Plants have three main parts:
   1. The Roots
       Roots support the plant and take in water and minerals from
       the soil through their root hairs.
   2. The Stem
       The stem supports the plant.  It has veins to transport materials
       inside the plant.  Xylem carries water, phloem carries food.
   3. The Leaves
       The leaves carry out photosynthesis to make food for the plant.

Respiration is the process where our bodies use oxygen to burn food for energy.  The waste products are carbon dioxide and water.

     Sugar + Oxygen => Carbon Dioxide + Water + Energy

Photosynthesis is the process where plants use sunlight to turn carbon dioxide and water into sugar and oxygen.

     Energy + Carbon Dioxide + Water => Sugar + Oxygen

Phloem carries food (sugar) down from the leaves to the roots, where it is stored.  This operates by gravity.

Xylem carries water and minerals up from the roots to the leaves by TACT.  TACT stands for...
  Transpiration - Water evaporating out of the leaf pulls up the
                          water behind it.
  Adhesion - Water molecules stick to other molecules (such as
                    the wall of the xylem).
  Cohesion - Water molecules stick to each other.
  Tension - The surface of water tries not to break.  This is what
                 allows you to fill a cup above the actual rim.

There are two basic types of plant stems:
  1. Herbaceous stems are green and flexible.
  2. Woody stems are rigid and made of wood.

Most herbaceous plants are annuals.  This means that they only live for one year.  The flowers that you plant along your walk - that you have to re-plant every year - are annuals.

All woody plants (trees, shrubs, etc.) are perennials.  This means that they live for many years.  Woody plants have growth rings.  Each ring represents one year of growth.  Wider rings indicate wetter years, while narrow rings indicate dry years.

Stems contain three types of tissue:
  1. Xylem is a type of vein that carries water.
  2. Phloem is a type of vein that carries food.
  3. Cambium is the growth tissue of the plant.  This allows the
      plant to grow and can make new xylem and phloem.

Plant Behaviors are called tropisms.  There are basically two types: positive (when the plant grows towards something) and negative (when a plant grows away from something).  Some examples are:
  - phototropism: The plant grows towards the light.  [positive]
  - geotropism: The roots grow towards gravity. [positive]
                         The stem grows away from gravity. [negative]
  - thigmotropism: The plant responds to touch.