I think it is the Epidermis
Answer:
Nitrate
Nitrate is the form of nitrogen most used by plants for growth and development. Nitrate is the form that can most easily be lost to groundwater. Ammonium taken in by plants is used directly in proteins. This form is not lost as easily from the soil.
Explanation:
At the very end of the cladogram at the very top of a clade in the middle of a clade along the main trunk of the cladogram
Wouldn't the population then become a mix of both turning into purple.