Put the beaker in a light-proof box with a circular hole at one of its sides, to admit light.

Leave it for few days.

Observation:

The stem inclines towards the source of light, while the roots incline away from it.

Conclusion:

Tropism is due to the unequal growth rates at the two sides in both the root and the stem. The side of the stem away from light grows more rapidly, while in case of the roots the opposite occurs.

Explanation of phototropism:

Boysen Jensen found that:

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

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

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:

He exposed an Oat coleoptile to unidirectional illumination.

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

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:

Date: 2015-12-17; view: 692