1. Subcellular fractionation of maize roots resulted in the isolation of the following enriched fractions: cell wall, dictyosome, smooth-membrane and rough-microsomal fractions. In addition, extracellular polysaccharide of the root slime was isolated. 2. Maizeseedling roots were incubated in vivo with d-[U-14C]glucose, and the pattern of incorporation of radioactivity into the polysaccharides of each fraction was investigated. 3. The differentiation of maize-root cells with respect to the synthesis of specific extracellular polysaccharide directly relates to the polysaccharide synthesized and transported within the membrane system of the cell. A fucose-containing polysaccharide, characteristic only of root slime, was present only in the membrane system of the root-tip region of the root. Regions of typical secondary wall development within the root were characterized by an increased incorporation of radioactivity into xylose of polysaccharide within the membrane system. 4. The incorporation of radioactivity into glucan polymers in the membrane fractions was very low in all regions of the root. Since in regions of secondary wall development greater than 60% of all radioactive incorporation was into a glucan polymer, it can be inferred that this polymer, most probably cellulose, is not synthesized or transported within the compartments of the membrane system. It is suggested that synthesis of cellulose occurs at the surface of the plasmalemma. 5. Maize-root cells contained 40 times more rough endoplasmic reticulum than dictyosome membrane. The relative specific radioactivities of each fraction indicated that polysaccharide was concentrated in the region of the Golgi apparatus, which showed a 100% increase in specific radioactivity compared with the rough endoplasmic reticulum. The Golgi apparatus can thus be regarded as a localized focal point on the synthetic and transport system of polysaccharide by the intracellular membrane compartments.
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