The "green stuff" in leaves is chlorophyll, the pigment that absorbs sunlight for photosynthesis. As fall approaches, the days get shorter and the temperature drops, leading to:

* Less sunlight: Chlorophyll production slows down, and the green color fades.

* Breakdown of chlorophyll: The existing chlorophyll breaks down, revealing other pigments that were present in the leaves all along but masked by the green.

Here's what happens to those other pigments:

* Carotenoids: These are responsible for the yellow and orange colors we see in leaves. They're present throughout the year, but become visible as chlorophyll degrades.

* Anthocyanins: These are responsible for the red and purple colors in leaves. They're produced in the fall as the leaves break down sugars.

Ultimately, the green stuff in leaves breaks down and is either:

* Recycled: Some of the nutrients from the chlorophyll and other pigments are transported back to the tree for storage.

* Decomposed: The rest of the leaf matter decomposes on the ground, releasing nutrients back into the soil.

So, the green stuff in leaves doesn't magically disappear; it breaks down, allowing other pigments to shine, and ultimately contributes to the nutrient cycle.