S2 Lumen Primary Wall Secondary Wall -- S1, S2, S3 Primary Wall very thin and stretchable present when cells enlarging. Microfibrils randomly arranged. Compound middle lamella CML = primary walls of 2 adjacent cells plus true middle lamella. true middle lamella (pectinaceous) SEM Photo by T. Zimmermann. Cell Wall Layers

Cell Wall Layering Viewed with Polarized Light Secondary Cell Walls are Layered XS of Softwood

Three-Layered Secondary Wall S1 S2 --Largest layer S3 S1 and S3 layers near constant width from tracheid to tracheid. S2 thickness varies. So thick-walled fibers and tracheids will have proportionally more S2, as in these latewood tracheids Secondary Wall of Fibers and Tracheids

Microfibril Orientation S1 cell wall layer has microfibril angles degrees from long axis of cell S2 cell wall layer has microfibril angles degrees from long axis of cell S3 cell wall layer has microfibril angles degrees from long axis of cell

CELL WALL MODEL Cell wall model - softwood tracheid wwarty layer S1, S2, S3 secondary wall layers Pprimary wall layer MLmiddle lamellae Hardwood Fiber Walls Similar except NO warty layer P, S1, S2, S3. S 2 S1 S3 P

Chemical Composition of Wood % Dry Wt.D.P.Units Cellulose40--44% 5,000-10,000Glucose Hemicellulose(s) % Sugars (Var.) Lignin(s) % ,000Phenol- Propane (Var.) _______________________ U.S. softwoods usually have more lignin than hardwoods, while U.S. hardwoods have more hemicelluloses than U.S. softwoods.% Dry wts. from Kollman & Cote 1968.