denoting post-hoc test differences using letters

Is there code in R to automatically assign letters to different groups  
that are found to be statistically significant using a post-hoc test  
following an ANOVA? For example, let's say I found that relative rates  
of tree growth were statistically significant between tree species  
using an ANOVA and then I ran a TukeyHSD post-hoc test to determine  
which species were statistically different from one another. I can  
manually assign different letters for each species that is different  
from another, but is there code to do this automatically in R. I would  
like to add these letters to a table, but have many different ANOVAs  
and doing this manually will be quite time consuming.

Thanks,

Adrienne