Using AFM, we recently observed composite structures appearing on the inner surface of parenchyma cell walls. These structures appeared to be bundles of fibrils, termed the macrofibril (9). The size of the macrofibril varies from 50 to 250 nm in diameter. Figure 3.8

shows a large macrofibril “branching” or untangling at the end to form a set of smaller parallel fibers which appears to be the microfibrils. The macrofibril is also observed in our recent study on fresh parenchyma cells. The morphology of fibers in the macrofibril appears to be faceted, which differs from that of the microfibril (Ding, unpublished data).

The elementary fibril is the native form of the plant cell wall cellulose. Direct characteri­zation of the molecular structure of the elementary fibril has not been reported. Figure 3.4 is the recent model we proposed (9). In this model, the 36 cellulose chains are arranged in an Ip-like structure, forming a hexagonal geometry in cross section with approximate di­mensions of 3 x 5.5 nm. For this model, there are three distinct layers of chains: 6 group-C1 core chains, 12 group-C2 transition chains, and 18 group-C3 surface chains. The elementary fibrils are considered to be highly crystalline. However, due to the fact that half of the cel­lulose chains are surface chains and their interactions with the cell wall matrix polymers, elementary fibrils are expected to have some structural disorders.