Cell walls are deposited by layers upon synthesis. Generally, primary cell walls are synthe­sized when cells grow. Secondary cell walls are deposited when cell growth has ceased. Some cells possess only primary wall, such as the parenchyma cells; however, secondary deposi­tion may occur on most of cell walls when cells age. For example, thin layers of secondary deposition are commonly observed in mature parenchyma (Figure 3.6). Also, there is only one microfibril sheet in each lamella. These thin secondary wall lamellae are measured ap­proximately 10 nm, which appear to contain only one layer of parallel-arranged microfibril. Microfibrils are rotated approximately 50° with respect to each lamella. Secondary cell walls commonly consist of three anatomical layers: the outer (S1), middle (S2), and inner (S3) layers. The thickness of each layer varies in different cell types and tissues. The S2 layer is often thickest, and sometimes contains sub-layers. Another important structure in cells with

thick secondary wall (e. g., vessel cells) is the warty layer in the inner surface, this layer is comprised of granules and amorphous structures (Figure 3.7) that restrict access by rumen microorganisms (12).