[Hearing a friend tell the story of the coming of the corn] was the last
stroke of the hoe that fit my mind ...
Trad grammar analyses "hearing" as a 'gerund' functioning as predicator of the non-finite clause "hearing a friend tell the story of the coming of the corn", functioning as subject.
Modern grammar does not distinguish gerunds and present participles (and nor does it need to), simply calling them 'gerund-participles'.
Note that non-finite clauses don't normally have an overt subject, but we understand them as if they did. Here, the subject appears to be the writer.