Oscillating Growth Due to Negative Gravitropism

February 26, 2015

Oscillating Growth
Photographer: Mario Freitas
Summary AuthorMario Freitas

External stimuli are essential to the growth of plants, such as light, gravity, and touch. Recently, we let an identified weed grow in our home garden. It was 6 ft (2 m) tall when a strong wind bent it nearly flat on the ground. For several weeks it laid in this position. As a result, horizontal branches changed their growth direction, now pointing upwards. Once we lifted this weed to its original position, the growth direction changed again, becoming parallel to the stem. Thus, branches now display an elegant sinusoidal-like shape. Changes in inflection indicate the day we lifted the weed (left photo). The photo at right shows the entire plant.

Plant roots have positive gravitropism, growing towards the center of Earth. But leaves and stems do the opposite -- negative gravitropism. Botanists don't yet fully understand how plants can perceive gravity and respond to it. One of the most well-accepted theories involves the hormone known as auxin. It stimulates cell elongation as its concentration increases in the lower sides of stems.

Note the very faint, waning crescent Moon visible at the right top of the photo at right. Though widely accept by popular culture as an influence on plant growth, the possible influence of the lunar phase cycle on successful gardening hasn't yet been verified by scientific research. Photos taken in Curitiba, Brazil on the morning of January 15, 2015.