What changed Pa's opinion about Dave's studies in "Split Cherry Tree"?
Pa changed his mind about Dave's studies and the way in which things were being taught after he had the opportunity to meet Professor Herbert and learn more about what he was doing with the students. Pa didn't truly understand the ways in which education needed to change to deal with the changes in the world until he had the chance to observe some of the class activities and listen to Professor Herbert's explanations of them.
The single event that was most influential in changing Pa's mind was seeing the germs he had scraped off his teeth.
Pa put his head down and did as Professor Herbert said. "I see 'im," says Pa. 'Who'd a ever thought that? Right on a body's teeth! Right in a body's mouth. You're right certain they ain't no fake to this, Professor Herbert?"
"No, Luster," says Professor Herbert. "It's there. That's the germ.
From that point on, Pa was more willing to listen and accept Professor Herbert's ideas about different types of learning and new approaches to education that were needed. Pa came to realize,
I've larned a lot from you. My boy must go on. Th' worId has left me. It changed while I've raised my family and plowed th' hills.