[R] replacing "+" in string

[Nevil Amos]

I am trying to replace "+" in a string with another character 
I am getting odd results using sub and gsub
> X<-"one + two"
> gsub("+","plus",X)
[1] "plusoplusnpluseplus plus+plus plustpluswplusoplus"
> sub("+","plus",X)
[1] "plusone + two"
> X<-"one ~ two"

it seems to work fine with other characters:

> sub("~","plus",X)
[1] "one plus two"


How do I enable "+" to be recognised as a normal character?

thanks

Nevil Amos