Organ development begins only when an optimal interaction with this stimulus is obtained in this stimulus’s namesake “set point angle” model. An “altered response” to this stimulus can result from mutating ARG1, which prevents localization of PIN3. Cells that detect this non-auditory stimulus are found in the columella. Inverting the normal response to this stimulus allows gas exchange to occur in pneumatophores (“noo-MAT-oh-fors”). Specialized amyloplasts (“AM-uh-lo-plasts”) called statoliths contain starch granules that move in response to this stimulus, which directs the growth of root caps. Both light and this stimulus trigger asymmetrical elongation (10[1])of cells by altering cellular distribution of auxins. For 10 points, what stimulus names a form of tropism that allows plant roots to grow downwards? ■END■ (10[1])