Plant Growth and Structure


Summary of Lecture

Germination and Growth of the Seedling

Tissue Organization and the Plant

Primary Growth in the Root

  • Differentiation in the Young Root Tip
  • The Stele
  • Root Systems

Leaves

Primary and Secondary Growth in the Stem


Germination and Growth in the Seedling

Seeds have:
  • a multicellular embryo with meristematic tissues for the first leaves and root already established
  • oils for high energy food storage and to help prevent decay
  • a seed coat to prevent desiccation and provide mechanical protection
  • carbohydrates in cotyledons which when broken down will supply the energy needed for rapid uptake of water for germination.

 

Angiosperm seeds are an effective evolutionary adaptation allowing plants to weather rough conditions. If they germinate when the conditions are not right the species could perish.

An Australian medical journal reported that a woman had suffered from persistent sore throat until doctors discovered a pear or apple seed had germinated in her throat and put down roots.

While it was reporting this peculiar plant performance, the journal went on to mention that doctors in south Africa had found a little boy with a chrysanthemum seed growing out of one eye and a British woman who had a tomato plant growing out of her upper gum. It seems a seed had gotten caught under her dentures and found the temperature and moisture conditions ideal. (Gardens for All)

Requirements for Germination

Dormancy may be broken when conditions favor plant growth. All seeds require:

  • adequate supply of water
  • favorable temperatures
  • presence of oxygen

Depending on the species some special conditions for seed germination are:

How dormancy is Broken

After the uptake of water the plant hormone gibberellic acid is released. It acts on the aleurone layer found just below the seed coat to stimulated the enzyme alpha-amylase. The amylase mobilizes the starchy reserves of the endosperm. The resulting sugars quickly supply the meristematic tissues the energy they need to begin cell division.

 

The Seedling

In all seedlings a root develops first and grows down into the soil. Then the shoot must emerge from the embryo and search out light above the soil and leaf litter. When stems and leaves emerge above ground chloroplasts in them rapidly mature turning the plant green. Although there are differences from plant to plant the basic pattern is the same and is illustrated below.

The epicotyl which emerges from a seedling is an example of primary growth.


Tissue Organization and the Plant

 

All plants have a special tissue that continues to divide called apical meristem which is generally found at the tip of roots or shoots. The apical meristem in turn gives rise to just 3 types of primary tissue which can divide and differentiate into all the other plant tissues. These primary meristems are:
  • Protoderm
  • Ground Meristem
  • Procambium

 

Given the right environment each plant cell can develop into a new plant -- the ultimate in cloning. Such cells and tissues are totipotent.

 

  1. Protoderm matures to form epidermis. This flattened irregular layer covers roots, shoots, and leaves. Some epidermal cells specialize to become:

 

  • guard cells (always in pairs) to help in gas exchange, generally found in the lower epidermis of leaves.
  • root hairs which emerge from the epidermal cells just above the root tip help increase the absorptive surface area of the root.

     

    Many epidermal cells produce a waxy water proofing, the cuticle of a leaf for instance.

    • leaf epidermis produces a wax called cutin
    • stem and root epidermis make subrin

     

    Where secondary growth occurs the epidermis within becomes a special layer of cells called periderm. Periderm is responsible for the production of a waterproofing material called subrin.

 

  1. Ground meristem differentiates into 3 important tissues:
    1. Parenchyma
    2. Collenchyma
    3. Sclerenchyma

      Parenchyma cells are generally spherical or slightly irregular in shape with thin walls. Parenchyma forms a versatile tissue which can be used as storage in the cortex region of roots or it may be loaded with plastids such as chloroplasts in the spongy or palisade layers of leaves. Wounds are filled with parenchyma cells.

      Collenchyma with its thickened but flexible walls is primarily used for support without preventing growth. Collenchyma cells lack the hardening agent called lignin. They remain alive at maturity, their thick walls create a supportive cylinder just inside the epidermis of young stems or within vascular tissue of leaf veins.

