80523thumbWhen a seed germinates, the roots know that they want to go down, and the new stem knows that it wants to go up. Going up sounds simple enough. Stems just go towards the light. How do they know where the light is while they are still under the soil? Well, new stems go upward for the same but opposite reason that roots go downward; gravity. Roots go toward it. Stems go away from it.

Tropism is how plant parts respond to a variety of stimuli, particularly gravity and light, but also water, chemicals, mechanical stimulation, trauma and electricity. Response to gravity is geotropism. Roots exhibit positive geotropism by growing towards gravity. Stems exhibit negative geotropism by growing away from gravity. Stems exhibit positive phototropism by growing towards sunlight.

Roots are always figuring out where to go next by prioritizing their innate positive geotropism, their tropism for or against certain chemicals, and their tropism for moisture but against saturation. Until we see them migrating into lawns, displacing concrete or getting into a septic systems, their work underground is unseen. Tropisms above ground are quite visible and perhaps informative.

When a tall herbaceous plant falls over, it tries to get up. If unable to, it can at least curve new grow upward. Even cut flowers and vegetables in the kitchen can do that much. Snapdragons that are initially arranged leaning outwardly from the center of a floral arrangement can go vertical within a day or two. The fluffiest houseplants regularly get turned so than they do not favor one sunny side.

Trees are too big to move, but on rare occasion, it happens. The crossed pairs of Mexican fan palms commonly planted outside of In-N-Out Burger restaurants get planted at a lean while mature, but then grow vertically after installation, leaving an angular kink where the direction changed. Such a kink in an otherwise straight trunk of a tree that was not planted at an angle might indicate a sudden destabilization. A curved trunk indicates either a slow destabilization, or tropism to escape shade.

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17 thoughts on “Tropism Gives Plants Some Direction

    1. That is a weird one. You may have also noticed that insects tend to avoid trees under high voltage utility cables. Even though those are always them most disfigured, they are also often the healthiest.

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  1. Thanks Tony – you quite often see photo-tropism as shade-intolerant plants reach away from walls – I believe they are growing away from blue light reflecting from the shaded surfaces (what I call the shade signal).

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    1. They use more blue light than other light, so would not want to grow away from it, even if it reflected. If they were to grow away from blue light (or a more specific blue light that indicates or signals shade), such a response would be negative phototropism. I really never considered plants exhibiting negative phototropism because it seems so obvious that they would grow towards the light instead of away from it, which they could do for a shade signal. Nor do I know how they respond to infrared or ultraviolet light. They use in in floral color, so they must know that it exists.

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      1. Hi Tony, It’s really about very low spectrum blue down at the 400nm level, the rest of the blue is utilised in photosynthesis. Horizontal low blue light acts in tandem with a low Red:Far Red signal resulting in stem-bending growth away from the signal.
        Plants Feel Blue in the Shade
        https://www.sciencedirect.com/science/article/pii/S0960982216312659
        “new studies highlight the importance of blue light in the regulation of stem elongation and bending during shade escape”
        Lots of mysteries yet to unravel

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      2. Oh my! Thank you for this information. This means that it is BOTH positive AND negative phototropism. They grow towards most light and away from another.

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      3. Actually all tropisms have a negative and a positive. Negative tropism is growing away from a stimulus, while a positive is growing towards it. Climbing plants actually show a negative response towards light and therefore positive towards shade because shade means things like trees, and that is going to be your support for growing upwards (which is now positive phototropism!).

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      4. I am aware of the positive and negative tropisms, but would not have expected the differences to be that of different colors of light. There is only one kind of gravity, so tropism can be towards it or against it. However, light is composed of different colors, so tropisms can be different responses to different colors.

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