In addition to the middle lamella there are two layers, primary and secondary, in the walls of the libriform fibers. The primary layer is composed of strands of fibrils helically oriented around the fiber almost at a right angle to the fiber axis. The secondary layer is composed of a number of lamellae each of which is composed of fibrils that run parallel to the axis of the fiber.
Proceedings of the Iowa Academy of Science
©1935 Iowa Academy of Science, Inc.
Martin, John N.
"Structure of the Libriform Fibers in the Roots and Crowns of Alfalfa and Sweet Clover,"
Proceedings of the Iowa Academy of Science, 42(1), 82-82.
Available at: https://scholarworks.uni.edu/pias/vol42/iss1/17