This is more of a sight-gag than an actual joke — it connects the anxiousness of having one piece missing from a jigsaw puzzle with the anxiousness of waiting for the last leaf to fall from a tree.
This isn’t so much “ha-ha” funny, but more “interesting connection” funny.
So jigsaw puzzles grow on trees?
Cartoonist heard of monkey puzzle trees and decided to pun on a shortened version, “puzzle tree”. Which, I guess, makes that hairless ape the monkey.
Comics I Don't Understand 
This is more of a sight-gag than an actual joke — it connects the anxiousness of having one piece missing from a jigsaw puzzle with the anxiousness of waiting for the last leaf to fall from a tree.
This isn’t so much “ha-ha” funny, but more “interesting connection” funny.
So jigsaw puzzles grow on trees?
Cartoonist heard of monkey puzzle trees and decided to pun on a shortened version, “puzzle tree”. Which, I guess, makes that hairless ape the monkey.
https://owlcation.com/stem/The-Monkey-Puzzle-Tree-An-Ancient-and-Endangered-Plant
It’s a surreal play on Newton’s apple?
Doesn’t make any sense and isn’t funny but at least it’s original and clever.
Strange.
That looks more like a play mat than a jigsaw puzzle. I mean, there’s no assembled image.
A metaphor for life?
Young Newton?
Synchronicity . . .
He is waiting for the last piece of the puzzle to fall into place.
Waiting for the last piece of the puzzle to fall into place