primarily responsible for the thickening of a root or stem.
XYLEM • In plants, vascular tissue made of dead cells that transport water and minerals from the roots is called xylem. Water and minerals ascend from roots to shoots through the xylem. The xylem sap flows upward from the roots throughout the shoot system to veins that branch throughout each leaf. Leaves depend on this delivery method for their supply of water. Plants lose an astonishing amount of water by transpiration, the loss of water vapor from Leeds. A single plant can lose 125 L of water during a growing season. Unless the water is replaced, the leaves will wilt in the plant will eventually die. The upward flow of xylem sap also brings mineral nutrients to the shoots.
XYLEM • Xylem sap needs to rise more than 100 m in the tallest trees. To get to this height, it is either pushed up from the roots or pulled upward by the leaves. Root pressure pushes the xylem sap upward, especially at night. The root pressure at night sometimes causes more water to enter the leaves then is transpired, resulting in exudation of water droplets that can be seen in the morning on tips of grass blades or the margins of leaves. This is not the same thing as dew, which is condensed moisture produced during transpiration.
XYLEM • Root pressure can only force water upward a few meters, and it cannot keep pace with transpiration after sunrise. For the most part, xylem sap is pulled upward by the leaves themselves. This is accomplished by the transpiration-cohesion-tension mechanism, like sucking liquid through a straw. As moisture escapes the leaves by transpiration, one water molecule sticks to the other water molecules by cohesion, and the entire column of water rises. This transpiration pull can extend down to the roots only if the chain of water molecules is unbroken.
XYLEM • If an air pocket forms, such as when xylem sap freezes in the winter, the resulting air bubbles will break the chain. Air bubbles can also occur if there is an excess rate of evaporation of water from the leaves. This is common when the leaves are exposed to windy conditions, such as when plants are transported in the back of a truck. A plant can be killed in as little as 20 minutes of exposure to these conditions if the soil is not thoroughly watered before the trip.
PHLOEM • In plants, vascular tissue that consists of living cells that distribute sugars throughout the plant is called phloem. Organic nutrients (the products of photosynthesis) are translocated through the phloem. Phloem is arranged in sieve tubes that are positioned end to end. Between the cells are sieve plates, structures that allow the flow of sap along the sieve tubes. The main component of phloem sap is sugar (sucrose). This gives the sap a syrupy thickness. A sugar source is a plant organ that produces sugar by photosynthesis. Mature leaves are the primary sugar sources. A sugar sink is an organ that is a consumer or storage site of sugar. Growing roots, buds, stems, and fruits are sugar sinks. A storage organ, such as a tuber or a bulb, may be a source or a sink, depending on the season.
TRANSPIRATION • Gas exchange (transpiration) in plants occurs through structures called stomata. • The rate of transpiration is regulated by stomata, which are pores in the leaves. Carbon dioxide enters through the stomata into airspaces formed by the spongy parenchyma cells. This increases the internal surface area of the leaf by up to 30 times greater than what it appears when we look at the leaves. This increase in surface area improves the rate of photosynthesis however it also increases water loss through the stomata. Therefore, a plant requires a tremendous amount of water to make food by photosynthesis. By opening and closing the stomata, guard cells balance water conservation during photosynthesis.
TRANSPIRATION • A leaf may transpire are more than its weight in water every day and water may move through the xylem at a rate which is about equal to the speed of the tip of a second hand sweeping around a clock. If transpiration continues to pull sufficient water upward to the leaves, they will not wilt. But the rate of transpiration is greatest on a day that is sunny, warm, dry, and windy because of the increase in evaporation. Plants adjust to these conditions by regulating the size of the stomatal openings, but some evaporation still occurs when the stomata are closed. As cells lose water pressure, leaves begin to wilt.
TRANSPIRATION • Transpiration also results in evaporation cooling. This prevents the leaf from reaching temperatures that could damage enzymes involved in photosynthesis. Cactus plants have low rates of transpiration, but have evolved to tolerate high leaf temperatures.
NUTRIENTS • Watch a large plant grow from a tiny seed, and you cannot help wondering where all the mass comes from. About 90% of a plant is water which has accumulated within their cells. However, soil, water, and air all contribute to plant growth. Plants extract essential mineral nutrients from the soil, especially phosphorus and nitrogen. They also require other minerals as well. The symptoms of a mineral deficiency depend partly on the nutrient’s function. For example, a deficiency of magnesium, a component of chlorophyll, causes yellowing of the leaves, known as chlorosis.
SOIL QUALITY • Along with climate, the major factors determining whether a particular plant can grow well in a certain location are the texture and composition of the soil. Texture refers to the relative amounts of various sizes of soil particles. Composition refers to the organic and inorganic chemical components of the soil. In turn, plants affect the soil, taking part in a chemical cycle that sustains the balance of terrestrial ecosystems.
SOIL QUALITY • Soil originally comes from the weathering of solid rock. Rocks break apart over time from several mechanisms. Water can seep into crevices, freeze, and the expansion can fracture rocks. Acids dissolved in the water can also break down rocks chemically. Roots that grow in fissures can also cause fracturing. The eventual result of all this activity is topsoil, a mixture of rock particles, living organisms, and humus, the remains of partially decayed organic material.
