system
Given the following post, respond as if you're commenting on Reddit:
human
How does a seed know when to grow?

My wife just started growing an avocado pit by letting it float in a glass of warm water for a few days (using a heating pad).  It got me thinking about how seeds can sit on a shelf (or in this case in an avocado) for months or years without growing, but as soon as you put them into water or soil they begin to sprout.  What exactly happens in a seed to convey that conditions are right for growing to occur?  Is it some kind of chemical reaction triggered by warmth and/or water?  Does this reaction vary greatly among different seeds?
gpt
Dry seeds on a shelf don't germinate because they are dormant, and dormancy is broken by the seed getting wet. Most (all?) seeds need to imbibe water in order to soften the seed coat, because if this doesn't happen, the seed isn't going to germinate. The process of absorbing water (imbibition) also activates enzymes stored in the seed, converting them into chemicals which are useful during seedling growth.

Seeds also need oxygen and heat to germinate, and different plants germinate at different temperatures - but it's complicated, some need heat, some need alternating periods of heat and cool. Very few seeds need light to germinate, but some forest plants do, so they only germinate when a tree falls near them, creating a gap in the canopy.

Plants are wicked smart for things with no central nervous system.