1. Oat (Avena) coleoptile lose its ability to bend towards light, if the tip is cut off (1-2 mm of the tip).

2. This ability is restored if the decapitated tip is returned or fixed again to the tip with gelatin.

3. If the tip is separated from the remaining coleoptile with mica sheet, there will be no curvature.

The indicates that the tip of the coleoptile synthesizes chemical substances, that diffuse through gelatin, and affect the growth. It can not pass through mica sheet. These chemicals are termed Auxins, the main one is known to be Indole-acetic acid (IAA). Curvature towards light is the result of the unequal growth of the two sides which is due to the unequal concentration of auxins on the two sides of the coleoptile.

Went experiment to prove the validity of Boysen Jensen explanation:

1. He exposed an Oat coleoptile to unidirectional illumination.

2. He cuts off the tip and placed it on two Agar blocks separated by a metallic sheet.

3. He measured the concentration of auxins in each block.

Observation:

He found that a great amount of auxins accumulated in the Agar block next to the side of the tip away from light.

Conclusion:

Auxins move from the side facing light to the far (darkened) side.

Went found that these blocks of Agar will produce curvature if they are placed on decapitated coleoptiles which are not subjected to light. Auxins move away from light to the non-illuminated side. This leads to elongation of cells on that side thus curvature towards light occurs. The stem is known to be positively phototropic.

The difference in the behavior of the root and stem towards unidirectional illumination:

Accumulation of auxins in the dark side of the root produces an opposite effect, elongation is inhibited in the dark side, while the illuminated side continues to grow. The root curvature tends away from light, and the root is known to be negatively phototropic. The concentration of auxins required for the elongation of root cells is less than that required for the elongation of stem cells. So, the increase in auxins concentration will inhibit cell-elongation in roots, whereas it activates cell-elongation in the stem.

B. Geotropism:

Geotropism is the response of plant parts to the external stimulus of gravity, where they move away or towards the stimulus. The root grows vertically downwards, while the stem grows upwards.

It was believed that roots grow downwards in order to avoid light and seek nutrients, but this is incorrect.

Experiment:

Hang a potted plant upside for a time.

Observation:

The root grows downwards away from the soil, and towards gravity, while the shoot grows away from gravity.

Conclusion:

Growth of the root downwards is a positive response to gravity, while the growth of the stem upwards is a negative response to gravity.

Experiment:

1. Germinate a number of seeds in a soil moistened with water.

Observation:

1. The plumules grow vertically upwards, while the radicles grow downwards.

Experiment:

2. Place one seedling on its side so that the radicle and the plumule are horizontal.

3. Leave it for several days.

Observation:

2. The plumule grows upwards and the radicle grow downwards.

Conclusion:

The stems are negatively geotropic, while the roots are positively geotropic. Curvature is due to the variation in growth rates at the two sides of the plant organ, which is due to the unequal distribution of auxins.

Herman’s experiment:

Herman’s experiment showed that:

1. The total amount of auxins in the tips of Oat coleoptiles does not change by altering the position from the vertical to the horizontal.

2. There is a great variation in auxins distribution:

a. In the vertical position: Equal amounts of auxins move into the two halves from the tip.

b. In the horizontal position: More auxins diffuse into the lower half.

This variation in auxins distribution leads to unequal growth rates of the two sides of the Oat coleoptile.

Conclusion:

In the normal vertical position:

Auxins are equally distributed in the tips of both root and stem. As a result, the stem grows vertically upwards, while the root grows downwards.

In the horizontal position:

Auxins accumulate in the lower side of both the root and the stem:

In the stem: Auxins activate the cells of the lower which elongate more than those of the upper side. So, the tip of the stem curves upwards against gravity.

In the root: Auxins inhibit the elongation of the cells of the lower side, so the cells of the upper side elongate more. So, the root bends downwards.

The increase in auxins concentration –to a certain extend- inhibits the elongation of the root cells, while it activates those of the stem.

C. Hydrotropism:

Experiment:

1. Germinate a number of seeds in two glass troughs containing equal quantities of dry soil.

2. Spray water regularly in the first trough, but only at the sides in the second trough.

3. Leave the two troughs for several days.

Observation:

Roots in the first trough grow straight down, while roots in the second trough grow towards the water at the sides of the trough.

Conclusion:

In the first trough (A):

The roots grow vertically due to the equal distribution of water in the soil around the roots.

In the second trough (B):

Curvature of roots is due to the presence of water at the sides and its absence in the middle. Auxins accumulate in the side of the root facing water, and inhibits the elongation of cells of that side, while the cells of the far side continue their normal growth which leads to curvature of the root towards water.

The root is known to be positively hydrotropic.