<p>Fibers are a key component of wood, providing mechanical support to woody plants. Understanding their development is crucial for both tree function and industrial use. We investigated how the transverse surface area of wood fiber bodies increases in <i>Robinia pseudoacacia</i> L. Samples of vascular cambium and differentiating secondary xylem were collected and sectioned using a Tesla ultramicrotome. Zones containing cambium and wood fibers at different stages of differentiation were analyzed for cell transverse surface area, cell wall perimeter, cell width, and thickness. The shapes of fiber bodies were also examined and the obtained data were compared with mathematical considerations regarding the shape of geometric objects and the impact of shape changes on transverse surface area. Our findings show that wood fiber development involves a gradual shape change from rectangular to round, accompanied by increasing thickness and decreasing width. The increase in transverse surface area results from shape changes rather than cell wall growth in the transverse plane. This is supported by the nearly constant cell wall perimeter across the differentiation zones. Obtained results have important consequences for the analysis of xylogenesis in angiosperms, as they stay in sharp contrast to the common understanding of events occurring within differentiating xylem.</p>