Roots grow downwards regardless of the orientation of seeds due to a phenomenon called gravitropism. More specifically speaking, the fact that roots go downwards with the flow of gravity means that roots undergo positive gravitropism. In essence, roots are able to sense gravity and they therefore have the ability to always grow towards/into the ground. Roots have the ability to sense gravity through statoliths. Statoliths are specialized starch grains that settle on the lower portion of the root cap. As statolith aggregates, it causes the calcium to be redistributed throughout the roots, and this in turn causes the auxin, a plant hormone, to be transported to the lower side of the roots. The “lower side” means the side that is closest to the earth’s core, as opposed to the sky. As more and more auxin accumulates, cell elongation is inhibited on that side of the root, while the auxin-free high root side continues to elongate. Since one side is growing – the upper side – and the lower side isn’t elongating, the root starts to curve downwards. This continues to happen until the root is oriented completely southward. However, statoliths are not the only mechanism that helps the plant tell what is north and what is south. The cell also uses its dense organelles and the entire cells themselves, which are affected by gravity as well, to tell which direction is north and which direction is south. This occurs because gravity pulls down on the cells, and thus, the increasing density of the bottom of the cells helps to identify that that position is the bottom.

            In contrast, the upward growth of the shoots, since it defies gravity, is a result of a phenomenon called negative gravitropism. Though this process also includes the distribution of auxin, the difference between the roots and the shoot is that in the shoots, auxin (in small concentrations) promotes cell elongation. Furthermore, rather than being in the root cap, in shoots, the auxin is located for this process within the apical meristem.  In essence, auxin accumulates on the lower layer of the shoot, which causes those cells to elongate, whereas the cells that do not have auxin on the upper half, don’t elongate as fast. This causes the stem to bend upwards, regardless of the seed’s orientation. Also in step elongation, another hormone called Gibberellins help the stem elongate. This is believed to occur because of Gibberellins’ ability to weaken the cell walls of cells, and thus allow more proteins that help the stem elongate enter the cells. When shoots break the ground surface, a process called de-etiolation occurs; this helps to expand roots and slows down the elongation of the cells in the shoot. Furthermore, the plant then responds to the sun: the shoots start to grow towards the sunlight.

            Shoots bend towards sunlight, a process called phototropism. This occurs because the shoot, called a coleoptile, bends towards the sunlight. The bending occurs because of a faster rate of growth of the side of the coleoptile that is not receiving the sunlight than the side that is, which causes a bending affect towards the sunlight. Once again, the hormone that helps the darker side grow faster is Auxin. However, this time, the tip of the coleoptile, reacts to the sunlight and sends the Auxin to the side opposite of the sun, and then the above mentioned process occurs. However, without the tip, no auxin is distributed and there is no curving affect. Furthermore, the shoots bend towards the light because of an accumulation of inhibitory molecules that accumulate on the side that receives the sunlight. The molecules inhibit the elongation of those cells, and not the dark-side cells, causing the same bending affect towards the sunlight. The tip reacts to Blue-light to carry out phototropism. As the blue-light photoreceptors (receptors of light) receive the stimulus, in this case blue-light, a signal transduction pathway is initiated. The end result is the release of auxin.