Creating plots for all variables in a data frame and printing them with the variable name in the main title

Dear all,

I'm just trying to create plots for all variables in a dataframe (named 
"x") using the following:

png()
apply(x,2,hist)

Just as intended, it produces one plot for each variable. Unfortunately, 
the main title of each graph is "Histogram of newX[,i]" instead of 
"Histogram of name of variable". This makes it impossible to assign the 
graphs to the variables. Is there a way to change this and to make R use 
the correct variable names in the title of the plot?

Thank you very much,

Uli