Weird question: How do seeds know what direction to grow?

Let’s say you plant some seeds for a plant. The roots eventually come out but since its already in the soil, it just grows out everywhere. The green leafy stem needs to break through the dirt and grow outward towards the light, but how does the seed know which direction to grow in the first place?

I think a search for ‘gravitropism’ might answer your question.

geotropism is another term for that.

stems/leaf shoots go up due to it. roots go down due to it. they will make a U turn if need be.

Geotropism and phototropism.

The roots don’t just “grow out everywhere.” Seeds have a specific end that grows into roots, regardless of the position of the seed.

Didn’t you ever do that grade school experiment? You put a paper towel inside a glass jar, lining the sides. You put a seed between the paper and the glass, and add a bit of water. The seed starts to sprout upward. Then you turn the jar on its side for a few days, and the seedling takes a right-angle turn toward the new “up.”

Growth mediating hormones called “auxins”. These are produced in the growing tips of both shoots and roots, and are translocated back into the growing plant. This translocation is influenced by gravity, so more auxin accumulates in whatever part is “down”.

Envision: a simplified hypothetical plant consisting of a cylinder, pointed at each end, and lying horizontal to the ground. Auxins are produced in both tips, and translocated toward the “middle” of the cylinder. The auxins, while moving laterally, also move slightly “downward” so more of them accumulate along the “bottom” side of the cylinder. And now all you need to know is that auxins increase growth in shoots, while decreasing growth in roots.

Let’s say the left pointy end is the root. So as the auxins move from left toward right, and downward, they’re accumulating in the bottom, where they inhibit growth. The root grows more on the “top” of the cylinder than the bottom, causing it to curve downward.

The situation is similar at the right pointy end, the shoot, with auxins moving leftward and also downward, and also accumulating more on the bottom of the cylinder. But in shoots, auxins promote growth, so the shoot grows faster on the “bottom”, causing it to curve upward.

(Phwew! I never tried to describe this before without at least a chalk board. Hope it makes some sense.)

Of course, rotating the plant/seedling (as in the glass jar experiment) does not change the existing growth, but simply has an effect on the newer growth. In all events, roots grow downward and shoots upward.

ETA - this is geotropism, not phototropism.

How do they grow in space?



Yes, really. Or almost so, anyway.

The two systems mentioned upthread – geotropism and phototropism – control the major aspects of plant growth. Plants grow by elongation, and an asymmetrical distribution of auxins causes differential elongation of the cylinder that is the growing plant, causing curvature. As described above, geotropism involves this growth/elongation in response to gravity. Assuming that the question “in space” referred to “in a zero-G state”, geotropism would not come into play. (Of course, if the question really meant “in space”, then the answer would be that growth would not occur at all. :wink: )

Phototropism works much the same way, but the cause of asymmetrical distribution of auxin is light rather than gravity. So even in zero-G there could be some directional curvature if lights of proper wavelength were asymmetrically distributed in the spacecraft.

But without gravity, and with global (rather than directional) lighting, growth would not be much influenced by these two major (on earth) factors. So yeah, pretty much random.

And also, in the case of phototropism, the auxins that excite growth accumulate on the side AWAY from the light. This causes the cells on the dark side to elongate more than those on the light side. That causes the dark side to lengthen more than its opposite side, and that, in turn, causes the stem to bend toward the light. So cool! As someone said, there’s no place on earth like the world.

Agree! Way cool.