as.data.frame(do.call(rbind, lapply)) produces something weird

The following code:
--8<---------------cut here---------------start------------->8---
myfun <- function (x) list(x=x,y=x*x)
z <- as.data.frame(do.call(rbind,lapply(1:3,function(x) 
Winsemius' Corollary:

Kurt G?del's Incomprehension Theorem (one of his less well-known results)  has as one of its corollaries that writing `rbind` or `cbind` inside `data.frame` or `as.data.frame`  has probability of measure 1 of producing a result that will dissatisfy its author.
David.

>> c(a=paste("a",x,sep=""),as.list(unlist(list(b=myfun(x),c=myfun(x*x*x))))))))
>> z
>   a b.x b.y c.x c.y
> 1 a1   1   1   1   1
> 2 a2   2   4   8  64
> 3 a3   3   9  27 729
> --8<---------------cut here---------------end--------------->8---
> the appearance of z is good, but str() and summary betray some weirdness:
> --8<---------------cut here---------------start------------->8---
>> str(z)
> 'data.frame':	3 obs. of  5 variables:
> $ a  :List of 3
>  ..$ : chr "a1"
>  ..$ : chr "a2"
>  ..$ : chr "a3"
> $ b.x:List of 3
>  ..$ : int 1
>  ..$ : int 2
>  ..$ : int 3
> $ b.y:List of 3
>  ..$ : int 1
>  ..$ : int 4
>  ..$ : int 9
> $ c.x:List of 3
>  ..$ : int 1
>  ..$ : int 8
>  ..$ : int 27
> $ c.y:List of 3
>  ..$ : int 1
>  ..$ : int 64
>  ..$ : int 729
> --8<---------------cut here---------------end--------------->8---
> how do I ensure that the columns of z are vectors, as in
> --8<---------------cut here---------------start------------->8---
>> z <- data.frame(a=c("a1","a2","a3"),b.x=c(1,2,3),b.y=c(1,4,9),c.x=c(1,8,27),c.y=c(1,64,729))
>> z
>   a b.x b.y c.x c.y
> 1 a1   1   1   1   1
> 2 a2   2   4   8  64
> 3 a3   3   9  27 729
>> str(z)
> 'data.frame':	3 obs. of  5 variables:
> $ a  : Factor w/ 3 levels "a1","a2","a3": 1 2 3
> $ b.x: num  1 2 3
> $ b.y: num  1 4 9
> $ c.x: num  1 8 27
> $ c.y: num  1 64 729
> --8<---------------cut here---------------end--------------->8---
> thanks!
> 
> -- 
> Sam Steingold (http://sds.podval.org/) on Ubuntu 12.04 (precise) X 11.0.11103000
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> 
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David Winsemius, MD
Alameda, CA, USA

as.data.frame(do.call(rbind, lapply)) produces something weird

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