How do plants know when it’s Autumn? How do leaves know when to turn red and fall off?
In: Biology
the days are shorter.
the leaves don’t return enough sustenance to warrant maintaining them during winter so the circulation is cut off to them and they dry out and slough off like excess skin.
when the days are longer new leaves become useful again.
There are fewer hours of daylight. They just respond to reduced light time by shedding their solar collectors.
They react to changes in the air and changes in the amount of sunlight they’re getting.
They don’t have a brain to think and go “ok yes right now is the time to start dropping leaves”
Instead there are chemical reactions and processes that get triggered by a sustained drop in temperatures as well as the lower sunlight they get due to the shortening days.
Chemistry.
Similar to how your human cells respond to caffeine when you drink too much and get hyper and can’t sleep. Trees have an internal chemistry that react to the changes in weather. The cells detect it and start shutting down to go dormant.
This is not too dissimilar from your brain detecting increased levels of Melatonin (hormone for sleep) in your body and making you yawn and shut down for sleep. It’s all chemistry.
It’s quite fascinating to think about. Living things are just biological robots carrying out the instructions of their DNA. Are any choices really our own?, but that’s a question for another day.
Its the same reason you react when you smell something rotten, somewhere inside your dna you notice something different or wrong and you react accordingly
Plants are the same way, but their version of rotten food in this case is changing of the season, where they will be getting MUCH less energy from the sun and cant spend the energy it takes to support leaf growth, so the leaves slowly die. They know to do this based on how much sun they are getting, but more specifically how much LESS sun they are getting. Eventually, the amount of energy the sun is giving them becomes less than the amount of energy they are spending on leaves and they stop
This is a very simplified version, but hope it at least gives an idea
There is a chemical called auxin which plants produce, this chemical is broken down by sunlight so long nights and short days increase the amount of auxin and the plant “knows” winter is on the way.
In a similar fashion, horses grow their winter coat in response to decrease in hours of sunlight in fall.
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