my <- list(Low=c("21.1","126.8","78.3","32.4","34.5"),
High = ("301.3","17.3","20.3" ))

my

$`Low`
[1] "21.1"  "126.8" "78.3"  "32.4"  "34.5"

$High
[1] "301.3" "17.3"  "20.3"

as.data.frame(my)

Error in (function (..., row.names = NULL, check.rows = FALSE, check.names = TRUE,  :
  arguments imply differing number of rows: 5, 3

Low
21.1
126.8
78.3
32.4
34.5

High
301.3
17.3
20.3