by The Old Scott
Ever wondered why plants grow upwards instead of sideways? Or why roots grow down? Plant scientists have discovered part of the answer, and it is a plant hormone. Plant hormones are different from animal hormones, but work in the same way. "Auxin" is the hormone that controls plant growth, and the way it works is nothing short of miraculous.
Think of a sapling growing on a steep hillside. Many such baby trees bend out at first, but then turn straight up. This happens because more auxin goes to the underside of the branch tip than to its topside. The greater quantity of auxin underneath stimulates extra growth, and this unequal growth forces the branch to bend upward. When the branch tip is straight up, the amount of auxin on either side is equal, and all sides grow at the same rate.
Now, what makes roots grow down? Auxin. Scientists discovered that the same hormone invokes opposite reaction, depending on whether it is acting on above-ground or below-ground portions of the plant. How could this be? It turns out that when the concentration of auxin reaches a certain level, its effect is reversed. Instead of stimulating cell growth, it retards it. The hormone tends to pool on the lower side of the root tip, just as in branches, but growth of the affected cells slows down instead of speeding up—and the root turns down because of the pressure from growth of the cells on top
While research has solved some mysteries, it has only uncovered deeper ones. What causes the auxin to focus on one spot and not on another? Or what causes it to be dilute in one case and concentrated in another? Many believe the presence or absence of light is the answer—but if you sprout seeds in total darkness, the emerging shoot will still turn up, and the root will turn down. Why? Nobody knows for certain. What is known is that "something" makes sure that just the right amount of auxin goes to just the right place at the right time to make branches grow upright. "Something" causes root tissue to reverse its response completely when the concentration of auxin increases.
Somehow, every seed comes fully equipped with built-in instructions which will guide its development from first leaf to final form. Every seed is the marvelous creation of a vast Intelligence, which quietly brings wonders to pass in our amazing everyday world.
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Source: Ingenious Kingdom, Henry and Rebecca Northen, Prentice-Hall, Englewood Cliffs, NJ, 1970, pp. 136-137.