Gravity Tropism

Hello (yet again),
Today in BIOL1040, I came across an interesting problem: how do plants sense gravity and grow in response (separately from other stimuli, such as when a seed is deeply buried)? It seems very unlikely that plants have 'gravity' eyes, so my suggestion is that some mechanism can detect which direction requires the most energy to grow in. Another hypothesis is that the mechanism which facilitates growth orients itself via gravity and is then constrained to provide growth in the upwards direction by structure. These mechanisms might be proteins or hormones stimulate such growth. Plant cells can also restrict their growth to upwards or downwards based on the orientation of their cellulose microfibrils, and since turgor pressure drives this kind of elongation, perhaps the direction of growth is easier to achieve once the first cells are aligned.

Josh H