Texture of topsoil • The texture of topsoil depends on the size of its particles, which range from coarse sand to microscopic clay. The most fertile soils are loams, made up of equal amounts of sand, silt (medium-size particles), and clay. The fine particles provide a large surface area for retaining minerals and water. Coarse particles provide airspaces containing oxygen that can be used by roots for cellular respiration. If soil does not drain adequately, roots suffocate because the air spaces are replaced by water; the roots may also be attacked by molds that favor wet soil. These are common hazards for houseplants that are overwatered in pots with poor drainage.
Soil composition • Soil composition includes organic components as well as minerals. Topsoil has an astonishing number and variety of organisms. A teaspoon of topsoil has about 5 billion bacteria along with various fungi, algae, insects, and worms. The activities of all these organisms affect the soils properties. Earthworms aerate the soil by their burrowing and add mucus that holds find soil particles together. The metabolism of bacteria changes the mineral composition of the soil. Plant roots can release organic acids, changing the soil pH. Plant roots also reinforce the soil against erosion.
Soil composition • Humus consists of decomposing organic material formed by the action of bacteria and fungi on dead organisms, feces, fallen leaves, etc. Humus prevents clay from packing together and builds a crumbly soil that retains water but is still porous enough for adequate air ration of roots. It is also a reservoir of mineral nutrients that are returned gradually to the soil as microorganisms decomposed the organic matter. During heavy rain or irrigation nitrogen and phosphate is leached away from the soil and drained into the groundwater deeper down, making them less available for uptake by roots.
Soil conservation • Soil conservation is essential. It may take centuries for a soil to become fertile through the breakdown of rock and the accumulation of organic material, but human management can destroy that fertility within a few years. Before the arrival of farmers, the Great Plains of the United States was covered by hardy grasses that held the soil in place despite of the long recurrent droughts and torrential rains characteristic of that region.
Soil conservation • In the late 1800s, many homesteaders settled in the region, planting wheat and raising cattle. These land uses left the topsoil exposed to erosion by winds that often swept over the area. During drought seasons, much of the topsoil was blown away rendering millions of acres of farmland into what was called the Dust Bowl. This forced hundreds of thousands of people to abandon their homes and land, as found in the story, The Grapes of Wrath.
Soil conservation • In healthy ecosystems, mineral nutrients must be recycled by the decomposition of dead organic material in the soil. When farmers harvest of crop, essential elements are removed. To grow 1000 kg of wheat, the soil gives up 20 kg of nitrogen, 4 kg of phosphorus, and 4 kg of potassium. Each year, soil fertility diminishes unless fertilizers replace these lost minerals. Additional irrigation is also necessary. More than 30% of the world's farmland suffers from low productivity stemming from poor soil conditions.
Fertilizers • Fertilizers are essential. Commercially produced fertilizers are enriched with nitrogen (N), phosphorus (P), and potassium (K). They are labeled with a three-number code called the N-P-K ratio, indicating the content of these minerals. A fertilizer marked as 15-10-5 indicates the percentage of each mineral. Manure, fish meal, and compost are called organic fertilizers because they are of biological origin and contain decomposing organic material. Before plants can use organic material, however, it must be decomposed into the inorganic nutrients that roots can absorb.
Fertilizers • Whether from organic fertilizer or a chemical factory, the minerals a plant extracts are in the same form, but organic fertilizers release minerals gradually, whereas commercial fertilizers are immediately available but may not be retained by the soil for long. Excess minerals not absorbed by the roots are usually wasted because they are leached from the soil by irrigation. To make matters worse, mineral runoff may pollute groundwater, streams, and lakes.
Fertilizers • Agricultural researchers are developing ways to maintain crop yields while reducing fertilizer use. One approach is to genetically engineer “smart” plants that inform the grower when a nutrient deficiency is imminent, before damage has occurred. One type of smart plant will produce a blue pigment in the leaves when phosphate is being depleted in the soil. Therefore, the farmer can add phosphate without needing to add other minerals that would be wasted.
Soil erosion • Soil erosion is another main concern. Thousands of acres of topsoil is lost to water and wind erosion each year in the United States alone. Certain precautions, such as planting rows of trees as windbreaks, terracing hillside crops, and cultivating in a contour pattern, can prevent loss of topsoil. Crops such as alfalfa and wheat provide good ground cover and protect the soil better then corn and other crops that are usually planted in more widely spaced rows.
NITROGEN • Nitrogen is often the mineral that has the greatest effect on plant growth and crop yields. It is ironic that plants can suffer from nitrogen deficiency because the atmosphere is nearly 80% nitrogen. However atmospheric nitrogen is in a gas form (N2) that plants cannot use. For plants to absorb nitrogen, it must first be converted to of ammonium (NH4) or nitrate (NO3). These to absorb mobile forms of nitrogen do not come from the breakdown of rock. They are generated by the decomposition of dead vegetation by certain kinds of bacteria, called nitrogen-fixing bacteria.
NITROGEN • All life on Earth depends on these special bacteria that can perform nitrogen fixation. Several species of these bacteria live freely in the soil, while others live in plant roots in symbiotic relationships. One of the most important crops that has this symbiotic relationship is the legume family, including peas, beans, soybeans, peanuts, alfalfa, and clover. Nitrogen-fixing bacteria live in the nodules of these plants and generate more useful nitrogen for themselves and the soil than all industrial fertilizers. When farmers plant the right amounts of these legumes at the right time, the soil becomes enriched at virtually no cost to the farmer.
Crop rotation • Crop rotation improves the quality of the soil. In this practice, a non-legume such as corn is planted one year, and the following year alfalfa or some other legume is planted to restore the concentration of nitrogen in the soil.
