Have you ever wondered why plants grow upwards instead of sideways? Or thought about why roots grow down? These are constants of nature, but why are they so? Plant scientists have discovered part of the answer, and it is a plant hormone. (Of course, they first had to discover that plants had hormones, just as animals do. These hormones are sophisticated organic chemicals which regulate growth and development. Plant hormones are not identical to animal hormones, but fulfill similar functions.)
The particular hormone of which we speak is primarily a growth hormone. It is called “auxin.” And through its growth control function, auxin does, indeed, make plant stems grow up, and roots grow down. Let us see how auxin does this.
Think of a sapling growing on a steep hillside, for instance. Many such young trees bend out at first, but then turn straight up. This occurs because auxin accumulates more densely on the lower side of the limb tip. This encourages the cells on the lower side to grow more rapidly than those on the upper side, and soon this side of the stem stretches. This extra growth forces the upward turn, and this continues until the growing tip is vertical – at which time the amount of auxin on the two sides is equalized, and growth henceforth will be straight up.
But suppose the growing tip (or leader) is destroyed. What will happen then? If the leader is lost, several branches below it will receive additional auxin, and begin bending upward. If one outdistances the rest, it will become the new leader, and the others will resume their former subservient status, being deprived of the extra supplies of the hormone. But if two branches win the race in a dead-heat, the tree is like ly to become a “double” above that point, for both will become leaders.
But what makes the roots turn downwards? In this case, auxin’s effect is exactly the reverse of its effect on stem tissue. Scientists have found that root tissue is extremely sensitive to the influence of auxin, and responds in the same way, as long as the concentration of the hormone is quite low. But when the concentration reaches a critical balance point, the effect is exactly reversed: Growth is inhibited instead of encouraged.
The auxin again accumulates on the downward side of the growing tip, as with the above-ground parts, but instead of stretching the lower side to force an upturn, those cells are retarded, while those on the upper side grow normally. The net result is that root tendrils turn down.
But if we hoped to find a full answer in science as to why plants behave as they always have, we have sought in vain. We have deciphered one or two pages of the Maker’s instruction manual, but not the entire book. Our discoveries have simply uncovered more mysteries.
For example, it was formerly assumed that sunlight was what triggered proper growth patterns: That leaves and stems stretched up to the light, and roots turned away from it. But a simple experiment will show it is not that simple. Seeds which germinate in total darkness still send out shoots which grow upward, and roots which grow downward.
So it is not light, or at least not light alone, which makes the plant respond in the “natural” way. Perhaps gravity plays a part. At this point, the answer is simply not known. What is known is that something regulates the production of auxin, and directs the delivery to the proper places, in the correct strength. Something causes root tissue to reverse its response to auxin when the critical concentration is reached. Something orders extra auxin to be sent to potential new leaders, when the original leader is lost.
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 everyday world. It is only as we seek to penetrate the secrets of this creation that we are able to realize the awesome perfection of all its parts.
We follow haltingly in the ages-old footsteps of God, His wonders to behold. Truly, His works declare His glory! Praise His Name!