Response to Stimulus In Roots

The forces that influence the direction of growth of roots are moisture, gravity, and light. They grow towards moisture, towards the centre of the earth and away from light. That is, they are positively hydrotropic and geotropic, but (with qualifications) negatively heliotropic.

From the seedlings growing in the gas-jars one should be chosen with a straightly growing root, about an inch and a half in length, and the root marked off as before into millimetres. It must then be pinned in a horizontal position, through the seed-part, to a strip of cork which stands in a gas-jar, or better still in a larger, rectangular jar, with 2 inches or so of water at the bottom.

Responding to the double stimulus of gravity and moisture, the root makes a right-angled bend, so that the tip points directly downwards. What is significant in this is, that the bend, as instanced by the divergence of the ink marks, has only taken place in the actively growing region. When the experiment is repeated with a similar seedling that has had its root-tip cut off, no bend occurs.

The conclusion, then, is that the tip actually receives the stimulus and that this is transmitted by the protoplasm from cell to cell until, in the elongating region, and here only, there is active response and the bend occurs.

In some few plants it has been shown that the growing region of the root is itself sensitive to stimulus. In the Lupin the root makes the bend even when the tip has been removed.

By slipping soaked seeds in any position between blotting-paper and gas-jar the same result is seen. No matter how the seed is placed, the root grows down and the stem grows up.

Geotropism

A very simple way of showing the response of the root to the action of gravity alone, is to fix the root of a straightly growing bean seedling in the smallest possible hole in a cork that fits tightly into a gas-jar full of water. For the success and continuation of the experiment all connections, between root and cork, and between cork and glass, must be made absolutely water-tight by using paraffin wax.

When the plant has grown in a normal position for two or three days the jar should be turned upside down, being held by a clamp or an iron ring. If the apparatus is water-tight there will be no drip, and the plant may be left in this position for some weeks.

Inside the gas-jar the main root bends, in the growing region only, to grow vertic- ally downwards. The branch roots take up their normal position. The stem makes a definite bend, also in the grow- ing region, and grows vertically upwards.

The Klinostat

This response to gravity may also be shown by using a klinostat, or a clock, in which the minute hand has been replaced by a firmly fixed rod, projecting outwards from the face. Damp moss must be tied very tightly to this projecting arm, so that it is sure to rotate with the arm when the clock is wound up.

Pea seedlings are then pinned, in any position, to the moss, the clock is wound up, and the whole apparatus is put under a large bell-jar, so that the seedlings are in a moist atmosphere.

The shoots and roots continue to grow in the direction that they held when first they were pinned to the moss. If root and shoot were originally horizontal, they keep this position.

This is because the force of gravity is never concentrated on the actual tip of root or stem. These are solid cylinders.

No sooner does one point on the circumference of the tip respond to the attraction, than the rotation of the arm brings the next point under the influence of the gravitational pull. As this responds another turn brings yet another point into the direct range of the stimulus, and so on and so on, the ultimate result being that every point on the circumference is, in turn, equally influenced. The root and stem, consequently, grow on in their initial direction, the gravitational influence having been rendered null and void.

Heliotropism

Heliotropism is the particular tropism, or response, that plant-organs make with reference to the stimulus of light. Main stems grow towards the light, that is, they are positively heliotropic.

The general statement that roots are negatively heliotropic needs qualification. Apparently all aerial roots are negatively heliotropic : the adventitious roots of Ivy always grow on the side of the stem that is in the shade.

Many earth roots, but certainly not all, are also negatively heliotropic. This is true of the Sunflower and of most of the members of the Crucifer. This response may be seen when soaked Sunflower seeds are allowed to germinate in coco-nut fibre in a glass-fronted box. The “seeds “ are planted close up against the glass front, which faces a window with a good light. The roots grow inwards, away from the light.

If Broad Beans are used instead of Sunflowers, the roots generally grow inwards at first. Then they come to view once more. Although after this they do not actually burrow backwards into the darkness, they grow in an irregular way, as though irritated by the light. The same experiment may be repeated with black paper covering the glass front, so that all light is excluded. The root-tips are in both cases attracted backwards by the moisture ; they are attracted downwards by gravity. If the box be tipped slightly forward the glass front makes a diagonal in a parallelogram of forces and the roots grow diagrammatically, closely pressed against the glass. If the front is not darkened and is allowed to face the light, a deeper tilt is necessary to make the roots grow against the glass.

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