how to avoid change string to number automaticlly in r

I was trying to save some string into a matrix, but it automatically
changed to numbers (levels). How can i avoid it??

Here is the original table:

  trt    means  M1 0   12.16673  a2 111 11.86369 ab3 125 11.74433 ab4
14  11.54073  b

I wanna to save to a matrix like:
J0001 a ab ab b

But, what i get is:
J0001 1 2 2 3

How can i avoid this?