What to do if the corresponding author is presumably dead? I need further information on a botany paper I found on jstor which is from the 1920s. So I am assuming the workers wouldn't still be alive. Sadly, there has no work been done in recent times on the subject. 

What is to be done if I require further/supplementary information if the corresponding author doesn't exist?
You could contact their institution (or its successor), explain your issue and ask if you can access its archive. There is at least a chance that supplementary information or even a specimen preparation can be found there. Obviously, going through an archive can be a big effort.