Help with "by"

Searched for "by" in forums and no hits, should be simple problem.

This is my line:

bio.mean<-by(Data,Plot, function(x) mean(AbvBioAnnProd))

I want to calculate the mean of "AbvBioAnnProd" in each Plot. What am I
doing wrong?

What is AbvBioAnnProd? I only see that you pass Data, Plot and the parts 
  of Data subscripted by Plot as x into the anonymous function, but 
there is no AbvBioAnnProd that mean can use so far ...
You need to tell us why R should know anything about that object.

Uwe Ligges

Thanks in advance,
-AL