Development and properties of tracheids and their cell wall layers in eastern hemlock (Tsuga canadensis L.)

Abstract

Changes in anatomical, physical and chemical properties within developing and one-year-old increments were investigated in a 52-year old eastern hemlock tree, felled in the latter part of July, 1968. Based on available previous knowledge, the zones of xylem maturation stages were determined by the aid of a polarizing microscope. In accordance with the cellular structures described microscopically 4 mm x 30 mm x 60 µm tangential microtome specimens were cut from both increments. These specimens were tested for tensile strength and analyzed for nitrogen, lignin and carbohydrate content. From the anatomical properties of the developing xylem, and from the changes in chemical constitution and physical properties, a model for wood formation was derived. The model for wood formation and the physical properties and chemical constitution of the partially developed tracheids were used to calculate physical and chemical properties of individual cell wall layers. Results showed that 59% of the middle lamella and primary wall was lignin, but these layers contained only 19% of the cells total lignin content. Lignin content of the secondary wall was higher in the S3 layer, (35%) than in the S1 layer, (25%). Carbohydrate distribution showed highest glucan content in the S₂ layer of the secondary wall, to which the next highest was the S₁layer. Hemicellulose monomers were mostly located in the middle lamella and primary wall regions. In the secondary wall the S₃ layer was the richest in xylan and araban contents. Interactions of measured tracheid properties showed that tensile test of minute specimens were affected by the glucan, mannan and galactan contents and by the number of cells within the specimens.