• The secondary wall layer is divided into three sublayers: the outer
layer of the secondary wall (S1), the middle layer of the secondary
wall (S2), and the inner layer of the secondary wall (S3) or the
“tertiary wall” (T).
• The inside of the S3 layer is covered with a thin membrane called
the “warty layer” (W). The central cavity of the hollow fibre is
termed the “lumen” (L). The middle lamella (ML) is located between
the P walls of adjacent cells and serves the function of binding the
cell together.
• The term “compound middle lamella” (CML) is used to designate the
combination of ML with the two adjacent P walls.