The Transverse Anisotropic Shrinkage of Wood and Its Relation to the Cell Wall Structure (I) : The Lignin Distribution in the Radial and Tangential Walls of Coniferous Wood Tracheids

This paper is concerned with the relationship between the transverse anisotropic shrinkage of wood and the lignin distribution in the cell walls. The experiment was made by use of two conifers, namely Sugi (Cryptomeria japonica) and Momi (Abies firma). The lignin content in the cell walls of tracheids was shown by the intensity of color reaction with phloroglucinol-hydrochloric acid on the cell wall. The absorption of light passing through color films of the stained cross section taken at a magnification of 400 times was measured by a microphotometer to determine the coloring intensity of the cell walls. In order to indicate the difference in the lignin content between the radial and tangential walls, a coloring intensity ratio (Ir1/It1) was proposed. On the other hand, the transverse shrinkage of small serial cross sections of 30 micron in thickness, including 10 to 20 tracheids free from ray cells, was microscopically measured, and the degree of anisotropic shrinkage was shown by an anisotropic shrinkage ratio (βt/βr). During the shrinkage measurement the sections were kept conditioned in a specially designed microchamber (Fig. 1). The results obtained are as follows: 1) The coloring intensity measured by the microphotometer is higher in the radial walls than in the tangential walls in either of green and air-dried mature wood tracheids (Photo 2, 3 and 4, and Fig. 3 and 4). 2) In green wood, the difference in the lignin content estimated from the coloring intensity is evident even in the eighth or ninth cells from the cambial zone for Sugi and the fourth or fifth cells for Momi, where the staining is more prominent in the radial walls than in the tangential walls (Photo 1). 3) The value of the coloring intensity ratio (Ir1/It1), being always greater than 1, is higher for late wood than for early wood in the same annual ring (Fig. 3 and 4). 4) In the small cross section cut from once air-dried sapwood and heartwood of Sugi, the relationship between the anisotropic shrinkage ratio (βt/βr) and the coloring intensity ratio corrected (Ir'/It') are diagrammatically illustrated (Fig. 5). The relationship is shown to be linear. In the early wood the linear lines are drawn with a slope of about 45°, and this suggests that the anisotropic shrinkage in early wood is closely connected with the difference of lignin content between the radial and tangential walls throughout the sapwood and heartwood. In contrast, the linear lines obtained in the late wood lie with an easy slope. This means that the dependence of differential distribution of lignin in the cell walls on the anisotropic shrinkage of wood is less in late wood than in early wood, and in this case the effect of another factor should also be considered.