boxplot via plot command

Hi folks,

I've just discovered that the following code leads to boxplot (surprisingly to me).
Can anybody explain to me why? Is this documented somewhere? I've never 
consider this option before.

x <- rnorm(300)
l <- c(rep("label1",100), rep("label2",50), rep("label3",150))

df <- data.frame(as.factor(l), x)
plot(df)


Thank you!
Antje