Observation of the Swelling Behavior of Coniferous Cells Using a Confocal Scanning Laser Microscope and a Digital Image Correlation Method

Abstract

The transverse swelling behavior of coniferous wood cells was observed using a confocal scanning laser microscope and a digital image correlation method (DIC). Since the laser microscope has great depth of focus and high resolution, it was possible to observe the deformation of cells precisely. In order to observe the swelling behavior of specimens (Douglas-fir, Pseudotsuga menziessi) on the stage of the microscope, moisture was supplied using a humidifier. Digital image data of the cells taken by the laser microscope were used to measure the swelling behavior using DIC.The outside diameter of latewood tracheids swelled or changed almost isotropically with increasing moisture content, but the inside diameter (lumen) shrank greatly and anisotropically. Earlywood tracheids expanded a little but underwent great changes in shape. Thickness expansion of cell walls was less than circumferential expansion (perpendicular to the thickness direction), so that we concluded that the anisotropy of expansion was mainly based on thickness swelling. In latewood, circumferential expansion of radial cell walls (changes in length of the middle lamella) was great, but tangential cell walls expanded less than radial walls (in some cases the expansion was nearly zero). The angles between a radial wall and the adjacent tangential wall increased both in latewood and in earlywood by swelling.