Have you ever wondered why things (trees, grasses, weeds, people) grow "up" instead of in some other direction? Probably not, but look around you and you'll see that "up" is the preferred direction for growth. I became aware of this when I was about twelve years old and the older neighborhood boys and I were having "strength tests." One test was to see whose arm could stay in the air for the longest time. The boys whose arms were not perfectly straight up soon dropped out. Bobby Holtz and I were the last two survivors when he announced that he had to get something in our tent. He walked into the tent with his right arm in the air and came out with his left arm up. We all laughed about that but I had won. But, it set me to thinking--why?
Look at the lower limbs of some trees. As they get longer, they tend to curve upwards. Seeking light, you say. No, that's not it. Every side limb on a tree is under a certain amount of stress as it grows. The "top" portion of a limb is under a tension stress and the bottom is under compression. This tension stress acts to inhibit growth whereas the compressional stresses stimulate growth by the re lease of chemicals. Because the bottom cells of a branch grow larger, there is a natural tendency for this branch to grow upwards.
Also, there are chemicals in every seedling that pre-program how this organism will grow. Plant one hundred pea seedlings in a random manner and watch as the first growth emerges. The root materials always point downwards and the stems always point upwards. Again, this is the result of certain chemicals built into the plant's makeup.
When my son Mark was in sixth grade, he entered a science-fair competition. I suggested to him that he grow some seedlings on an old 45-RPM turntable that was in the garage. When the first seedlings emerged, we turned the turntable on and let everything grow for a week. Which way did the seedlings end up growing? The "natural answer" would be to say "In an outwards direction, since the centrifugal force would force the seedlings that way. Instead, the se seedlings started pointing inwards towards the center of rotation - so as to act against the force of gravity. After a week, the seedlings at the outer edge of the turntable were growing inward at almost a 45-degree angle, compared to the vertical. The combination of gravity and centrifugal force made the seedlings grow in what the plants "thought" was an "up" direction. Plants don't think, but the chemicals in them act that way.
Grasses Growing Up
And you thought nature was simple!
June 18, 2003