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The Magnetic Field Apparatus (MFA) was developed specifically to test whether high gradient magnetic fields (HGMFs) can simulate gravity by providing a directional stimulus for plants grown in space. This space shuttle middeck-locker experiment was designed to imbibe dry flax (Linum usitatissimum L.) seeds on orbit, capture time-lapsed images of the emerging roots as they are exposed to HGMFs, and, at the appropriate time, chemically fix the biological material. One of the major obstacles in the development of this payload system was to determine exactly when was the ‘appropriate’ time for fixation. Ideally the emerging roots were to be fixed after they have passed the area of highest magnetic gradient (∼8mm), but before they have grown so long as to physically touch the sides of the chamber (∼12mm). Initiating the fixative delivery sub-system within this relatively narrow window of acceptability was obtained with a unique iterative control methodology.