      Sclerenchyma has the ability to very the shape and thickness of its cell walls. The mature cells are dead and can form either strong supporting fibers or extremely hard stone cells (sclereids) common in seed coats and nut shells. These cells contain lignin.

 

  1. Procambium gives rise to the following vascular tissue:
    1. primary xylem
    2. primary phloem
    3. vascular cambium (important in secondary growth)

      Primary xylem has the dual roles of support and water transport, although the latter is by far the most important. When mature the lifeless tracheids and vessel elements form hollow, continuous, conduits from root to leaf

      All vascular plants contain tracheids, long slender cells with tapered ends. Pits along the sides and at the ends of these cells line up permitting water to flow from cell to cell. The pits of pit pairs consist of thin primary cell walls only.

      Only Angiosperms have vessel elements. These shorter, wider, cells align end to hollow end to form uninterrupted pipelines. The cell wall at the end of vessel elements is often missing, and pits dot their sides for lateral movement of water.

      Primary phloem is living tissue which is involved in the active transport and bulk movement of sugars and other nutrients. It is a complex tissue that has two major components:

      1. sieve elements and
      2. companion cells

    The backbone of phloem function is carried out by sieve tube members which when arranged end to end form sieve elements. Because the end walls of sieve tubes have pores materials dissolved in the cytoplasm can stream from cell to cell. Sieve tube members, while alive at maturity, lack a nucleus so their energy and metabolic needs are supplied by companion cells which lie adjacent to them. Thus companion cells also play a role in conduction of food.

    For additional information link to:


Primary Growth in the Root

 

Differentiation in the Young Root Tip

The root tip to be successful must accomplish 3 tasks:
  • growth (root apical meristem)
  • penetration of the soil (root cap and region of elongation)
  • absorption of water and minerals (root hairs in region of maturation and mycorrhiza)
Primary growth involves the ability of a cell with a potentially rigid cell wall to:
  • divide -- root apical meristem
  • elongate -- see article about expansin

Roots can penetrate soil without destroying the delicate meristem cells because they create a protective cap. The turgor pressure helps elongate cells above the root tip which forces the tip deeper into the soil.

As the epidermal cells of the root tip mature many will produce hair-like extensions. These root hairs intermingle with soil particles and perhaps the hypha of fungi to create a large absorptive surface area.

  • The epidermis is a single layer of cells which first produce root hairs and when mature the wax subrin.

 

  • Cortex cells are thin walled and generally used for storage
  • The endodermis produces the water tight casparian strip
  • The stele is composed of conductive tissues such as xylem and phloem and gives the root support. The pericycle lines the outer edge of the stele and can produce lateral roots

 

  • Root Systems
    1. Adventitious -- roots emerge from the stem
    2. Aerial roots are generally adventitious and found above ground (Corn)
    3. Prop roots are usually aerial roots which help support the trunk (Mangroves)
    4. Tap roots are a dominant main root which can grow deep into the soil.
    5. Diffuse root systems as in grasses have numerous branches producing a shallow mat.


Leaves

under construction
Notice which cells of the leaf have chloroplasts and which do not.
Follow this link for a microscopic view of the stoma (plural stomata) and this link for a micrograph of a leaf vein.

Leaf cross section and anatomy is illustrated by Ross Koning at this web site.


Primary and Secondary Growth in the Stem

The primary growth in the stems of monocots and dicots differ in the arrangement of vascular bundles. In monocots the vascular bundles have a random appearance scattered throughout the stem, but in dicots these bundles are located in a circle near the outer edge just below the epidermis.

In all vascular bundles the xylem is located toward the center of the stem and the phloem faces the epidermis.


Secondary growth results in an increase in the plant stem's girth or thickness. This occurs mostly in dicot perennials.

Perennials also exhibit open growth and an indeterminate life span. It is possible for these plants to live for a very long time, perhaps as long as 10,000 years, constantly growing in girth.

 

Dicot stem cross section