October 2 – Happy Birthday, Julius von Sachs

Posted on October 2, 2014

Today's famous birthday was a botanist extraordinaire!

You probably know that botanists study plants.

You could look at plants as living things that don't move around or make sounds or, really, do much of anything – or you could look at plants as incredible living factories that convert solar energy, gas, and water into food. You could also look at plants as living things that DO move, actually...

Julius von Sachs, born in Prussia (Germany) on this date in 1832, advanced the study of plant physiology in many different arenas, but one area he added a lot to was the study of the way that plants move in response to gravity, light, and water.

These plant responses are called tropisms.


Roots grow downward, and stems and shoots and tree trunks grow upward. That seems so obvious, but notice what happens when you lay a potted plant on its side:

The roots bend in order to continue to grow downward, toward the gravitation pull of the Earth. (This tropism is sometimes called gravitropism.) This is called positive geotropism.

And the stem bends in order to continue to grow upward, away from the gravitational pull of the Earth. This is called negative geotropism.


You have probably noticed many times that plants tend to bend toward light. Because the sun moves through the sky during the day, outdoor plants often make cycles, always following the sun as it arcs from east to west each day.

(This tropism is sometimes called heliotropism. However, plants also bend toward artificial lights; with helio- meaning sun and photo- meaning light, the latter seems more correct.)


Another tropism is the bending toward a water source.

Here are some videos about tropisms and plant movements:

Also on this date:

Phileas Fogg's Wager Day

Great Books Week

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