Lens-shaped gypsum crystals are common in both modern and ancient evaporite deposits, but laboratory experiments demonstrate that the presence of certain types of dissolved organic material is the major factor promoting growth of this particular habit rather than warm saline water conditions. A very widespread type of soluble organic material results from green plant decomposition. Experiments utilizing diffusion-controlled growth of gypsum within sediments of various types, and also utilizing evaporation-controlled solution growth indicate that this organic material promotes the growth of lenticular gypsum, but only under alkaline conditions. In acid conditions, both in the presence or absence of added plant material, only elongate prismatic gypsum develops. The type of sediment within which gypsum grows has only minor effects on its morphology unless the sediment is contaminated with soluble organics. These observations help to explain why lenticular gypsum forms in some instances, and prismatic gypsum in others, within natural sedimentary environments.