Let us first look at a somewhat similar case in non-living things which will, perhaps, help us to understand the process in living plants.
Take a small “thistle-funnel” and tie tightly over the wide opening a piece of bladder; then pour some very strong solution of sugar into the funnel and place it in a glass of pure water. Mark the level of the sugar with a label (see fig. 21, S). Leave this for a short time, and you will find that the water has entered the funnel tube and run up it for quite a long way.
Fig. 21. “Thistle funnel” covered with bladder B, filled with sugar solution up to level S, and placed in a jar of water. After a time the water is seen to have risen to W.
You should take another similar tube and do everything in the same way, except that you leave out the sugar solution. Then you will find that the water remains inside the funnel at just the same level as in the outer jar. This is the usual behaviour of water, and in the first case, where the water rose inside the funnel, the rise was due to the influence of the sugar, which has the power of drawing in water. Now we can compare the skin of the root hairs (see fig. 9) to the bladder membrane covering the funnel, and it has been found that inside the cells are substances which have the same power of attracting water as we found was possessed by the sugar. So that the entrance of water into the roots depends chiefly on the attraction of the substances within its cells.
That a large amount of water enters the root in this way you can see if you cut off a quickly growing plant (a vine is very good if you can get it) just near its base, and attach to the cut-end a long glass tube in place of the shoot you have cut away. You must fasten this tube by a very well-fitting indiarubber tube, which you bind tightly so that it will allow no leakage, and support the glass. Pour a few drops of water down the tube to keep the cut-end of the plant from drying up at the beginning of the experiment. Then mark the level reached by the water, and do this every day as it rises in the tube. You should find that for some time it steadily rises day by day (see fig. 22).
Fig. 22. Plant P, which has been cut off near the root, is attached by the indiarubber tube I to a tall glass pipe, which is supported by stand S. On the glass are marked the levels reached by the water rising from the root.