<span>The levels of cell organisation in increasing size in a tree, as with all living organisms, are: organelle, cell, tissue, organ, organ system. Organ systems combine to make an organism. </span>
I don't know much about bio, but for this your answer is skin. This is because it is the outer layer.
Hope this Helps!
~JaredEkey
Wouldn't the population then become a mix of both turning into purple.