The two-cell-layered endosperm of lettuce (Lactuca sativa L.) is characterized by thick cell walls and dense cytoplasm. The periodic-acid-Schiff's(PAS)-positive cell wall forms numerous peg-like projections which extend into the cytoplasm. The dense cytoplasm contains organelles of protein and lipid storage. The protein bodies are numerous and appear to be interconnected by narrow extensions of their envelopes. Spherosomes are also numerous; they occupy a peripheral position in the cytoplasm. Other organelles typical of plant cells (nuclei with prominent nucleoli, mitochondria, microbodies, dictyosomes and various vesicles) are also found in the ground cytoplasm of the endosperm cell. Germination of the seeds began after 14 h imbibition in light, and by 24 h 35-40% of the seeds had germinated. The cell walls of endosperm from seeds germinated in light for 12-15 h were extensively broken down as shown by the decrease in PAS staining of the wall. Cell-wall breakdown increased with the duration of imbibition, with the exception of the wall adjacent to the integument which showed no evidence of digestion. The structural complexity of the endosperm cell wall is correlated with the role this tissue plays in restricting embryo growth. Cell-wall breakdown is correlated with radicle protrusion, although a causal relationship between these two events is not proved.