[R] superscript

[David Douterlungne]

Dear R-users. 
 
I'm struggeling to fix the superscript of a label of a figure axis. For some reason R doesn't recognize the "hat" symbol. 
   
 
  plot(1,1,xlab="ligth intensity (PAR)",ylab=expression("mass Pteridium rhizomes" (gr/0.25m^2)))
 
 
A very similiar scriptline does not give any problem at all: 
 
 plot(1,1,xlab=expression("balsa plot basal area" (m^2/ha)),ylab="light intensity (PAR)")
 
 
Someone?
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s. It's easy!

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