Portrait of Julius von Sachs (1832-97), German botanist. Sachs studied under Purkinje, obtaining his PhD in 1856. He studied the way in which plants reacted to their environment and their response to light, water and gravity (tropisms). He also showed the process of transpiration, the movement of water through the roots, stem and leaves. His most important discovery was that chlorophyll is contained in discrete bodies within the plant cell, called chloroplasts. Sachs showed that plants used chlorophyll as a catalyst, turning carbon dioxide and water into tissue components in the presence of light, a process now known as photosynthesis.