Seeds sprout up, why?

[Gene_the_Geek]

Please help me with this: You plant a seed, no matter what direction, and the root goes down and the stalk heads up. Just how does this happen?

I’ve gone to the University of Google, called the conservatory, even got in touch with a botonist and haven’t heard anything conclusive. Having luked around here, I’m sure somebody knows the answer.

[David_Simmons]

@Gene the Geek

Please help me with this: You plant a seed, no matter what direction, and the root goes down and the stalk heads up. Just how does this happen?

I’ve gone to the University of Google, called the conservatory, even got in touch with a botonist and haven’t heard anything conclusive. Having luked around here, I’m sure somebody knows the answer.

It seems that seeds are responsive to gravity.

[Roches]

It’s called geotropism; have you Googled that term specifically? The phenomenon occurs due to gravity, because it doesn’t happen in space. I easily found a few sites that suggest different theories, most of which have to do with something settling to the bottom (with respect to the center of the earth) of the plant’s cells due to gravity. In one case it was auxin, a plant growth hormone. In another, it was small deposits called ‘statholiths’ (sic; Google only gets a few hits for this, but gets many more for ‘statolith’). According to this theory, these deposits exert pressure on the cell wall, causing it to stretch; this causes the plant to grow vertically upwards (wrt the center of the earth).

[GilaB]

IANAbotanist, but according to my college textbook Plant Physiology, second edition(1998), by Taiz and Zeiger:

The only way that gravity can be sensed is through the motion of a falling or sedimenting body. An obvious candidate for intracellular gravity sensors in plants are the large, dense amyloplasts that are present in many plant cells. These specialized amyloplasts [starch storage organelles] are of sufficiently high density relative to the cytosol [the fluid filling the cell, in which the organelles sit] that they readily sediment to the bottom of the cell. Amyloplasts that function as gravity sensors are called statoliths, and the specialized gravity-sensing cells in which they occur are called statocytes (p 569).

So no matter which way you orient the seed, the statolyths in those specialized cells sink to the bottom, orienting the cells that they’re in as to which way is up.

[Roches]

One comment about the page David Simmons linked to: the text refers to auxin, a plant hormone that causes growth along the longitudinal axis (i.e. it makes stems, etc. lengthen). The graphic refers to ‘IAA’, which stands for indoleacetic acid. These terms are equivalent, though there are other hormones that may act as ‘auxins’ besides IAA. It can be confusing to use an acronym without defining it and to use a different term in a graphic than the term used in the text.

On a quasi-related note, humans also produce IAA/auxin, as one of the waste products of amino acid metabolism. The well-known neurotransmitter serotonin is broken down into a derivative of IAA/auxin, 5-hydroxyindoleacetic acid or 5-HIAA. (Monoamine oxidase inhibitors, a class of antidepressants, partially prevent this from taking place.)