[R] What does "rbind(iris[,,1], iris[,,2], iris[,,3])" do?

[Sasha Pustota]

It's in the Venables & Ripley MASS (ed 3) book in the section on
principal components.
The context is as follows

> ir <- rbind(iris[,,1], iris[,,2], iris[,,3])
> ir.species <- factor(c(rep("s",50),rep("c",50),rep("v",50)))
(then they use brush(ir) which I guess is not an R function)

and then

> princomp(log(ir[1:4]),cor=T)
(there is no [1:4] in the text which is probably a typo)

>From what I can see,

> iris[,,1]

is the same as

> iris

and "rbind(iris[,,1], iris[,,2], iris[,,3])" duplicates iris 3 times
while "mangling" the row index. What happens to the index and what is
the purpose of that?

I also seem to get the same result with "rbind(iris[,,1], iris[,,1],
iris[,,1])". What is the difference with "rbind(iris[,,1], iris[,,2],
iris[,,3])"?