I. Plant Tissues
  • Angiosperms and to a lesser extent, gymnosperms are the groups that now dominate the plant kingdom. These seed-bearing vascular plants have complex aboveground shoot systems, which consists of stems, leaves, flowers, and some other structures. They also have complex root systems that typically grow downward and outward through soil.
  • There are three major categories of tissue systems in these plants. A ground tissue system makes up the bulk of the plant body. A vascular tissue system distributes water, dissolved minerals, and photosynthetic products through roots and shoots. A dermal tissue system covers and protects plant surfaces exposed to the surroundings.
  • The simple plant tissues (parenchyma, collenchyma, and sclerenchyma) are each composed of not more than one type of cell.
  • Complex plant tissues are made of two or more types of cells. Xylem and phloem and dermal tissues are like this.
  • Overview of the plant body - a typical flowering plant will be composed of the following parts:
    • Shoots which are comprised of aboveground parts - stems, leaves, flowers, and other structures. Stems provide structural support and conduct water and solutes.
    • Roots are specialized structures that penetrate soil and spread downward and outward. A root system absorbs water and dissolved minerals and anchors the aboveground parts. It is also responsible for storing food and releasing it when needed by the plant.
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  • Three Tissue Systems
    • Ground Tissue Systems is the most extensive and makes up most of the plant body.
    • Vascular Tissue System includes the xylem and phloem and is responsible for transporting water, dissolved minerals, and sugars.
    • Dermal Tissue System covers and protects the plant's surfaces.plt 6.png
  • Three Types of Simple Tissues
    • Parenchyma - makes up most of the soft, moist, primary growth of roots, stems, leaves, flowers, and fruits. The cells are pliable, thin-walled, and many sided. Often their cell division heal wounded plants. Mesophyll is a type of parenchyma in leaves that is photosynthetic.
    • Collenchyma - provides flexible support for primary tissues. Its living cells often form patches or cylinders near the surface of lengthening stem. These tissues have unevenly thick cell walls and they are infused with pectin, which is a gluelike polysaccharide that binds cellulose fibrils together and imparts pliability to the tissue.
    • Sclerenchyma supports mature plant cell parts and also protects many seeds. Most of these cells have extremely thick cell walls with lignin in them. Lignin makes the cell walls waterproof and even stronger. These tissues can flex and twists without stretching and are usually used to make cloth, rope, paper, and other commercial products.plt 7.png
  • Complex Tissue - made of two or more types of simple tissue
    • Vascular Tissue - composed of xylem and phloem and functions in the distribution of substances throughout the plant.
      • Xylem conducts water and dissolved mineral ions. The cells are dead at maturity, and their lignified walls interconnect. They are a water-conducting pipelines and also strengthen plant parts.
      • Phloem conducts sugars and other solutes through the plant. The cells are alive at maturity and are loaded with sugar in leaves and unloaded in areas such as growing roots and shoots that are using sugar.plt 8.png
    • Dermal Tissue - this is called epidermis and covers the surfaces of plants. In most plants this epidermis is a single layer of cells. These cells can excrete a waxy covering called a cuticle that protects the leaf from water loss and in a small part from infection. Stem and leaf epidermis contains a large number of specialized cells, such as guard cells that change shape in response to changing conditions to create or close a gap called a stoma.

  • Differences in Monocots and Dicots
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  • Structure of Stems - stems are made of the three tissue systems, but the tissue systems are arranged differently in monocots and dicots. The dicot arrangement is in a ring around the perimeter of the stem, while the arrangement in monocots is more scattered.
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  • Structure of Leaves - leaves vary greatly in size, shape, surface details, and internal structure. Leaves can resemble blades, spikes, cups, needles, feathers, tubes, and other structures. They can be different colors and even different odors. Some are edible and even more are poisonous. The typical leaf has a flat blade and petiole, or stalk, that attaches it to the stem. There are simple leaves and compound leaves as seen below.
    • Leaf Epidermis - covers the leaf surface exposed to air. May be smooth, sticky, slimy, hairy, scaly, spiky, have hooks, glands, and other surface specialties. Most plants have more stomata on the lower surface than the top of the leaf.
    • Mesophyll - Photosynthetic Ground Tissue. In Dicot plants there are two basic arrangements to this section of the leaf. Cells in the Palisade Mesophyll are usually located toward the top of the leaf closest to the sunlight and contain vastly more chloroplast per cell. They are arranged in columns and closer together than the Spongy Mesophyll. Spongy Mesophyll layer is usually located toward the bottom of the leaf and contains a more random arrangement with more space between cells to facilitate gas exchange. Monocot leaves do not have this arrangement of mesophyll cells because of their shape and arrangement.
    • Veins - The leaf's vascular bundles contain the xylem and phloem. In dicot leaves there is usually one or more main veins with smaller veins branching lacily away from the main veins. In monocot leaves there is a parallel arrangement as seen in corn plants and grasses.
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  • Structures of Roots - roots contain all three tissue systems. All roots elongate from the root tip which is protected by a cap of cells called the root cap. Rapidly dividing cells push the root tip through soil, sometimes as far as 175 feet. Each root as small root hairs which are extension of the cell membranes of epidermis tissue. These root hairs increase surface area and enable the plant to absorb more water and minerals. In monocots most root systems consists of a fibrous root system with most of the roots having the same size and all branching from the stem. In dicots a primary root forms called a tap root and then smaller branching roots project from this.
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II. Plant Nutrition and Transport
  • Water Transport through Plants - only a fraction of the water absorbed by plants is used for growth and metabolism. Most of the water is lost during evaporation from the stomata by a process called transpiration.
    • In a plant's vascular tissue, water moves through the pipelines called xylem. The botanist Henry Dixon came up with a good way to explain water transport in plants. It was his cohesion-tension theory.
      • The drying power of air causes transpiration which puts the water in the xylem in a state of tension. This tension is projected from the veins in leaves, through the stems, and into the young roots where water is being absorbed.
      • Unbroken, fluid columns of water show cohesion (the attraction of water to itself due to hydrogen bonding), and resists rupturing as the tension pulls them up the tiny tubes of xylem.
      • As water transpires it continues to place tension on water molecules, pulling them along.
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  • Conservation of Water - this is important to plants because they can lose up to 90% of the water they absorb through evaporation.
    Plant utilize special structures to minimize water loss.
    • Cuticles provide a barrier that keeps water in but allows sunlight to penetrate the surface and reach the mesophyll layers of the leaf.
    • Stomata are openings regulated by the shape-changing guard cells. When these are open gas can enter and leave the leaf as well as water. When they are closed nothing can enter or leave; especially water. Most plants keep their stomata open during the day to take advantage of the sunlight eventhough they lose water.
    • CAM plants such as cacti open their stomata at night and keep them tightly closed during the day. They perform the process of photosynthesis a little differently which allows them to reduce water loss in their extremely dry environments.
  • Nutrient Transport through Plants - phloem distributes organic products of photosynthesis.
    • Glucose is produced by photosynthesis in the leaves and some stems and converted to sucrose for transport. The site of sugar production is called the source.
    • Sugars are transported from a source (mature leaf cell) to a sink (a cell with low sugar levels like rapidly dividing root cells). Translocation is the process used to do this. High pressure drives this process. Sometime the pressure can be five times that found in automobile tires. Pressure at the source is always higher than at the sink so fluids are forced to areas of lower concentration and pressure.
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