by jckkeith » Mon Mar 08, 2010 7:19 am
Sunflowers follow the sun
This phenomenon is known as positive phototropism (photo = light, tropos = turn), and although it is not fully understood yet, is commonly associated with the plant hormone auxin (indoleacetic acid). Auxin is derived from the Greek word auxein, which means to increase, and in low concentrations, it causes the elongation of plant cells by entering them and signalling the cleavage of cell wall microfibrils, allowing the cell to expand longitudinally by taking in water.
Auxin is produced in the elongating tip of the stem. Experiments by Charles and Francis Darwin in 1880 proved that only when the tip of a seedling was exposed to light could bending of the stem take place. When the tip was removed or covered by an opaque cap, no bending took place. When the base of the stem was covered with an opaque shield, however, and the tip was left uncovered, bending took place as per normal.
Auxin produced at the tip diffuses down the stem. Experiments in 1926 by Frits Went showed that when diffusion of auxin is uniform for all sides of the stem, elongation occurs uniformly and the stem simply increases in height. However, when diffusion is not uniformed, and one side of the stem receives more auxin than another, that particular side elongates more than the other, causing the stem to bend.
However, the situation is much more complex. Although auxin is able to cause bending, there is no evidence to show that illumination of one side of the tip causes asymmetrical distribution of auxin. There is, however, an asymmetrical distribution of certain substances that may act as inhibitors that become more concentrated on the lighted side of a stem. So a reasonable answer now is that we are not entierly sure.