You can see a cat----- the tree.

in
on the tree
在树上.是指树上本来就有的东西,比如果实,花等.
举例：The apples are on the tree.
苹果在树上.
in the tree
在树上.是指本身不属于树的东西藏在树里面了.比如人或动物爬到了树上.
举例：The cat is in the tree.
猫在树上.