1. Light
2. Gravity
3. Water

Positive hydrotropism is when plant roots grow towards water.

Positive hydrotropism in the roots is beneficial for plants as it allows the roots to absorb more water, which is an essential medium for transport and a key reactant in photosynthesis.

IAA is a plant growth factor that controls plant cell elongation. It is a type of auxin.

1. IAA is produced in cells in the tip of the plant shoot
2. IAA is transported down the plant shoot
3. Light stimulates IAA to move from the light side of the shoot to the shaded side
4. IAA becomes concentrated and stimulates more cell elongation on the shaded side of the shoot
5. The shoot bends towards the light

1. IAA is produced in cells in the tip of the plant root
2. IAA is transported along the plant root
3. Light stimulates IAA to move from the light side of the root to the shaded side
4. IAA becomes concentrated and inhibits cell elongation in the shaded side of the root
5. The root bends away from light

1. IAA is produced in cells in the tip of the plant root
2. IAA is transported along the plant root
3. Gravity moves IAA from the upper side of the root to the lower side of the root
4. IAA becomes concentrated and inhibits cell elongation in the lower side of the root
5. The root bends downwards, towards the force of gravity

Plant growth factors are chemicals produced in plants that influence their growth and development.

Plant growth factors differ from animal hormones because:

1. Plant hormones usually act locally at the site of production, while animal hormones usually act on distant organs.
2. Plant hormones are produced in a variety of tissues throughout the plant, while animal hormones are produced in specialised cells within glands.

Plant growth factors are sometimes called plant growth regulators, or simply plant hormones.

1. Auxin binds to cell-surface membrane
2. Hydrogen ions are actively transported from the cytoplasm into the cell wall
3. The cell wall becomes more plastic
4. Cells elongate and the plant grows