The shear stress effect on directional expansion of pro embryogenic masses (PEMs) and suspensor cell development of somatic embryos of Norway spruce (Picea abies) at the proliferation stage was studied by a direct and quantitative image analysis system. The experimental system allowed for detailed observations of the effect of hydrodynamic shear stress in rotating and deforming liquid cultures of proliferating Norway spruce somatic embryos. Briefly, somatic embryos at an early development stage comprised only of clusters of meristematic cells without suspensor cells were fixed on an alginate film. The alginate film was affixed on the bottom of a flow cell and the somatic embryos were subjected to laminar flow through the chamber of the flow cell. Magnified images of the cell clusters were collected every 24 h. The image data was processed based on a normalized cross-correlation method, capable of measuring morphological and size features of individual cell clusters in both temporal and spatial domains. No suspensor cells developed in the cell clusters under shear stress of 140 s(-1) for the duration of the experiments. Cell Clusters in the control cultured in stationary liquid conditions developed suspensor cells after 5-9 days in culture. Furthermore, the radial growth of meristematic cell clusters was inhibited by shear rates of 86 and 2 140 s(-1), corresponding to shear stress of 0.086 and 0.14 N/m(2), compared to growth under stationary conditions. The shear rate showed a significant negative correlation to growth rate. Control group showed no preference for direction during growth under static conditions. Biotechnol. Bioeng. 2010;105: 588-599. (C) 2009 Wiley Periodicals, Inc.