Vents and cowl vent 67 Chevy C10

[RTHarvey]

Where does the air come from that flows thru the vents in the kick panels of a 67 chevy PU? The reason I ask, is because pretty often, like when I park near a Bartlett Pear tree, when I am driving along and open the vents, I get a storm of leaves or flower petals or other detritus. I would like to try to minimize this--I have seen screens that go under the cowl vents--is this where vent air comes from? I guess I could just go look and see if I can see where it comes from, but I though someone might just know off the top of their head. ?