The rate at whichabsorb water under different conditions may be measured by transferring a strongly growing plant to a gas-jar containing water. A Fuchsia, because of its strong , is excellent for the purpose.
A wedge-shaped portion is cut out of a well fitting cork. In the remaining part two holes are bored for the passage of a thistle funnel and a glass tube bent at right angles, whose short arm is the depth of the cork into which it fits. From the tip of the wedge the smallest piece possible is cut off to give room for the Fuchsia stem. The divided cork, with its equipment, is then fitted into the gas-jar, which is full of water.
Unfortunately there are a great many connections to make air-tight, and unless the apparatus is absolutely air-tight it is useless. The most satisfactory means of attaining this result is to mould and press just-melted, or just-solidifying, paraffin wax into all the junctions, between the cork and the glass, and the cork and the stem, particularly, but also all over the cork, because it is porous. It is a tedious process, and it is unlikely that the first attempt will be successful.
The thistle funnel is filled with water. When the tap is turned on water will enter the already full gas-jar and fill the long horizontal arm of the bent tube.
The whole leafy crown of the plant must be cut off, so that an inch or so of main stem projects above the soil. A long piece of glass tubing, of just the right bore to fit closely, is then at once carefully put over the projecting bit of stem. A 2-inch length of rubber tubing, previously slipped over the glass tube, is so adjusted that it completely encloses the connection between glass and stem. This joint is made airtight by tightly binding the rubber tube at each end.
The plant-pot should stand in a saucer of water, the tube being supported by a clamp. The rise of water in the tube, due to the pumping action of the, is fairly rapid. It increases on gently warming the root or on the addition of warm water to the soil. There is also a certain periodicity in the rise ; more water is pumped up by night than by day. As the days pass the pumping action gradually slackens. In about ten days’ time, if a Fuchsia has been used, it stops altogether. The stoppage in the root’s work is due to a stoppage in the supply of food, because the plant is now leafless, and make the food. The work as long as the food store they contain holds out. Once this is exhausted no further food is forthcoming, because of the absence of , and the work of the root comes to an end.
Incidentally here is a proof that raw materials, as such, are quite useless as food. Before they can be of any service to the plant they must be worked up, in the leaves, into definite compounds which are the actual food of the plant.
Here, too, is the explanation of the fact that, in late years, so many Oak trees have been destroyed in England by the armies of caterpillars that have eaten their leaves throughout the summer and early autumn.
Later in the growing year root-pressure is of a negative character, because of the very active transpiration of the leaves. In early spring, when root-pressure is at its maximum, sap is seen to exude from the cut ends of many, as from an Apple tree, pruned in the spring (which is the wrong time of year to choose) or from the cut stems of Vines. But if an actively transpiring plant be cut in the summer time, even quite near the root, there is no exudation of sap. The amount of water supplied by the root has not been sufficient to make good the loss due to transpiration through the leaves.
Root-pressure alone, then, cannot account for the ascent of water in high trees.
In such lowly animals as the Earthworm oxygen, needed in breathing, is taken in through the moist skin of the whole body. In the higher animals, the breathing apparatus becomes more and more specialised, until finally the extremely complex human mechanism is evolved. In plants, no matter what their degree of development, there is not the slightest trace of specialised respiratory organs. As in the Earthworm, so in plants both high and low, the interchange of gases in breathing takes place through the outer layers of every part.