如果能把X和mature的数据贴出来可以更准确的解答，下面以一个小例子说明，不知道是不是你要表达的意思：
> X <- cbind(a=c("aa","bb","cc","dd"),b=c("ee","ff","gg","hh"))
> X
     a    b   
[1,] "aa" "ee"
[2,] "bb" "ff"
[3,] "cc" "gg"
[4,] "dd" "hh"
> mature <- 1:4
> mature
[1] 1 2 3 4
> target.var <- apply(cbind(X,mature),1,paste,collapse="")
> target.var[1]
[1] "aaee1"