Plant Structure

One of the distinguishing characteristics that you may recall from the diversity unit is that all plants carry out the process of photosynthesis. This process is essential not only for the survival of a plant, but for the survival of all living things. The organelle whose duty it is to carry out this daunting task is the chloroplast. The image on the next slide highlights all the components of a plant cell. You should be able to describe the function of all the organelles shown below. If not, then please take the time to review.

Kind hearts are the gardens, Kind thoughts are the roots, Kind words are the flowers, Kind deeds are the fruits, Take care of your garden And keep out the weeds, Fill it with sunshine Kind words and kind deeds. Henry Wadsworth Longfellow

Please read the handout NOW.

Roots There are many analogies equating roots with positive attributes like strength, stability, and support. A person who has strong roots implies that he or she has a solid foundation that is built upon firm principles. It is no surprise then to learn that those qualities that are assigned to people are the same qualities that roots provide to plants. In addition to anchoring plants in soil, roots also absorb and transport water and minerals to the stems.

Not all roots are the same.  Tap roots grow deep into the ground in search of nutrients. They are generally large and fleshy and store food which is necessary for perennial plants (plants that last more than two growing seasons) which experience periods of dormancy.  Fibrous roots do not dig deep into the soil; rather they arrange themselves close to the soil surface and collect water and nutrients before they sink deep into the ground.  Adventitious roots are found in climbing plants like ivy. These roots develop from other plant tissue like stems and leaves. Adventitious roots can help support a plant.

Roots contain tiny little projections called root hairs. The root hairs absorb water and nutrients from the soil. Much like alveoli that increase surface area for gas exchange, the root hairs amplify the surface area which increases the rate of absorption.

Stems Stems provide the structural and physical support to leaves and flowers. The stems hold the leaves and flowers in a position that will allow for nutrient gathering and reproduction. Xylem and phloem cells transport materials within the stem from areas of plenty to areas of need. The xylem and phloem cells are arranged in vascular bundles. Within these bundles, the phloem cells are always situated closer to the outside of the stem while the xylem cells are located closer to the centre of the stem.

There is a thin layer of tissue that separates the xylem from the phloem in dicot plants. Stems also store nutrients for future use. Rhizomes, tubers, and bulbs are all types of stems that have been modified to suit the needs of a plant.

Tubers grow underground and develop buds called eyes Tubers grow underground and develop buds called eyes. These buds grow into new shoots above the ground. Bulbs are small stems that grow underground. They usually have short and thick leaves. Rhizomes grow just below the surface of the ground and are thick and fleshy. They can survive through the winter and develop buds in the spring.

There are two main types of stems.  Herbaceous stems are soft and green and woody stems are more complex and are hard. Most trees and plants that can survive through winter have woody stems. Woody stems grow thicker over time. Each year a new layer forms and the stem of the tree widens. This is the result of new xylem being formed each spring. The older xylem no longer transports fluids and fills with oils and other chemicals to become heartwood.

The vascular cambium appears as rings when looking at the cross section of a woody stem. The rings can be counted to determine the age of the tree. The outer part of a woody stem is called the bark. The bark is composed of phloem cells and cork tissue. These cells and tissues protect the stem from water loss.

Leaves The primary site of photosynthesis occurs at the leaves. Leaves are positioned on trees to receive and absorb the sun's rays. This is necessary to drive the process of photosynthesis. Leaves have many variations in their shape, size and structure. A compound leaf contains a leaf that is divided into many smaller leaflets. A simple leaf has just one blade.

Compound Leaves Simple Leaves

A leaf is made up of many specialized cells and tissues as illustrated in the image below:

1. Cuticle - The cuticle is a waxy, water resistant covering that protects the leaf from excessive absorption of light and evaporation of water. 2. Upper Epidermis - These cells appear on the top of the leaf and are transparent and colourless. They allow light to pass through to mesophyll cell where most of photosynthesis takes place. The epidermis lack chloroplasts therefore no photosynthesis takes place. 3. Palisade Mesophyll - These cells are arranged close together and contain chloroplasts. These photosynthetic cells form the bulk of plant leaf. 4. Spongy Mesophyll - These cells also contain chloroplast, but not as many as the palisade cells. The spongy mesophyll cells are not as densely packed together. This allows more surface area for gas exchange.

5. Lower Epidermis - These cells appear on the bottom of the leaf and are transparent and colourless. They allow light to pass through to mesophyll cell where most of photosynthesis takes place. The epidermis lack chloroplasts therefore no photosynthesis takes place. 6. Stoma - Stomata (pl.) are small openings located on the underside of the leaf of most plants. These small openings allow for oxygen, carbon dioxide, and water vapour to move into and out of the leaf. 7. Guard Cells - Guard cells are specialized epidermal cells that contain chloroplasts. Guard cells regulate the opening and closing of the stoma. When weather conditions are hot and dry, the guard cells collapse and the stoma closes.

The process that regulates the opening and closing of the stoma involves the movement of ions and water. This animation shows how the shape of the guard cells affects the condition of the stoma. Well it would if it worked! Grrrrrr.

8. Xylem - Xylem are specialized structures that transport water and minerals from the roots to the rest of the plant. 9. Phloem - Phloem cells are specialized structures that transport carbohydrates produced in the leaves to all parts of the plant. 10. Vascular Bundle - The veins of the leaf contain both xylem and phloem cells.

Homework Roots contain specialized cells and tissues. Determine the role of the: Epidermis, Cortex, Endodermis and Vascular Cylinder. If you use the Internet, be sure to use academic sites.