What is remarkable about the beans in Mattie's backyard? What is the literary termed used?

Mattie has purple beans from purple vines. I do not see a specific literary term used to describe them.

It was covered with vines, and the crazy-quilt garden kept right on going on the other side, except without the car parts. The purple beans appeared to go trooping on down the block, climbing over anything in their path.

"They're originally from seeds she brought over with her in nineteen-ought-seven," Mattie told me. "Can you picture that? Keeping the same beans going all these years?"