[neilep (OP)]

Dear Seedy Experts,

Seeds are great.......dig a hole...chuck em in....voila !...plants grow !!

But how do seeds know which way up to grow ?...doesn't seem to matter which way their orientation is !!

[kdlynn]

i have often wondered this. i know some bulbs must be planted one way or another but seeds don't seem to care

[neilep (OP)]

i have often wondered this. i know some bulbs must be planted one way or another but seeds don't seem to care

I agree about bulbs...especially light bulbs !! *sheepy slaps own face*

[kdlynn]

oh yes those always have to bein the right way. and they never grow... but i've never planted one either... hmmm...

[another_someone]

I agree about bulbs...especially light bulbs !!

Is this because the light bulbs will float away unless you put them in the right way, while the heavy one's will stay put no matter which way you plant them? []

[kdlynn]

lol

[another_someone]

Don't know what cues plants use to decide which way is up (I suspect they don't all use the same), but the following are possibles that I can think of:

•   Magnetism: some bacteria can use magnetism to indicate way is up (although this would not be possible around the magnetic equator), but I am not sure that plants would use the same.
•   Heat: cold nights and warm days are up, while a more steady temperature would be down.  I suspect the differential is not that great over the short height of the average seed to make this a plausible mechanism.
•   Light: especially in seed that are not too deep in the ground, there must be some light getting through from the surface.
•   Gravity: the most obvious, but cannot be ruled out.
•   Water: they may not actually care that much as to which way is up, but care more where they can get their next drink, so they smell water from the water table below.

[dentstudent]

Most plants when in the earths atmosphere conform to a stimulus to gravity called geotropic growth coined in 1869 by the scientist A.B. Franck. He identified three kinds of geotropisms: positive, negative and transversal. Positive and negative geotropism are the growth directions of a plant's stem and roots. Transversal geotropism is the direction of growth at 90° to the plant's axis. All other directions are plagiotropic. In summary, a plant's main stem and roots exhibit positive/negative geotropism, side roots and certain leaves are transversal, and leaves that branch from the main stem at angles are plagiotropic.

Some plants also exhibit phototropism. I'm sure you've seen flowers that follow the path of the sun? But the underlying force for growth is gravity.

[neilep (OP)]

THANK YOU GEORGE......all very moot points....specially Gravity which then leads me to thanks STUART for his concise and fascinating post.

Would you happen to know which direction seeds have grown when experiments have been conducted in space ?

[dentstudent]

You might need an astro-biologist (if there is such a thing!) for this, but from memory, grasses have been grown in space, but they did not grow as strongly, and there were problems with reproduction. Whether this was down to the reduced gravity or other environmental factors I have no idea. The stems tended to become more phototropic (grew towards a light source) and the roots seemed to grow in any which way they chose. There would of course be no requirement for them to be phototropic, as they would normaly be underground.

[neilep (OP)]

I petition we send Stuart into space to conduct further investigations...I'll come along...we can pull faces at the earth.

THANK YOU very much Stuart for your very interesting post.....it's all good stuff.

[dentstudent]

We could pull down our pants and create two new moons!

[Heronumber0]

Maybe I am missing the point here, in a fascinating topic,  but what is the mechanism for responding in the opposite direction to gravity? Is it in the way that growth hormones are distributed in the growing shoot? Is it that more hormone is distributed on the lower side of the shoot 'bending' it upwards due to greater growth on the lower side, against the force of gravity? In that case where in a plant is there a gravity receptor? Am I being silly here?

[another_someone]

Maybe I am missing the point here, in a fascinating topic,  but what is the mechanism for responding in the opposite direction to gravity? Is it in the way that growth hormones are distributed in the growing shoot? Is it that more hormone is distributed on the lower side of the shoot 'bending' it upwards due to greater growth on the lower side, against the force of gravity? In that case where in a plant is there a gravity receptor? Am I being silly here?

Not silly at all - a perfectly valid question, that hopefully someone knows the answer to.

[dentstudent]

I do know that the currently accepted concept behind this is the "Statolith theory", which describes the specialist cells (statocytes) and their growth response in relation to gravity. However, not being a biologist, I've no full concept on how it works. Therefore, here is an excerpt from Wiki...

