Are We Going Up Or Down?

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People can often sense from the movement whether an elevator is going up or down. But when pilots are flying in clouds or over open sea at night, they can soon become confused about whether they're going up or down because they lack a visual horizon. People need external references to determine which way is up. A disoriented diver usually checks the direction of the bubbles since they are always sure to head upwards.

It's interesting that plants have an innate ability to grow the right way up. This capacity, which we seem to lack, is called geotropism - an ability to sense gravity. Plants have cells that contain little granules. These do for plants what bubbles do for divers - help them decide which way is up. The little granules settle at the bottom of the cell, setting the reference point for which way is down. The biochemical signals that cause the cells to grow in a certain direction are determined by the position of these granules. The parts of the plant that will be roots are triggered to grow downwards and the stem of the plant to grow upwards.

In addition to its ability to sense gravity, the plant exhibits what's called phototropism. This is an ability to grow towards a light source. Also, plants respond to the presence of water, so roots grow towards it.

So next time you plant your bulbs, even if you get confused, they always know which way is up.