Tropisms - Making Plants Do Your Will


Tropisms - Making Plants Do Your Will


Plants are amazing, aren't they?  When you think of the sheer magnitude of what they are capable of, you can't help but be in awe!  Yes, they clean the air, prevent erosion, provide food, and are beautiful to look at - all without being able to move.  Or can they?  The word "Tropism" stems from the greek word, tropos, meaning "a turning."  It's a biological phenomenon that occurs in plants in response to a stimulus, allowing them to survive.  Darwin pegged it when he said that "it is not the strongest of a species that survives, nor the most intelligent that survives.  It is the one that is most adaptable to change."  And that's exactly what plants are capable of, and pretty quickly! 


Darwin believed the finches in the Galapagos were originally one species, but over millions of years they evolved different shaped beaks to more easily accommodate their diet and other environmental factors. But a plant will adapt itself amazingly fast to accommodate its environment, and it won't take generations - they have evolution built into them. There's a simple explanation for it - a plant growth hormone called auxin becomes unevenly distributed throughout the plant, and the plant grows in the direction it receives the most of what it needs.  When you think of what this means the ideas will flood your mind!


Tropisms allow us to control how plants grow!  You know how you'll turn a houseplant every few months to keep them growing evenly?  Well, this is called a heliotropism.  When plants receive light mostly on one side, they will start growing in that direction, where the most auxin is being produced.  Now think of all the plants that are trained to grow in a certain direction.  Bonsai, for example, are trained to grow in the most aesthetic direction. 


Curly bamboo is also trained using heliotropism techniques.  Bamboo is not naturally curly, nor does it grow in a zig zag pattern, but this is easily accomplished when you control where the light falls on it.  Many people start with a small bamboo plant and a shoebox, putting the plant inside with open box side facing the light source. This way the plant cannot get light from the top or sides - in order for it to thrive it must grow in the direction you want.  Simply turn the plant as you'd like for the direction to change.  That simple.


Another way to train plants is using gravitropism, which is the turning or growth of a plant in response to... you guessed it - gravity.  Again, Darwin was spot on!  He pointed out that roots grow in the direction of gravitational pull while stems grow in the opposite direction.  Turn a houseplant on its side and leave it there, keeping all other conditions the same.  Chances are you'll see the plant growing upwards anyway, rather than straight out as it was doing before (there are a few mutants).  Voila!  Another way to train a plant!  Auxin is again, the one to thank - in both roots and stems, it accumulates towards the gravity vector and has the opposite reaction on each end of the plant.  Upward growth of plants against gravity is called "negative gravitropism", while downward growth of the roots is called "positive gravitropism."  Pretty nifty huh?


That doesn't begin to cover it!  There are plants who react to heat, known as Thermotropism, and will curl their leaves up in cold temperatures to remain warm!  Rhododendron is a common example.  And Thigmotropism is when a plant responds to touch or contact, like when roots feel an object they grow away from it!  You can see this on NY streets where old trees planted in openings in the street stop growing roots where the cement reaches and instead move further to the side where there is more freedom, However, not all plants act the same to a roadblock.  Brunnichia ovata (redvine), uses thigmotropism to curl around supporting objects and incorporate them in their growth.  Touched cells produce auxin and transport it to untouched cells, and then some untouched cells will elongate faster so cell growth bends around the object.  Amazing!  


So how will you use tropisms to modify your funky flora?  Art and science unite!!!

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