I read it as a joke of increasing absurdity. Sniffing pepper makes people sneeze so that one works (besides the trying to make a plant sneeze part). He got a bit distracted by experiment 2 and mixed up the involuntary reactions sneezing and tickling and tries to tickle it with a feather. He then gets further distracted between experiment 2 and 3, and tests feet which are commonly ticklish.
When I break it down like this you can see that it is a very funny joke, as one can always tell from a detailed explanation of the joke.