要旨 : Lignification is an important differentiation process in plant cell walls in which lignin is deposited in the polysaccharide matrix. The actual chemical structure of the lignin polymer is still unclear for several reasons : it has multiple structural units and bonding patterns ; it is entangled with the polysaccharide matrix and, therefore, very difficult to isolate ; and the lignin structure may vary among the different cells and tissues in plants. The controlling mechanism of lignin

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... tion is also still controversial. Timeofflight secondary ion mass spectrometry (TOF -SIMS) has been used to investigate the structure and formation of lignin in individual cells ; herein, those lignin studies are reviewed. TOF -SIMS is a rapidly developing analytical method that can provide mass spectral information with submicron lateral resolution. The main polymer components of plant cell walls (cellulose, hemicellulose, and lignin) are detected as fragment ions in the TOF -SIMS spectra. Because an understanding of fragmentation behavior is essential in order to discuss the original structure, such behavior is described in detail. Subsequently, practical results on the relative amounts and the structure of the lignin in specific tissues are introduced. The latest advances in cryo -TOF -SIMS, which allows the analysis of frozenhydrated samples, are also described. Cryo -TOF -SIMS is a powerful visualization tool, especially for watersoluble compounds of nearly equal distributions in a living system. The heterogeneous distribution of the many chemical components in plants should provide useful information for innovative discussions of the various mechanisms in plants.