Ever wonder how tiny little seedlings know to send their roots one way and their shoots the other? This behavior is called Gravitropism. While a lot is known about this process, much of it is still a mystery.
Within the tips of growing roots or shoots are specialized cells called statoliths. These cells have one outstanding visible feature. (Visible with a microscope, that is.) Statoliths have tiny grains of starch in their cytoplasm.
A current theory of gravitropism goes like this: The grains of starch, being heavier than the rest of the cell, will settle to the bottom side of the cell. (This action can be observed with a good microscope.) Somehow, and this is the part that is yet unknown, this action results in release or redistribution of the plant hormone auxin on the same side of the statolith. Auxin encourages growth in plant shoots. Thus if a plant shoot is growing sideways the bottom part will receive more auxin and cause the shoot to grow upwards. Plant roots behave the opposite way : Auxin slows growth in plant roots. That has the net result of roots growing downwards.
Although this simple model was proposed in the 1930s, the process still isn’t fully understood. Many organelles, such as the Golgi apparatus and the mitochondria, are thought to be involved in the signal transduction from the starch grain to the production of auxin.
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One reply on “Seeds are sprouting”
Two questions:
is it possible to see “depth” or 3D with a microscope? where is the bottom of the cell? is it the part of the cell that’s farthest south, so to speak, or is it the part of the cell that’s the farthest from the lens?
you sort of addressed this, but I’m still curious about plants that grow sideways. I’m also thinking about plants that develop branches…some grow sideways, and then gravity takes over and they start growing down. Does that mean that some plants have less auxin?