Statoliths are a specialised form of amyloplasts involved in gravity perception by plants.

These specialised amyloplasts are denser than the cytoplasm and can sediment according to the gravity vector. They are found in a special subset of cells of the root cap (a tissue at the tip of the root) called statocytes. Statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensing channels. The gravitropic signal then leads to reorientation of auxin efflux carriers and subsequent redistribution of auxin streams in root cap and root as a whole. The changed relations in concentration of auxin leads to differerential growth of the root tissues. Taken together, the root is then turning, following the gravity stimuli. Statocyts are also found in the endodermic layer of the inflorescence stem. The redistribution of auxin causes the shoot to turn in a direction opposite that of the gravity stimuli.

[dentstudent]

Here's another explanation.....the terms "geotropic" and "gravitropic" are synonymous.


www.microgravity.org.uk/subjects/Plantgrav.htm

Plants have always had a prominent role in gravity related biological research, due to the fact their ability to detect gravitational fields underlies their ability to grow and reproduce.

Terrestrial (earth based) plants grow roots, which normally bend downwards (termed a positive gravitropic response) and shoots, which characteristically bend upwards (negative gravitropic response), a plants ability to do this correctly is fundamental to its survival, leading us to the conclusion that plants can not only sense the direction of gravitational fields but also send signals to the growing areas of the shoot and root that change the direction of growth accordingly. Our understanding of this system is at an advanced stage, due to the successful collaboration of ground and space based biological research programs.

At a cellular level, a collection of cuboid shaped cells in the plants root sense changes in its orientation using cellular inclusions called Statoliths. The plant then produce signaling molecules, which carry the signal from the root cap at the end of the root (where the signal is sensed) to the area of growth. A class of plant signaling molecules called auxins (which play a similar role to hormones in the human body) carry the signal from the root cap to the zone of elongation; representing an important component of the gravitropic growth response. The gravitational curvature (a growth curve due to gravity) is accomplished by differing the growth rate of each side of the root or shoot, causing it to bend upwards or downwards.

One area of particular interest to researchers looking at gravitropic responses and plant signaling in general is the way these auxins are transported across plant membranes. The model organism for most biological study is the plant Arabidopsis, its genome is relatively small and has been fully sequenced and the use of mutant strains of the plant has greatly facilitated genetic studies.

In the future it is hoped that space based research programs can continue to complement ground-based studies; and micorgravity research facilities such as those on the International Space Station will provide a base for cutting edge research that would not have been possible on earth; over time this research will help us continue to push back the frontiers of the genetic and molecular processes that underlie plant biology. The ultimate goal would be a complete understanding of plant growth, which would have massive implications both in space and on earth, especially in developing nations where plants represent a cheap way of providing everything from food and clothing to the basis of an economy, at a lower cost to the environment and the nation than man made materials; whether we achieve this depends in part to how willing we are to realize the huge potential offered by the unique research environment of space and microgravity.

[Andrew K Fletcher]

And here is a perfectly reasonable explanation of how plants use gravity.
http://www.thenakedscientists.com/forum/index.php?topic=1982.0
Andrew

[neilep (OP)]

THANK YOU ALL again..(and it's great to see ANDREW) back on the site.

EXCELLENT POSTS and thorough information have been gathered and planted in my head ...cheers !

[Andrew K Fletcher]

Thank you Neil

Seeds know which way to grow because gravity is driving their growth! Gravity provides a pumping mechanism causing fluids to rotate / circulate Micro gravity in orbiting crafts above the Earth do not provide zero gravity environments so results with plants in orbiting projects cannot address the absence of gravity as most people believe these projects do.

To remove seeds from the effects of gravity is probably impossible, but reducing the effects of gravity is indeed possible and the resulting experiments in plants, cells and small organisms together with inevitable experiments on mankind are proving that life requires gravity in order to function. The trouble with proving this concept is that space travel will be set back until a device to induce an effect similar to gravity has been put in place. Experiments with magnetism and centrifuge are being investigated, but I fear if we were to send a human on a long trip the results would prove fatal for all concerned.

Gravity is life, without it there is no life, so perhaps the sciences should begin to examine how gravity interacts with seeds plants animals and humans a little closer?