"Oh," you reply, "I say 'ouch' because it hurts; and teacher and the Physiology say my arm pulls my hand away because my head tells it to."
"Yes, but how does the head make the arm do the pulling? What's the connection?" says the science man.
Well, I guess we'll have to tell him we don't know, won't we?
But all the root's brains aren't in the tip, any more than all our brains are in our heads. Scattered through our bodies, you know, are little brains, the ganglia, that control different parts of the body. So it is with roots. For instance, a root at a short distance from the tip, is sensitive to the touch of hard objects in such a way that it bends toward them instead of turning away, as the tip does. The result is that when a root comes to a pebble, say, under ground, the sides of the root press close up to the sides of the pebble—turn around corners sharply, by the shortest route—and so get over the obstruction as soon as possible and resume their course in the soil.
BUT THEY COULDN'T CHANGE ITS MIND
Some sprouting seedlings were attached to a disk like that, and when the roots started to grow down, the disk was turned to make them point upwards. But, no Sir! The roots just wouldn't grow upward. They turned downward. Every time!
And different parts of a plant's root system respond in different ways to the pull of gravity, and some don't respond at all. The tap-root, for example, which always grows down, has roots growing out from it horizontally. They just won't grow any other way, and yet this is also supposed to be due to the influence of gravity. Then, from these horizontal roots, grow out a third set, and they don't seem to pay any attention whatever to gravity. They grow out in all directions—every which way—so that if there is a bit to eat anywhere in the neighborhood they are reasonably sure to find it. You see it works out all right.
When a plant first begins to peep into the world out of that wonder box we call the seed, it's the root, as we know, that does the peeping; it comes first. And its first business is to get a firm hold in the soil. So a lot of fine hairlike fibres grow right and left and all around and take a firm grip. There is an acid in the root that dissolves whatever the root touches that has any food in it—including pebbles and old bones—and so makes a kind of sticky stuff that hardens. In this way these fibrous roots not only get good meals for themselves and the rest of the plant, but they hold the plant firmly in the soil, against the strain of the winds. They also give the tap-root something to brace its back against, as it were, while it pushes down for water, for the moisture in the damper portion of the soil beneath.