Why do seeds grow upwards? Surely gravity would pull them towards the earth's core? Can anyone shine any light on this? Do seeds that are planted deeper grow more quickly.
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7$\begingroup$ While the general question is good, still, the part about "Surely gravity would pull them towards the earth's core?" could be applied widely to many other things. Why aren't you pulled toward the core? Why aren't you sinking into the dirt? Why don't you grow downward? Why does water evaporate upward into the air? Why do particles get ejected away from the sun? Gravity attracts, yes, but that does not mean that everything is always moving toward it at all times. If that were true, everything would collapse into tiny points and be like black holes. $\endgroup$– AaronNov 1, 2018 at 19:48
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2$\begingroup$ one could argue that we do grow downwards, but for every bit we grow downwards, the earth pushes us back up $\endgroup$– user371366Nov 3, 2018 at 5:07
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$\begingroup$ @Aaron Nicely considered and said! Yay! $\endgroup$– stormyNov 6, 2018 at 21:41
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6 Answers
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It is a reaction to gravity, it is called geotropism or gravitropism. The roots tend to grow in the direction of the gravity (positive geotropism) and the stem is going against gravity (negative geotropism).
The mechanism works with auxins, or plant growth hormones. They are pulled by gravity toward a side of the plant and will either stimulate or inhibited growth in that area.
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2$\begingroup$ For a longer view, reflect that plants which didn't evolve some mechanism to grow upwards (and their root systems downwards) would be pretty much restricted to floating on ponds and such. $\endgroup$– jamesqfNov 2, 2018 at 5:14
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2$\begingroup$ is correct, but doesn't provide any support or citation. $\endgroup$ Nov 2, 2018 at 13:40
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$\begingroup$ @jamesqf or having a 50/50 chance of getting it right anyways. $\endgroup$ Nov 4, 2018 at 10:38
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Because of a force known as geotropism - it is a reaction to gravity. The upward growth of shoots from seeds is known as negative geotropism whereas the downward growth of roots is known as positive geotropism.
The act of a seed to decide which way is up, or to orient itself, is geotaxis - it detects which way is down and up, in other words, because of gravitational force. see here https://www.sciencefocus.com/nature/how-do-seeds-know-which-way-to-grow/
And no, seeds which are planted deeper don't grow faster - each kind of plant seed needs to be at an optimal depth in order to germinate. Ever noticed you get lots of weed germination in freshly dug areas? That's because seeds sitting too low down in the soil which might have been there ages are moved closer to the surface by the act of digging, which then means they are now able to germinate and grow.
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In addition to the geotropism mentioned in the other answers, experiments on the ISS have shown that plants will grow oriented in a manner such that “upward” (i.e. the stem, leaves, etc.) is toward a light source, even in the absence of gravity. Of course, this depends on the plant being able to determine which way is toward the light source. Although I have no source for this, I imagine something like a thermal gradient could be used before the plant breaks the surface.
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1$\begingroup$ Interesting. Do you know if there were any experiments where plants were grown on the ISS with an equal distribution of light and heat? Ex: either completely dark or completely light from all directions. $\endgroup$– AaronNov 1, 2018 at 19:49
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$\begingroup$ Not to my knowledge, but it’s certainly possible. $\endgroup$– alex_dNov 1, 2018 at 22:27
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$\begingroup$ @Aaron Check out the last site I have just now included in my answer concerning zero gravity and growing plants on the ISS. I hope I've sent enough sites to support what I am trying to say... $\endgroup$– stormyNov 6, 2018 at 21:47
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$\begingroup$ @alex_d I meant to make sure I pinged you as well. Thanks! $\endgroup$– stormyNov 6, 2018 at 21:49
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$\begingroup$ @stormy and alex_d: note also that I have since asked a question about this on space.SE here: space.stackexchange.com/questions/31762/… $\endgroup$– AaronNov 6, 2018 at 22:22
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Gravity sure contributes to this behavior (that is to the growth vector being "up"), but none of the other answers mention moisture gradient, which is also a major contributor to the seed growth vector.
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It is probably because the plants which did this were more successful in catching light with their chlorophyll than the ones which did not do this.
More successful in growing and maturing and sprouting flowers: bigger chance of more baby plants next generation et.c.
Consider the converse: a plant that grows deeper into ground. It will clearly need some other way to get energy than by photosynthesis to survive and thrive underground. As far as I know this is not at all as common among plants.
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$\begingroup$ Sorry I don't have any sources. I am just Darwinguessing. $\endgroup$ Nov 3, 2018 at 20:24
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Note added; Gravity is but one factor in a 'sea' of complex factors that have effected everything living in this thin blue line on Earth. Atmospheric pressure, temperature, humidity, the medium's porosity, the Earth's spin counterbalances the gravity and a myriad of things I didn't list or was able to mention a myriad of things we humans just do not understand about life, gravity. I am trying to say that there is no ONE thing controlling the direction of the cotyledon nor the root growth. Roots do not grow and grow towards the earth's core. Roots stay within the top 4 to6" of topsoil for a reason and gravity has nothing to do with it other than gravity is part and parcel of all the systems that put pressure on an organism to make it adapt to its environment and file that information away in the seed of a plant, within every living cell. As DNA. It is weird we share 99% of the DNA messages with all animals all plants all life. What does that tell us?
Geotropism isn't quite right because it certainly is not the only factor plants have taken into consideration. Again, roots grow horizontally to the force of gravity, not vertically. The fact that gravity is a constant tells us that there has to be many forces that shaped the physiology plants display. We animals are able to run and jump against and with gravity forces.
