Lesson 9: Plant control systems

Lesson 9: plant control systems

Plant Control Systems

• Animals have the ability to sense and then respond to their environment by thinking and acting

• Plants cannot think, however, they do Respond to their environment as well

• These responses to the environment are called Tropisms in plants

Phototropism

• The growth of a plant toward a light source

• Phototropism allows the plant to maximize the amount of light that is absorbed by the leaves which in turn maximizes the amount of Photosynthesis within the leaves

• Since the plant can not move, Phototropism is accomplished in an interesting way In order to bend towards the light a plant will have the cells of its stem grow at different rates

• Cells on the side of the stem that is further from the light will grow longer than the cells on the side of the stem that are closer to the light

The Darwin's' Experiment

• In 1880 Charles Darwin and his son Francis performed experiment to prove that plants will grow toward a light source

• In their experiment they not only proved that plants respond to a light source, they also proved that it was the tip of the plant that would sense, respond and then send

The Boysen-Jensen Experiment

• Decades after the Darwin's' Experiment a Danish scientist named Peter Boysen-Jensen continued on with Darwin's idea

• He was interested in the actual signal that was sent from the tip of the plant in response to light

• He cut the tip of a plant off and replaced it with both Gelatin and Mica

• The Gelatin-tipped plant grew toward the light as the signals were able to diffuse through it

• However, the signal could not diffuse through the Mica and the plant grew straight up and did not respond to the light proving that the signals are sent from the tip of the plant to the rest of it

Auxins: Plant Growth Chemicals

• Fritz Went's Avena Experiment:

• In 1926 a Dutch Scientist named Fritz Went confirmed the hypothesis that a growth chemical signal is produced in the plant tips and are sent to the rest of the plant

• He extracted the chemical by removing the tips from young stems and then placing the tips in Agar (a growth material)

• The chemical signal diffused into the Agar

• He then took the chemical filled Agar and put them onto the cut tips of small Oat seedlings

Fritz Went's Results:

• Went found that the plants grew in different ways! Plant tip Completely covered by chemical soaked Agar = Plant Grew Straight Up

• Plant tip Partially covered by chemical soaked Agar - Plant Grew Away from the Side with the Chemical Soaked Agar

• Plant tip Completely covered by Non-chemical soaked Agar - Plant Did Not Grow At All

• Went concluded that the chemical produced in the plant tips stimulated Growth

• This chemical was called Auxin (Greek for "To Grow")

Action of Auxins:

• Auxins are produced in the tip of the plant as it responds to a light source

• After Auxin is produced in the tip of the plant it travels to the shaded side of the stem

• Active transport moves the Auxin into the cells of the shaded side of the plant

• The Auxin then causes the cells on the shaded side of the plant to grow longer than those cells on the lighted side causing the plant to bend toward the light

Gravitropism

• Gravitropism is a plant growth response to Gravity (an environmental stimulus)

• Positive Gravitropism:

• Positive Gravitropism occurs in the Roots of plants which the roots will grow downwards with gravity

• Negative Gravitropism:

• Negative Gravitropism occurs in the Stems of plants in which the stem will grow upwards against gravity

Gravitropism and Auxin

• Gravitropism also relies on the ability of the plant signal Auxin to elongate the cells on one side of the stem or the roots

• This is accomplished in opposite ways in the Stem vs. The Roots

• Gravitropism in the Stem:

• A plant will respond to gravity by releasing Auxin from its tip (ex. If a plant was on its side)

• The Auxin will be sent to the lower side of the stem stimulating the cells on that side of the stem to elongate

• The stem will bend to grow upwards against gravity

• Gravitropism in the Roots:

• A root will respond to Auxin in the opposite way

• The Auxin will be sent to the lower side of the roots inhibiting the cells on that side of the root from elongating

• The roots will bend to grow downwards with gravity

Nastic Response

• Some plants have the ability to respond to Touch Ex. Venus's-Flytrap and the Mimosa Plant

Cells and Complex Responses in Plants

• Imagine how difficult it must have been for ancient scientists to solve the questions about life works without some of the modern technologies we take for granted today

• The invention of the microscope and the discovery of the cell has allowed modern scientists to explain and even predict observable events such as the closing of a Venus's-flytrap!

• These responses. Performed by many co-ordinated cells, combine to create a fantastic whole: a living, respiring, moving, multicellular organism/