Subject: What parts of a plant cell actually have the medicinal value?

I am working with a group of 6th graders on a research project about cells, 
their functions and how that knowledge affects our lives in a healthy way. We 
understand that different parts of the plant have medicinal value, but we 
would like a deeper understanding of exactly what parts of a plant cell have 
the medicinal value.  We searched, but could not find an answer. 

Re: What parts of a plant cell actually have the medicinal value?