Solving Vegetable Growth Questions: Gravity & Light

[fluidistic]

3 days ago I put a seed in a wet soil and today when I wake up I saw a 4 or 5 cm vegetable. It leads me to the following question(s) : How did it know to grow "upward", that is where there is light. Is it because the seed can "feel" the light? Or because the seed can feel the gravitational acceleration direction?


If it is the former then what would happen if I have a vertical jar without bottom, full of soil and I put 2 light bulb, one at the top, one at the bottom? In what direction would the vegetable grow? Another related question : If instead of a vertical jar I have a horizontal one and I put a light bulb at the left and another one at the right of the jar, in what direction would the vegetable grow? (Assuming that it is perfectly at the center of the soil, so that the seed receive an equal amount of light on each side). I could also ask the same question if I have a spherical soil without any jar and light coming from all directions.

If it is the latter what would happen in case I put the seed in a vertical jar with no bottom, in what direction would grow the vegetable?

Last question : is it the same for any seed/vegetable/tree?

Thanks a lot!

 

[Ouabache]

I admire your scientific spirit. My suggestion is try these simple experiments and see what happens. That would be more fun, wouldn't it?
In the meantime you may want to read up on geo/gravitropism and phototropism.
By the way, I've heard several of your queries were incorporated in experiments made on the U.S. space shuttle missions. You might research their results.

 

[fluidistic]

I admire your scientific spirit. My suggestion is try these simple experiments and see what happens. That would be more fun, wouldn't it?
In the meantime you may want to read up on geo/gravitropism and phototropism.
By the way, I've heard several of your queries were incorporated in experiments made on the U.S. space shuttle missions. You might research their results.

Ok thank you very much. The problem is that doing these experiment here in my flat is too much trouble. I really want know and I'm willing to spend time on it, but I can't do an accurate experiment here. I don't have 2 light bulbs I could use and maintaining the soil in a jar without bottom seems really hard.
Thanks once again, I will look for what you've mentioned.
Edit: By the way I've more questions if the answer to my first question is the former. For example, what is the limit of the difference (in percentage) of intensity of the light bulbs to make a difference. My English is not good enough, I show an example : in the situation where the jar without bottom is horizontal, I put a light bulb at 10 cm from an extremity and another one at 9 cm from the other extremity. Will the vegetable grow in the direction of the closer light bulb? If no, then I'd repeat the experiment but instead of the light bulb at 9 cm, I would put it at 8 cm, etc. I'd do the same for all the vegetables I could in order to see if there are difference and maybe trying to understand why.

 

[Ouabache]

If you are limited to one light bulb, here is a possible experiment:
How about using a tin can instead of a jar (jar implies that light can also go through the side confounding your question about light's effect), add soil, plant seed and water as before. Then cover with plastic sheet or cellophane and hold plastic in place with rubber bands. Now invert the can so that the open side is facing down and sealed side up. Set up the light bulb under the can and wait for germination.

As long as you provide ambient light around your experiment you don't need two light bulbs to do your second experiment with both sides of the can open to light. With ambient (room) light reaching both sides of the can, you could conduct your experiment with both sides of the can open and resealed with plastic wrap. You can use more than one sheet of plastic if soil is too heavy. Or use a smaller can.

Another easy experiment to try is to allow seed to germinate in soil in a pot (jar or can) but put the whole setup inside of a cardboard box. Put a pin hole into one of the sides of the box and as the seedling germinates, see what happens.

You may be able to devise more intuitive experiments after you have read about the tropisms I mentioned.

 

[fluidistic]

Perfect! I will probably try something these days.
Thanks a lot.

 

[Biophreak]

Root cells are affected by gravitropism the most- that is, they will grow towards a center of mass, namely the center of the earth. Stems are most strongly affected by phototropism, which is to grow in the direction of light, you can observe through time-lapse photography that a flower will actually follow the sun across the sky throughout the day. However, for a freshly germinated seed, there is no available light that is penetrating through the dense soil, so the stem is driven by a negative gravitropism, or an 'antigravitropism' (though that is not a real term, to my knowledge).

For your question regarding light, plants will always grow roots down and stems up. Placing growth bulbs on the sides will cause the leaves of the plant to face the bulbs, while the stem will continue its vertical primary growth.

In regards to the experiment where you place the plant in a horizontal jar: What will happen is the roots will grow down, and the stem will grow upward. Once the stem reaches the impenetrable ceiling, it will arbitrarily choose a direction either left or right in which to grow, and it will crawl along the ceiling of the container until it reaches the edge and is able to continue its vertical growth. This is not based on the light, because the light is simply not penetrating that deeply into the soil. You will find that the likelihood of the plant choosing to grow left is equal to the likelihood that it will grow to the right. An interesting experiment that I have seen is that you can construct a maze beneath the soil for the plant, and the stem will actually navigate through the maze to break the surface of the soil after repeated bends. Granted, the maze has to be short enough that the plant can actually navigate it before it run outs of nutrients, but the plant in this experiment had no problem bending 3 even 4 times before breaking the soil/atmosphere interface.

 

[Monique]

You should tell my orchid these scientific facts, it keeps shooting out roots in the wrong direction. At first I didn't recognize them and thought that they were flowering stems, finally I cut them off because they were just a waste of the plant's energy. Now it is again starting to grow roots that are pointing up in the sky, if they grow a few more centimeters I'll try if I can get them to bend down into the pot.

Orchids might just be an exception though, since it is a parasitic plant that grows up in trees. Maybe the roots don't feel gravitropism?

I've got another orchid that is starting to form flowering stems, it will be my first orchid to bloom after it has lost its original flowers, so I can't wait for the results (can you imagine my disappointment, waiting for weeks and then finding out that the spikes of the other plant were just plane roots).

 

[jim mcnamara]

Orchid species typically have very thick roots with a spongy cortex/epidermis for absorbing rainwater and runoff.

Roots normally grow every which way - up, down, out, and around. To be anthropomorphic about it - the plant is searhing for a bark surface to anchor on.

Orchids are not bonsai, trimming aerial roots can make the plant very unhappy.

 

[Monique]

Orchids are not bonsai, trimming aerial roots can make the plant very unhappy.

I realize that, but this was the only growing root that already had made three side-roots and was starting to bud off even more branches. When I saw that it was preparing to grow more side branches I decided to take it off. The plant had been dormant for a long time and it doesn't have a very healthy root system. It really should be putting its energy in growing some new healthy roots that can be functional. It is starting to grow new leaves and I can see growth buds on the older roots, so it seems to be getting back to life.