[R] sampling problem - new to R

[baldeck]

I have a data set of individual trees and the plots that they are in:

Tree      Plot
56749    1
63494    1
87375    1
37494    2
92753    3
34847    3
38747    4 etc...

So each plot is represented once for every individual that occurrs in it. 
Plots get different numbers of rows because there can be a different number
of individuals in each plot.

I want to make a data frame that consists of one individual from each plot. 
I would like to randomly choose one individual from each plot that is
present in the data set.  I will have to do this to multiple data sets which
may contain different plots, and may contain up to 1200 plots, so I can't
choose the plots by hand.

Please help me with this.  I'm an ecologist and I'm in Panama, with no one
around who is educated in R.  Whoever solves this problem for me will be
acknowledged in any resulting publications.

Thanks!
-Claire
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