What is the difference between a shrub and a tree?
Some biologists maintain that there is no genetic difference between shrubs and trees - some species may have expressions in tree form and shrub form.  Other scientists believe that the physical form is essential and that shrubs are distinct from trees.
Below is an instruction that describes a task, paired with an input that provides further context. Write a response that appropriately completes the request.
### Instruction: What is the difference between a shrub and a tree?
### Input: 
### Response: Some biologists maintain that there is no genetic difference between shrubs and trees - some species may have expressions in tree form and shrub form.  Other scientists believe that the physical form is essential and that shrubs are distinct from trees.