Specifying column names in a data.frame changes spaces to "."

Let's say I have a data.frame, like so:

x <- c(1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10,1:10)
df <- data.frame("Label 1"=x,"Label 2"=rnorm(100))

head(df,3)

returns:

  Label.1    Label.2
1       1  1.9825458
2       2 -0.4515584
3       3  0.6397516

How do I get R to stop automagically replacing the space with a period in the column name? ie, "Label 1" instead of "Label.1".