AFM measures attractive or repulsive forces between a probe or “tip” and the sample. The height imaging (z-axis) measures the topography and the phase imaging detects varia­tions in composition, adhesion, friction, viscoelasticity, and perhaps other properties. Phase imaging is particularly useful for mapping variations in sample properties at very high res­olution, often with superior image detail. AFM is also able to image samples under fluids that could directly visualize biomacromolecules under physiological conditions (37). AFM has been used increasingly to map cell wall surface structure. Cell walls of various plant species, both from the Monocotyledonae and the Dicotyledonae, have been imaged using AFM (9, 38-42). Early AFM studies of cell walls mostly relied on extensive sample prepara­tion processes using grinding and/or chemical extraction (37-40, 43), which could disrupt the native structure of cell wall fibrils (34). Recently, fresh cell walls from intact tissues have been imaged in water or in a partially hydrated state resulting in varied observations. In one case (42), celery epidermal peels were imaged under water with sequential dehy­dration conditions (e. g., using various concentrations of ethanol). The microfibrils were found to be smaller, uniformly distributed, and highly parallel in the never-dried speci­men. However, the microfibrils were found to be distinctly larger and more disorganized after dehydration treatment with ethanol or air. These investigators then concluded that dehydration processes could significantly affect the structure and arrangement of primary cell wall microfibrils. Our previous study (9) revealed an accurate measurement of maize cell wall dimensions with nanoscale resolution. Figure 3.9 shows a high-resolution im­age of maize parenchyma cell wall taken by AFM phase imaging. For example, a single microfibril can be measured in maize primary walls and it is approximately 3-5 nm in diameter and tens to hundreds of microns in length (supporting our previously proposed model).