Why is it callled kernel "tree"?
 

Tried to google this question but no straight answer found. So why is it called tree?

my guess is that linux starts off with one main file and braches off to all the other stuff like module loading, disk access, to plug and pray stuff.

however, if you look on the web for 3d representations of the kernel ( check slashdot for past linux stuff ) you will see it looks more like a nebula in space, but then if you look on think geek for the linux kernel source poster you will see they have shown it in circles and little line drawings for each file

but it is still called a linux tree no matter what :)

The filesystem starts at the root and branches off into multiple directions. :p
Just like a real tree. :D

I'm assuming you mean kernel tree as in source code? As in "get the latest kernel tree"?

It's referred to as a tree because of the directory structure. You have a "root"/top-level directory (like linux-2.4.20), and then each subdirectory underneath is a "branch". A "leaf" is a subdirectory that has no other sibdirectories in it.

All of that naming scheme is derived from computer science, and the different ways of organizing data.