What are statoliths, and what role do they play in plant growth?

They are a membrane bound clusters or set of starch grains just like amyloplasts in plant cells. They acts like a gravity sensors. Starch statoliths are the main components which are found in root tips at cells. They are very much close to the vascular bundles in shoots. Statoliths pressurize on the membrane systems which are located inside the cell eg endoplasmic reticulum which results in differential growth.

cells spend most of their time in interphase

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