The changing leaves of autumn are really awesome to look at, but they're also a really striking
example of nature taking extreme measures to protect itself.
You’re probably familiar with photosynthesis — it's the process plants use to turn carbon
dioxide, water, and light energy into sugars and oxygen.
And you probably also know that photosynthesis depends on a pigment -- a colored compound
called chlorophyll -- but you may not realize that plants contain lots of other pigments,
as well.
Some of the most important are the carotenoids -- yellow, orange, and brown pigments that
give color to things like corn, carrots, pumpkins, and sweet potatoes -- and anthocyanins, which
give red and purple color to cherries, berries, pomegranates, and red apples, to name a few.
All of these pigments play an important role in the plant’s functions, but there’s
usually far more chlorophyll in a plant than anything else, because photosynthesis is a
plant’s number one job.
However, many trees are less active in the winter because they grow at northern and southern
latitudes that get less sunlight during those months.
These trees are called deciduous, from the Latin word that means “to fall off”.
Since deciduous trees don’t do much photosynthesis in the winter, it doesn’t really make sense
to spend a bunch of energy maintaining big green leaves.
So when the days get shorter and the temperature gets cooler, they send less of their limited
resources to the leaves, and start using what water and nutrients they have to keep the
rest of the tree alive.
The chlorophyll in the leaves breaks down, and the green color gradually goes away.
And when that happens, the other pigments -- which were there all the time -- are better
able to show off their colors, before the leaves die entirely and fall off the tree.
So, the leaves aren’t really changing pigments -- they’re just losing their strong green
pigment to reveal the other colors in the tissue.
After the tree stops the supply of food and water to the leaves, all that’s left is
for the tree to cut them off.
The tree forms a special layer of weakly-bound cells near the base of the leaf’s stalk.
Then another layer of cells at the very bottom of the stalk expands to push the leaf away.
Eventually the leaf can be knocked off easily, even by light wind.
And then it’s your job to rake them up.
Thanks for asking, and thanks to our Subbable subscribers who keep these answers coming!
If you have a quick question, let us know on Facebook or Twitter or in the comments
below, and don’t forget to go to YouTube.com/scishow and subscribe!