The light to dark gradient background effect is caused by the Anode heel effect inherent to all x-ray tubes. It is just more visible with the low exposures used for floral radiographs. I try to take advantage of it when composing an image. This Orchid radiograph shows the effect well.
Different films produce a variety of background textures.
Many times the background grain is not visible until enlarged. This Delphinium radiograph uses the anode heel effect, and solarization in combination to add dramatic texture and lighting effects. This equipment was designed for specimen radiography and is not suitable for diagnostic imaging of patients. I prefer the Cymbolic Sciences Fire 1000 or the Kodak LVT film recorder. Finally a traditional darkroom is used to make hand enlargements on a fine art silver rich paper.