system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How does plant seeds know to root downward and to sprout upwards?
gpt
Plants have the ability of sensing gravity and orienting themselves accordingly. This is called gravitropism. In the roots, there are some small pouches in the cells which produce and store starch and basically pull down the roots through some specialized signaling mechanisms. Roots have positive gravitropism (they grow towards higher gravity) and stems have the opposite. 

Additionally, both roots and stems orient themselves according to the amount of light. This is called phototropism. Stems are attracted by the light (phototropism), while roots are repelled by it.