What is the name of the food-conducting tissue in plants?

The ability of plants to grow and produce depends inescapably on the soil, from which their roots absorb the water needed for photosynthesis, as well as the mineral elements required for growth and cell functions. Roots also anchor the plant in the soil so it can hold itself upright and serve as a storage area for excess food. Roots therefore perform both mechanical and transport functions and contain structural features serving each.

Water absorption by roots is basically an osmotic process. As roots transport mineral nutrients into their xylem, the solute concentration in the xylem increases. This causes osmosis of water from the soil into the xylem through the membranes of intervening cell layers. Transpiration pull, caused by evaporation of water from the leaves, removes water from the root xylem causing water to move into the root from the soil. In terms of water potential, the removal of water from the root by transpiration pull reduces the water potential in the root xylem, creating a water potential difference between the soil water and the root xylem that drives inward water flow. The structure of roots gives them a rather large surface area to compensate for the slowness of uptake by any one root and provide an adequate total uptake of water. This osmotic absorption mechanism enables plants to take up water from soil that appears macroscopically to contain no liquid water and cannot be used as a water source by animals.

Regions of a root:

• Root cap - functions to protect the apical meristem and to penetrate the soil as the root elongates.
• Apical meristem - the growth region of the root. The lower side produces new root cap cells as old ones are rubbed off. The upper side produces new growth cells for root growth.
• Elongation region - cells produced in the apical meristem begin to elongate, producing an increase in the length of the root. Most of the growth in length of the root occurs here.
• Root hair region - as cells reach their maximum length, many epidermal cells develop lateral protrusions called root hairs. These serve to increase surface area for better absorption.
• Mature region - the region where cork begins to replace epidermal cells. This greatly reduces the ability of this area of the root to absorb water and minerals. Root hairs are not replaced in this area when lost.

Tissues of a root:

• Vascular tissues:
  • Xylem - water-conducting tissue. The term "xylem" comes from the Greek xylon, wood.
  • Phloem - food-conducting tissue. The term "phloem" comes from the Greek phloios, bark.
• Endodermis - the inner layer of the cortex. This selectively permeable layer filters absorbed soil water passing into the cells to the xylem.
• Cortex - serves as an area for food storage.
• Epidermis - a single layer of fatty cells serving to protect the cells beneath.

Types of roots:

• Primary - The primary, or seed root, is the first root formed by a sprouting seed.
• Adventitious - Roots originating from something other than root tissue, such as the lower portion of the stem.
  • Aerial - Adventitious roots developing above ground with specialized functions such as the climbing or hold-fast roots of some vines.

Types of root systems:

• Tap Root System - Smaller roots are organized around a large central root.
• Fibrous Root System - A mass of small adventitious roots as large as the primary root.

What is a perennial plant?

Stems: support flowers and leaves and provide transportation within the plant.

Most plants produce ordinary leafy stems with appendages such as leaves, and flowers growing from their nodes. These plants are called caulescent. A plant with basal leaves (either a very short stem or no above-ground stem) and a leafless flower stalk is called acaulescent. A young stem or twig of a flowering plant is marked by the presence of nodes, the points on a stem where a leaf or leaves are attached. The intervals between the nodes are called internodes. Although the root and stem share many common structural features, the root bears no appendages comparable to leaves, and consequently, has no nodes or internodes.

Three main stem types:

1. Herbaceous stems have very little woody tissue. The epidermis is very similar to that of leaves, being a single cell layer that secretes a waxy cuticle onto its outer wall.
   • Monocot stems have numerous vascular bundles scattered throughout the stem The xylem of each vascular bundle is located in the half of the bundle toward the center of the stem, while the phloem occurs in the half toward the outside surface of the stem. Each bundle is surrounded by a sheath of elongated, heavy-walled sclerenchyma fiber cells, which provide mechanical support for the stem.
   • Dicot stems have vascular bundles arranged in the form of a cylinder, around a central core of parenchyma tissue called the pith. The tissue outside the vascular bundles is called the cortex. The outer edge of the cortex, just beneath the epidermis, functions mainly in the mechanical support of the stem.
   • Herbaceous stems live for different lengths of time:
     • Annual plants - live for only one year.
     • Biennial plants - short-lived plants that produce only vegetative growth the first year, then bloom the second year.
     • Perennial plants - live for an indefinite period of years and bloom every year after the first.
2. Woody stems have a high percentage of woody tissue. A continuous vascular cylinder develops in a young woody stem. Leaf traces, strands of vascular tissue, extend from gaps in the vascular cylinder at the stem nodes and travel outward toward leaves and twigs. These gaps are soon closed by secondary growth, resulting in a solid cylinder of xylem and, to the outside of it, phloem. Between the xylem and the phloem lies the vascular cambium, a thin sheet of dividing cells which causes the growth in diameter of the stem. The xylem portion of the stem, as it becomes more massive by further secondary growth, becomes what we recognize as the wood of a tree. The tissues outside the wood and the cambial layer are called the bark. Its inner part comprises the tree's phloem, while the outer bark consists of multiple layers of tough protective tissue called cork.
   • Read about woody dicot stem morphology.
   • Read about the morphology of tree rings.

