system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do seeds know which way is up?

So I was doing some gardening with my 9yo daughter and we moved on to planting some seeds. She was studying them carefully before dropping them into the holes in the compost. I asked her why and she said that she was trying to make sure she planted them the right way up, otherwise they wouldn't grow upwards and flower.

As cute as it is, this got me thinking... How does a seed know which way is the right way to grow?
gpt
ELI5: Plants have cells that can tell which way is down. The roots know which way is down. The leaves/stems decide which way is up by sensing the light.

In the most simple of terms, plants have cells called stratocytes in the tips of roots that respond to gravity. They give a signal that releases a plant hormone that directs the growth of new cells.

In the shoots and stems, a similar process occurs which directs growth upward. Mature plants don't really have this ability anymore and thus use the angle of the light to "decide" where to grow. This is called phototropism.

A seed in utter darkness will know to grow roots downward, but a plant in utter darkness will not know which way to grow upward because there is no light for it to chase after.