This question is a very good one because it causes us to pause, step back to think about the larger picture, a picture we humans have just begun to imagine.Geotropism seems to be a pretty minuscule reason plants grow upwards and roots grow outwards. No one thing could ever be pointed to as the reason a life form as evolved to survive with or against. If seeds are planted too deeply (more than 3X the widest diameter of the seed) that seed will more than likely damp off with fungus and decompose. Animals are the same, we sit still too long in one place and we start decomposing, grins!
I have a feeling that people think plants are dumb like rocks. In fact, they were here first, before animals. Lots more time to perfect their survival abilities.
If it was up to gravity, the roots would grow and grow and grow downwards. That is not at all what happens. 95% of all plant roots are within the top 6" of soil. The reason is water doesn't infiltrate the subsoil well at all. Plants 'know' this fact. Roots grow where there is air and moisture and temperature stability. There is absolutely no reason to grow any deeper roots with the exception of a few plants that need find deep water sources, chemistry for photosynthesis and/or need support. AT the same time in the desert environment, cactus don't waste any energy on deep roots! They instead, figured out how to store water. They keep their roots very shallow and very small. Deep roots would be a waste of energy and resources. They can live to 150 years, 50 feet high but their root system is just enough to keep the 2 to 10 ton plant from falling over in a wind.
There are a few plants that are programmed to have tap roots that are for support and scouting that do grow very deep. 1000's of feet deep and 1000's of feet horizontally. They don't have the ability to store water.
The cotyledons grow upward against gravity. They don't even need light to be persuaded. Plants have had more time than us animals to evolve. Gravity is like a 'positive' stressor, makes plants and animals more 'sturdy'. But gravity certainly isn't at all the main director of plant direction of growth.
It is said that animals share 99% of DNA with plants. That number has qualifications but what does that tell us? Other than plants were here first, DNA or the blueprint for life started earlier plants. Before that DNA was in virus and bacteria. Plants may be stationary but they are by no means dumb as rocks. There is life in rocks that never sees light, O2 or any organic matter; bacterium and worms. They eat rock. They procreate once every 1000 years.
Dr. David Attenborough stated in a documentary called "The Earth's Core" that 50% of all life is below the surface of the soil, in the crust and mantel of the earth. I have a hard time with that statement, but heck, David Attenborough? I filed that away to deal with later. I am thinking 'numbers' of individual lives, not biomass?
Plant DNA guides them via experience of billions of years. As long as this planet hasn't stopped spinning, lost its oceans or heaven forbid is below 150 ppm atmospheric (manmade or not) CO2, or have temperatures too low for too long, plants will thrive. Our current CO2 at 400 ppm is getting too low. 2000 ppm was dandy.
Plants even communicate. Via chemistry. Some amazing instances of plants getting together for survival, such as when an animal population gets too large? Plants that evolved to handle some grazing by those animals as a mutually beneficial symbiosis, will become toxic to those same animals. Herds have mysteriously died within days. Then it was discovered the plants had communicated and all switched their DNA to temporarily be toxic to the animals in their environment.
Plants are not at all dumb as rocks. They are just not as well understood as animals. Cotyledons grow upward in pure darkness. When a light is turned on off to the side, that plant will grow towards the light. If that light is turned off and another turned on that same plant will easily change directions. Trying to get at light necessary for further growth and survival of that plant.
There are some incredible documentaries on the life of plants. If aliens show up they will be hard pressed to decide who is smarter; humans, other animals or the plant kingdom!
Also watch; "The Mind of Plants", a documentary I thought was excellent.
David Attenborough's documentary, "The Earth's Core" is a must.
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3$\begingroup$ This answer doesn't address the OP's question. They asked why seeds grow upwards, and you have pointed out that indeed they do grow upwards. You talk about plants being 'intelligent', but don't explain any of the mechanisms involved in plant growth. You've also contradicted yourself (perhaps it's a typo?), you said "cotyledons grow upward against gravity", but then "gravity certainly isn't at all involved in plant direction of growth". Finally, I would like to see some citations for the "animals share 99% of DNA with plants" claim as well as the other anecdotes. $\endgroup$– KevinNov 2, 2018 at 21:31
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1$\begingroup$ Some of this is right, some of it is wrong ("But gravity certainly isn't at all involved in plant direction of growth." is wrong), and some of it is confusingly nonsensical ("Then it was discovered the plants had communicated and all switched their DNA"). Please clarify this! $\endgroup$ Nov 2, 2018 at 21:31
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1$\begingroup$ "But gravity certainly isn't at all involved in plant direction of growth" - this sentence is totally incorrect. $\endgroup$– BenNov 3, 2018 at 10:08
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1$\begingroup$ Also, I want to +1 your answer but I do not know enough personally, and the other comments have dissuaded me from doing so. However, I'm not going to vote it down either. If it is any consolation, I thank you for providing more information, and that is the most important thing. $\endgroup$– AaronNov 6, 2018 at 22:26
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1$\begingroup$ I am not merely nit-picking when I say that it is wrong that plants are not affected by gravity. It has a very significant effect on plant growth. Not that gravity forces the direction in which the plant grows - rather plants use gravity to detect and respond to their orientation, with some parts of the plant having mechanisms which direct their growth towards or away from the earth. This is the meaning of the word "geotropism" - which can be positive or negative - towards or away from the earth. While this is not the only thing plants detect, it is an important factor. $\endgroup$– BenNov 7, 2018 at 13:36