3. Shrub has several woody stems growing from a single base.
   • Suffrutescent - semi-shrubby stems that turn woody in the lowest parts and remain alive over the winter when the higher parts die back.
   • Fruticose - shrubby stems that are woody more or less throughout and commonly have several main stems but no main trunk.

Stem modifications:

Stolons: stems trailing above ground, which often root at their nodes and tend to produce new plants if the stem is broken.
Rhizomes: horizontal, underground stems that produce new shoots at their tips.
Tubers: thick, fleshy underground stems that serve as organs for food storage and reproduction.
Corms: fleshy, upright, underground stems with papery modified leaves or scales.
Bulbs: fleshy, upright, underground stems with fleshy leaves or scales.
Twining: slender stem branches that wrap and cling for climbing support.
Spines or Thorns: sharp and stunted stem branches. Some thorns may be modified leaves or leaf parts.

What type of soil water is most important to plants?

To complete the picture of how roots obtain water and minerals we must consider the structure and properties of soil. Soil is the combined product of the weathering processes by which rocks are degraded and the biological processes by which organic material is formed at the earth's surface.

Soils contain the following materials:

• Mineral particles, generally classified by particle size.
  • sands are made of the largest particles, from 2mm down to 0.05mm (50 micrometers) in diameter.
  • silts are composed of particles ranging in size from 50 micrometers down to 2 micrometers in diameter.
  • clays particles have a diameter less than 2 micrometers.
  • loams contain a combination of all three particle classes, thus a combination of the properties. The ideal loam contains about 20% clay and about 40% each of silt and sand. With a higher proportion of one of these components, the soil is called a clay loam, silt loam, or sandy loam.
• Inorganic ions, including the nitrates and phosphates needed by plants.
• Living cells, including algae, protozoa, bacteria, and various animals from earthworms to nematodes.
• Dead organic matter, humus, decayed by soil bacteria and fungi.
  • mineral soils make up most soils in humid, temperate climates and contain relatively small amounts, 1-10%, of humus.
  • organic soils are produced in bog conditions, consist primarily of organic matter, and have a spongy texture very retentive of water.
• Water, found in the pores between soil particles.
  • gravitational water drains freely through the largest pores (macropores). Except in soils with impeded drainage or where the water table comes close to the surface, gravitational water is present only just after rains or irrigation.
  • capillary water is found in the smaller soil pores (micropores). Micropores retain their water against gravity, yet hold most of it weakly enough that roots can absorb it. The finer textured the soil, the greater the amount of micropore space and the more useful water can be stored. When the micropore space is fully filled with water, as after a rain, the soil is said to be at its field capacity for water. When evaporation and removal by roots deplete a soil's capillary water, leaving only water that roots cannot take up, the soil is at its permanent wilting point. This represents the lower limit of soil moisture that will support plant activity.
  • hygroscopic water is water that humus and the surfaces of the soil's mineral particles bind by forces such as hydrogen bonding. Hygroscopic water and the free water in the smallest micropores are held too strongly for roots to absorb. This makes them unavailable to plants.
• Air fills the spaces left when gravitational water drains into the subsoil.

A soil's ability to supply mineral nutrients, chemicals other than carbon, hydrogen, and oxygen, is the principal factor in its fertility. Essential elements are those that are required for the growth of healthy plants. Macronutrients are elements required in relatively large amounts. Micronutrients, or trace elements, are needed only in minute amounts. Despite the small amounts needed, the requirement for most micronutrients is just as absolute as for macronutrients.

• Botany 210 - Texas A&M

Phloem is the food-conducting tissue in plants.
Food is mainly produced in plant leaves and must be carried downward to the rest of the plant. Remember that something "flows" downhill. Flow - Phloem

Perennial plants have herbaceous stems, live for an indefinite period of years, and bloom every year after the first.

The most important type of water in the soil is capillary water.
This is the water that is most often available for uptake into plants.