Walter直至他父亲死后才知他父亲是文盲,而Walter到底怎样发现的仅由“ I would see X’s wher

Walter直至他父亲死后才知他父亲是文盲,而Walter到底怎样发现的仅由“ I would see X’s where his signature should have been.”一句话来判断.偏偏X’s是什么意思又搞不懂,查不出,所以请教.
附录:
As the son of a doorman, Walter Dean Myers grew up in Harlem. He became a popular author. But as he tells his son, Christopher, there was one person that he always wanted his writing to impress: his father.
“I was working at 14 and I saved my money up, hoping to buy a typewriter. After finding out about it, he went out and bought me a really good one,” Walter says. “That was very important to me.”
Today, Walter’s bibliography (书目) includes nearly 100 books. He collaborates (合作) on some of them with Christopher, working together as writer and illustrator (插图画家). Their connection is one that he never had with his own father.
“He never said anything good about my writing,” Walter says. “That really hurt and bothered me a lot.”
Trying to make a good impression on his father, Herbert Dean, Walter started to use some of the stories that he had heard around the house in his writing. “I even would take his ghost stories and publish them,” Walter says. “And I would show them to him, but he would never comment on them. So I thought he hated me. ”
“Did you ever ask him about it?” Christopher asks.
“No. When he was dying, I took him a book that I’d just finished. He picked it up and looked at it, and then he just laid it down. And then, after he died, I went to his house and went through his papers. I would see X’s where his signature (签名) should have been. The man couldn’t read. I mean, that was why he never said anything about my writing. It just tore me up. I mean, I could have read him a story at the hospital.”
“Right,” Christopher says.
“I could have read something to him. But he was ashamed of the fact that he couldn’t read. And that was — that was a barrier between us all my life.”

X就是字母X,意思是该他签字的地方,由于他是文盲,签字只好以叉叉代替.