in the tree通常表示的是方位、位置,在树上、在树干上、在树枝间等都用in the tree表示.
例句：
The monkey was swinging in the tree.猴子在树上荡来荡去.
on the tree也可以当成in the tree来用,此外还专门特指生长在树上.如树上结的果子、开的花等等.
例句：
The apples rotted on the tree.苹果在树上就烂了.
A few flowers still remained on the tree.树上还剩几朵花.