Warm Up 12/2

1. Compare growth due to apical and lateral meristems in dicots.
Apical meristems allow roots and stems to elongate.  Stem apical meristems allow the production of new leaves and flowers.  Lateral meristems allow the stems and roots to grow thicker.

2. Explain the role of auxin in phototropism.
Plants use hormones to control their growth and development.  Auxin is a growth promoter, which causes the secretion of hydrogen ions into the cell walls that loosen connections between cellulose fibers and causes cell expansion.  Phototropism is the directional growth of the plant in response to the source of light.  Shoots can typically detect light.  Auxin is produced in the shoot tips and is concentrated in the shaded side of the plant.  Growth on the shaded side causes the plants to bend towards the sun.

3. Explain how water is carried by transpiration stream, including the structure of xylem, vessels transpirational pull, cohesion, adhesion and evaporation.
Transpiration causes a flow of water from the roots through the stems to the leaves of plants.  This flow is called the transpirational stream.  The process starts with evaporation of water from the cell walls of spongy mesophyll cells in the leaf.  The water is pulled out of xylem vessels and through pores in spongy mesophyll cell walls by capillary action.  Cohesion is when water molecules are weakly attracted to each other via hydrogen bonds.  Adhesion is water molecules forming hydrogen bonds with the xylem cell wall.  Low pressure or suction is created inside xylem vessels when water is pulled out.  This is called the transpirational pull.  The xylem has a specialized structure to facilitate transpiration.  The inner lining is composed of dead cells that have fused to create a continuous tube.  These cells lack a cell membrane, allowing water to enter the xylem freely.  The outer layer is perforated, allowing water to move out of the xylem and into the leaves.  The outer cell wall contains annular lignin rings which strengthens the xylem against the tension created by the transpiration stream.