LIGNIFICATION DURING SECONDARY WALL FORMATION IN COLEUS: AN ELECTRON MICROSCOPIC STUDY

Abstract

The fine structure of lignin deposition was examined in developing secondary walls of wound vessel members in Coleus . KMnO 4 , which was used as the fixative, selectively reacts with the lignin component of the cell wall and thus can be used as a highly sensitive electron stain to follow the course of lignification during secondary wall deposition. Lignin was first detected as conspicuuos electron‐opaque granules in the primary wall in the region where the secondary wall thickening arises and as fine granular striations extending into the very young secondary wall. As the secondary wall develops lignification becomes progressively more extensive. In cross sections the lignified secondary wall appears as concentric, fine granular striations; in tangent al or oblique sections it is seen as delicate, beaded fibrils paralleling the long axis of the thickening. High magnification of tangential or oblique sections shows that the fibrillar appearance is due to the presence of alternating light and dark layers each approximately 25‐35 A wide. It is assumed that the light layers are the cellulose microfibrils and the dark regions contain lignin which fills the space between the microfibrils. KMnO 4 , by selectively reacting with lignin, thus negatively stains the cellulose microfibrils revealing their orientation and dimensions.