Lewis and Margaret Henderson were surely unlettered, and I suspect that most of their children were, too. I have only been able to find handwriting samples for two.
Caswell C. Henderson was the most worldly of the siblings, having migrated to New York City in his mid 20s, engaged in local politics, and secured a patronage job at the United Customs House while in his 30s. From his 1893 marriage certificate to Emma D. Bentley:
And from a letter he wrote in 1926 to his sister Sarah:
Sarah Henderson Jacobs Silver was also literate, though her handwriting and grammar reveal the limits of her schooling. This signature appeared on the marriage license of her niece, Minnie Simmons Budd.
Most families grow exponentially, but Lewis and Mag’s descendants underwent a bit of a bottleneck in the third generation. Of their nine children, only two — Ann Elizabeth and Loudie — had children that survived to adulthood. Those two produced five children (barely, as one died at age 19) who reached majority. Of the five, I have only found the signature of one. Jesse “Jack” Henderson, Loudie’s son, affixed his name to his Social Security application in 1936: