[R] How do I sample "cases" within a matrix?

[Silvia Lomascolo]

Hi R community,
I am trying to obtain a sample from a matrix but sample(my.matrix) doesn't
do what I need. I have a matrix of 1287 interactions between the species in
columns and the species in rows and I want to obtain a smaller matrix with
say, 800 interactions, that may or may not have the same number of columns
and/or rows (i.e., some interactions may not be retrieved in a smaller
sample). For example, my original mock matrix M is


     [,1] [,2] [,3] [,4] [,5]
[1,]  140  100   90   40   20
[2,]  126   90   81   36   18
[3,]   84   60   54   24   12
[4,]   70   50   45   20   10
[5,]   42   30   27   12    6

The command sample(my.matrix) samples whole cells from my matrix, such that
if the interaction between species 1 and 1 is included in the sample, they
always show 140 interactions.  But what I want is to sample "cases" within
each cell.  My sample matrix S could have =<140 interactions between species
1 and 1, =<100 between species 1 and 2, etc. Again, if some combination is
absent from the sample matrix, that's OK.

pla<- c(10, 9, 6, 5, 3) #abundance of pla species
pol<- c(14, 10, 9, 4, 2) #abundance of pol species
m<-pla%*%t(pol) #matrix of interactions according to pla and pol abundance
m
     [,1] [,2] [,3] [,4] [,5]
[1,]  140  100   90   40   20
[2,]  126   90   81   36   18
[3,]   84   60   54   24   12
[4,]   70   50   45   20   10
[5,]   42   30   27   12    6

sample(m) #doesn't give me what I want...

I have searched this forum for an answer but all questions regarding
sampling from matrices refer to sampling whole rows or columns, not "cases"
within a matrix.  
Thanks in advance for any help! Silvia.
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