Plants: Control Systems (Part 4) Flashcards
Define tropisms.
Long-term
Plant responses in which the plant grows towards or away from a stimulus.
Plants respond to the environment (stimulus).
What is phototropism? Why? How? Mechanism?
The growth of a plant towards a light source.
Why? Maximizes light absorption for photosynthesis, which fuels plant growth
How? Plant cells respond to light by growing at different rates. When cells on the away side of the sun grow more elongated than the other side (to sun), the stem curves. ∴ Cell Elongation.
Mechanism:
Charles/Francis Darwin concluded that the tip of the seedling detects light, transmits that info to the stem, and the rate of growth of stem cells is affected.
Boyson Jsensen tested the presnce of a chemical signal, finding that the chemical could pass through gelatin but not mica.
(Hormone) In 1926, Frits Went confirmed that a chemical he named auxin (meaning to grow) was produced in the plant tip.
Auxin is actively transported through the cells towards the shaded side of the stem, causing cells there to grow longer than cells on the lighted side, resulting in bending towards the light.
What is gravitropism? Why? How? Mechanism?
The growth of a plant in response to the force of gravity. Gravity is the stiumulus.
a) Negative gravitropism: stem grows towards light and against the force of gravity
b) Positive gravitropism: roots grow into ground and towards the force of gravity
Mechanism:
Occurs as soon as seed germinate and the response of the stems and roots is consistent regardless of how the seed is placed when it’s planted. Auxin is responsible for the plant growth.
i) Stems:
When placed on its side, more auxin collects in the cells on the lower side. These cells then grow longer, resulting the stem’s curve up.
ii) Roots:
here, increased auxin concentration inhibits root growth. When a root’s placed sideways, auxin collects along the lower side but cell growth is inhibited here. Cells on the upper side continue to grow longer, resulting in the root growing down.
*Another theory of positive gravitropism is that dense strach grains in the root tip cells may settle at the low point in the cells signaling the direction of gravity and influecing the direction of growth.
What is nastic response?
A plant’s response to toucn. It is reversible, and as such, is not a tropism.
The stimulus sends an electrical signal to certain leaf cells resulting in a drop in turgour pressure (water). This causes the leaf to collapse, and wilt.
Ex: Venus Flytrap, Mimosa
What is thigomotropism?
The rapid growth of certain plant cells in response to structure.
It is seen in plants that use tendrils to wrap around supports or other plant stems.
Ex: Tendrils of a pea plant that come in contact with a chain-link fence will wrap around it, gaining support as it grows.
What are sleep movements?
Not a tropism.
Changes in turgor pressure in response to hours of light (day and night)
Less likely to be damaged. Ex: winds.
