Here are the starch statoliths that settle out of the cytoplasm to the "bottom" of the columella cells, allowing them to detect the direction of "up".
Posted on 12/01/2009 6:39:06 PM PST by GodGunsGuts
If you go back to my original comment, you will see I said:
“It will bend to keep growing towards the maximum source of sunlight.”
That is, in a shorter form, exactly what you keep stating to me, and yet you say I am wrong.
If you put a source of sunlight (or artificial light) say, to the side of a plant, it will turn and grow in that direction.
Is this not true?
If you win , nothing. I get the prize in the form of learning something new.
If you lose, however, you win the prize.
I don't recall saying you were wrong, other than pointing out that “towards the Sun” isn't a constant direction; it is variable. And heliotropism isn't keeping the entire plant moving in one direction, it is twisting the leaves so that they face east in the AM and west in the PM.
And yes, if you put a grow light to the left of a plant, it will display Phototropism and grow (its stems) towards the light and to the left.
If you track the light from one side to the other over the course of a 12 hour period, the stems will still be growing towards the left due to its initial Phototropism; but its leaves will track the light as it moves because its (leftward growing) stems will twist to keep the leaves facing the light; this is Heliotropism.
If you move the light from the left of the plant to the right, its stems will start growing in the other direction and it will look like one confused plant!
Phototropism will point the plant stem/wood structure in the direction where it is getting the most sunlight; and Heliotropism will point the leaves towards the sun by causing the stems to twist through hydraulic pressure.
So I guess Phototropism is the long term strategy of growing the entire plant structure towards the sunlight; while Heliotropism is the day to day tactic of facing your leaves to the sunlight.
I imagine temperature and water might do the trick via chemical signals that activate transcription factors that transcribe genes to produce the proper protein signaling molecules to start the ball rolling; but I just say that as a former farmer and current molecular biologist who took botany a LOOOOONG time ago. ;)
When the seed planted underground sprouts, it responds to gravity before it can respond to light, because it hasn’t been exposed to light yet.
What I’ve seen is that plants bend towards the light but you can make some interesting shaped plants if you lay them on their side, or like with this new way of growing tomato plants, hang them upside down.
What I’ve noticed is that plants respond more strongly to gravity, wanting to grow up, than they do towards sunlight.
I.E. Plants and trees grow towards the maximum source of sunlight (following it by maintaining an average position with respect to it’s daily movement).
If the source of light is rigid, they grow straight for it.
The plant’s ability to use (or need to use) is likely miniscule, and may only be used in a very brief period when it is underground.
A seed sprouts due to moisture and temperature (change). The temperature change is a direct result of sunlight. The sprouting seed pursues the source of that warmth to reach the surface. In that case it may be following infra-red rather than visible solar.
The use of ‘gravity’ to determine direction of growth is an unproven theory, and seems to have very little effect compared to the source of heat and light.
If you can point me to a documented source which can prove that seeds use ‘gravity’ to determine direction, I would be glad to read it.
http://www.plantphysiol.org/cgi/content/full/133/4/1677
As discussed in our previous Update (Chen et al., 1999), different cells are specialized to carry out these successive phases of gravitropism in monocots and dicots. For instance, in roots, gravity is perceived mainly by the columella cells of the root cap, whereas the differential growth response associated with gravistimulation occurs in the elongation zone (EZ; Figs. 1A and 2). In shoots, cells located in specialized tissues at the periphery of the vasculature, including the endodermis of hypocotyls (Fig. 1B), and the bundle sheath parenchyma in inflorescence stems and cereal pulvini perceive gravity and generate a signal that is transported laterally to the more peripheral tissues.
Here are the starch statoliths that settle out of the cytoplasm to the "bottom" of the columella cells, allowing them to detect the direction of "up".
Thank you for the detailed info and pictures.
These starch statoliths may assist the roots in determining which direction is ‘down’, but does that mean they also tell the ‘sprout’ which direction is ‘up’? Could it be that the ‘sprout’ goes by infra-red radiation coming from ‘above’?
It uses those starch statoliths to determine which direction it is going to send up its cotyledons. IIRC they did some trick with the starch statoliths to confuse the plant, and it sent shoots the wrong way. This gravotropism is also why NASA scientists were interested in sprouting seeds in zero gravity.
If you have a hypothesis about infra-red detection, we need an experiment, and data to support that; then find your mechanism and the signaling pathways. Or if the work has already been done you need a good reference to it.
Gravotropism has the hypothesis, the experiment, the data that supports it, as well as a known mechanism. I am unaware of the exact signaling pathways, but no doubt some sort of cascade of phosphorylations or some such modifying proteins, transcribing different genes, etc. Plant cells are often interconnected via cytoplasm, so signals are regional and often need no specific inter-cellular mechanism.
So, going by the rest of your response, even a seed is 'intelligent'.
It senses heat,light,nutrient,water, and as you have justly defended, gravity.
Gravity detection may be(or is) the leading sensation used by the seed until it reaches the surface. Does it remain the leading 'sense' after that?
A seed is capable of carrying out a biological program of expressing particular genes, devoting resources, and detecting directionality such that the cotyledons grow “up” because the start statholites settled “down”.
Directionality is established by the starch particles. Up and down are opposite sides of the same coin. If a plant can detect ‘down’ then the program has established (by which I mean “know”) which direction is ‘up’.
Gravitropism is just used to grow the cotyledons ‘up’ as far as I know, but like I said - one botany class, long ago; used to skip it sometimes for a midafternoon break with my girlfriend who didn't have class at that time.... pretty little thing.
Certainly no attributes of intelligence there.
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