Which way is up?

[Jesse Duke]

The corn is starting to pop up here in Pennsylvania. Does anybody know how the seed knows which way is up? If it wasn't able to sense where "up" was, only a small number of them would make it above the surface. Could it sense light?....even though the human eye wouldn't be able to with that much dirt covering it? Or is it gravity? How does a tree know which way is up after its seed pops above the surface? You would think that if it was the Sun then all the trees in the southern hemisphere would lean north and all the trees in the northern hemisphere would lean south and the closer you were to Earth's poles, the more lean the trees would have. Could trees have an internal gravity sensor? 

[WDH]

Trees are phototropic.  They respond to light.  Some trees will bend and twist to find a hole in the forest canopy.  Cherry is one that comes to mind. 

[Stephen1]

Watch plants in your house, they respond to light, they bend to the window. 

[Don P]

https://en.wikipedia.org/wiki/Gravitropism

[Jesse Duke]

I suspected that gravity had more to do with it than light....even though light is clearly a factor. Thanks so much for posting this....gravitropism..wow! I wonder what they speculate is the cause of that. Gravity has been known about for a long time but it wasn't until recently that its carrier, the graviton was discovered. I wonder if gravitropism has an equivalent counterpart to the graviton. 

[WDH]

Trees grow in response to hormones, too.  One hormone, auxin, is a growth regulator.  It is produced by the bud in the apical meristem and is translocated and evenly distributed down the stem around the bole.  If the bole gets bent to one side, say due to mechanical injury, auxin will no longer be evenly distributed and will stimulate the cambium to put more wood in the growth ring on one side than the other which gradually moves the tree (through push as in compression wood in conifers or pull as in tension wood in hardwoods) back into the preferred vertical position.  I am sure that it is a complex interaction between gravity, light, and growth hormones that regulate plant growth.

[Jesse Duke]

.  I am sure that it is a complex interaction between gravity, light, and growth hormones that regulate plant growth.

Thanks WDH. This stuff is fascinating. Since the underground seed isn't exposed to any light.....and since growth hormones probably can't sense which was is up, I wonder if an underground seed is stimulated by gravitropism and nothing else. It sure makes you wonder what causes the gravitropism. Gravity itself is a bit like that. We know that the more mass something has the more gravity it produces but don't understand why mass goes to all the trouble of producing gravity at all. The key word in your response certainly is "complex". There's some stuff we'll never know but it sure is fun trying.

[Edvantage]

 If fabric was stretched on a frame filled with soil and suspended over a grow light. If the top side was covered with plywood. Would the seed sprout towards the light? Maybe I'll have my kids try it for an experiment. Be cool to grow a stalk of corn upside down. Maybe in a tube used for concrete form with light at bottom?

[trimguy]

I would be interested in seeing some pictures of that.

[Treeflea24]

 Does anybody know how the seed knows which way is up? 

Inertial guidance. Each kernel has 3 accelerometers that are set orthogonal to each other to form a euclydian coordinate system. These sense and track and translational changes in velocity that the seed experiences. There are also three similarly arranged gyroscopes that are spun up while they are still on the ear. These gyroscopes sense and track any rotational motion, and adjusts the accelerometer data based on the rotational orientation. By tracking these six degrees of motion the seed is able to retain a reference on its position relative to its starting point.

Acorns & walnuts tried developing a similar system, but the extreme accelerations from the fall would exceed the system's ability to track and record data. This is why most acorns and walnuts (and other high-density, tree-grown seeds) are found with GPS antennas protruding up and out of the ground.

[WDH]

Exactly.  

[Bandmill Bandit]

Exactly.  

My Understanding is that seeds respond to the Earths magnetic field and that how they know which way is up. A combination of soil temp, moisture activates the seed which then responds to the Earth's magnetic field and turns so that the seed is aligned with the poles and it germinates up ward based on the magnetic indication the seed reads from the magnetic and electrical force of the poles, moon and sun in that order. It's been many years since I studied that and I may have the sun and moon reversed but I don't think so.
There was a company in Saskatchewan that made a "seed magnetizer" that you put at the discharge end of your grain auger so that the seed flowed through it as it was loaded onto the truck that transported seed to fill the drills. I laughed the first time I heard the sales guy tell me about it. He gave me one to try for a field of wheat the first season and I did end buying one and used it till I quit farming. It speeded up crop emergence by a minimum of a day and some times more. Soil moisture conditions affected that a lot. With good moisture and warm soil it could speed up emergence by 36+ hours.      

[CUT N RUN]

Does anybody know how the seed knows which way is up?

Inertial guidance. Each kernel has 3 accelerometers that are set orthogonal to each other to form a euclydian coordinate system. These sense and track and translational changes in velocity that the seed experiences. There are also three similarly arranged gyroscopes that are spun up while they are still on the ear. These gyroscopes sense and track any rotational motion, and adjusts the accelerometer data based on the rotational orientation. By tracking these six degrees of motion the seed is able to retain a reference on its position relative to its starting point.

Acorns & walnuts tried developing a similar system, but the extreme accelerations from the fall would exceed the system's ability to track and record data. This is why most acorns and walnuts (and other high-density, tree-grown seeds) are found with GPS antennas protruding up and out of the ground.

WHAT THE ????   i,ve read this twice , and have decided to re-read it later after a couple of beers

[thecfarm]

Treeflea24 and WDH went to the same school.  ;D  
Good information.

[Tacotodd]

Very interesting. I always wondered but every time I asked a "knowledgeable" person (like my extremely intelligent HS biology teacher) all I got was a answer that equated to "I don't know"!

BTW, that same teacher ALWAYS had a fall leaf collection part of class that went back about 20yrs worth of teaching for him.

[Jeff]

Heat.

[DMcCoy]

x2.
Apical meristematic tissue determines direction.

[RPF2509]

I'm fairly sure that starch particles within the cell use gravity to determine up.  The response to light is governed by growth hormones.