Warm-Up 12/5

Apical meristems	Lateral meristems
Occur in the tips of stems and roots	Occur between xylem and phloem in stems
Produces soft tissues	Produces hard xylem tissue: wood
Lengthens roots and stems	Widens stems to support the weight of tall plants
Allows plants to develop special structures like leaves and flowers	Allows trees to grow tall, helping them to compete effectively for light.
Found in all phyla of plants	Absent in mosses and horsetails.

2. Plants use hormones to control the growth of roots and stems. When a plant releases a growth hormone in response to an external stimulus we call the resulting directional growth a tropism.

One type of tropism is phototropism: growth in response to light. Phototropism may be either positive (towards the light) or negative (away from the light).

Phototropism requires the absorption of light by proteins known as phototropins. Phototropins change to a new conformation (a new shape) when they absorb certain wavelengths of light. The new shape causes phototropins to act as ‘on switches’ for a gene that regulates the activity of auxins.

Auxins cause cells to become longer. Therefore, by releasing auxins on one side of a stem but not on the other side, a stem will bend because one side becomes longer than the other.

When a stem detects directional light it moves auxins from its sunny side to its shady side, which causes the shady side to bend toward the light. Bending toward light allows plants to absorb more sunlight and be able to photosynthesize at a faster rate.

Auxins cause cells to become larger in the following way: 1) they cause cells to actively transport hydrogen ions out of the cell, making the outside acidic; 2) the acid outside the cell makes the cell wall softer; 3) softer cell walls make the cells more stretchable; and 4) stretchy cells are bigger because the internal pressure inside the cell causes the cell wall to bulge out.

3. The xylem is a system of long hollow tubes responsible for replacing water lost during transpiration and photosynthesis. The xylem is made of two kinds of cells: tracheids and vessels. Xylem cells die before they are functional: after they die they become long, narrow tubes with pores at each end that allow water to pass through them.

The xylem sap moves from roots, through the stem, to the leaves without any energy being spent by the plant.

Three processes cause water to rise up the xylem tube:

Root pressure: Water moves into roots by osmosis because the roots have high concentrations of solute. This causes a positive pressure that forces sap up the xylem towards the leaves. Root pressure is highest in the morning before the stomata open and allow transpiration to begin.

Capillary action: The xylem is a long tube that is microscopically thin. When water molecules contact the surface of the xylem there is adhesion. Adhesion tends to pull water molecules upward by a process called capillary action.

Transpiration pull: When water molecules evaporate from leaves the water potential drops at the stomata. The low pressure then pulls new water molecules towards the stomata from the xylem vessels. As these water molecules move they pull on water molecules behind them due to cohesion (caused by hydrogen bonding). The pull is transmitted from one water molecule to the next, all the way to the roots.