l1 <- list(c(1933:1935),c(1950:1954), c(2012:2013))  #groups of years
l2 <- list(c(19:21),c(19:24),c(22:26))               #groups of plot numbers

> l3
[[1]]
[[1]]
[1] 19 20 21

[[1]]
[[2]]
[1] 19 20 21

[[1]]
[[3]]
[1] 19 20 21

[[2]]
[[1]]
[1] 19 20 21 22 23 24

[[2]]
[[2]]
[1] 19 20 21 22 23 24

[[2]]
[[3]]
[1] 19 20 21 22 23 24

[[2]]
[[4]]
[1] 19 20 21 22 23 24

[[3]]
[[1]]
[1] 22 23 24 25 26

[[3]]
[[2]]
[1] 22 23 24 25 26