[R] How to split data for NMDS plots

[Trey]

I have a data matrix that contains site data for paired infested and control
plots.  For example:
       ab     ac  ad af ag ah
1     .024   x   x   x   x   x
2     .9      x   x   x   x   x
3     1.5    x   x   x   x   x
4     2.3    x   x   x   x   x
5     1.0    x   x   x   x   x
1c    .75   x   x   x   x   x
2c    2.1   x   x   x   x   x
3c    1.7   x   x   x   x   x
4c    10.2 x   x   x   x   x
5c    2.1   x   x   x   x   x

where: 1-5 stand for infested sites, and 1c-5c stand for control sites.  Row
1 headings are the species within each site.  Numerical data exists for all
columns, the x's are used here just to fill out the matrix.  What I seek to
do is to split out the "infested" sites from the control "sites" in my
subsequent graphical plot of the ordination.  I know there is a simple way
to do this, but I have not discovered it.  Once I split them I can create
unique graphical parameters for each to display within the plot; I know how
to alter graphics and how to plot sites alone in the graphical output. 
Anyone know how to split these out?
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