The Acid Reaction of Roots

In spite of the intervention of the primordial utricle there is, as a matter of fact, some slight exosmosis from root-hairs. In other words, some of the acid of the cell passes out into the soil. By means of this acid certain substances in the soil, which are insoluble in water, are brought into solution and so made available for absorption by the root.

Really air-tight the water will not run back along this arm when the funnel tap is turned off.

When the Fuchsia roots absorb water from the gas-jar, the loss in the jar is made good by a flow of water into it from the horizontal tube. A scale marked out in inches is attached to the tube, so that the time taken by the water in travelling backwards may be read accurately : thirteen minutes, let us say, for the first inch ; eleven minutes for the second ; twelve minutes for the third, and so on. The average time for a one-inch backward flow may then be calculated. With this as a standard the variations in rate, induced when the apparatus is put under different conditions, are noted.The Acid Reaction of Roots

If the leaves are blown with bellows, in imitation of a breeze, the water travels backwards along the tube more quickly than before, because the roots are now absorbing water at a greater rate. They also absorb more quickly if the gas-jar stands in warm water. On the other hand, the rate of absorption decreases if the gas-jar is cooled by being packed round with ice. It decreases, too, if salt solution is added through the thistle funnel.

The apparatus may be used indoors and out of doors ; in rain or in wind ; in sunlight and in shade. Under the varying conditions the rates of absorption may be accurately compared.

The actual volume of water absorbed under these different conditions may, of course, be calculated by measuring the diameter of the glass tube, for 1.7r = volume.

Many theories have been put forward in an attempt to explain the rise of water in the vessels of the wood, especially in tall trees.

It has been suggested that the root acts as a pump, forcing the water upwards. Such a force can certainly be demonstrated in the root at certain times. Again a young potted Fuchsia is a good plant to use for such a demonstration, because of its sturdy stem. Tree saplings are good, but are not so readily available. The time to choose for this demonstration is the beginning of the summer term, some time in May.

The Fuchsia chosen should have one main stem, not one that branches into two just above the surface of the soil.

By using blue litmus solution the proof of this is simply and strikingly obtained. The solution should be in two test-tubes, one of which is to serve as a control. Into the other tube the roots of a plant dip. A tuft of Sweet Alyssum or a Broad Bean seedling acts excellently. It is not long before the litmus solution surrounding the root changes colour. On comparing it with the control, it is seen to be decidedly pink, proving, beyond doubt, that acid has entered the litmus solution from the root.

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