German botanist whose experimental study of nutrition, tropism, and transpiration of water greatly advanced the knowledge of plant physiology, and the cause of experimental biology in general, during the second half of the 19th century.

Sachs became an assistant to the physiologist Jan Evangelista Purkinje at the University of Prague, where he received the Ph.D. in 1856. In 1859 he was appointed assistant in physiology at the Agricultural Academy of Tharandt in Saxony. Two years later he was made director of the Agricultural Academy at Poppelsdorf near Bonn. In 1867 he accepted the chair of botany at the University of Freiburg-im-Breisgau. The following year he became professor of botany at the University of Würzburg, where he remained for the rest of his life.

Sachs had a strong interest in the movement of water in plants. In his book on plant physiology, Handbuch der Experimental Physiologie der Pflanzen (1865), he discussed how root hairs remove water from the soil and deliver it to other cells of the root. In 1874 he announced the first part of his imbibition theory stating that imbibed (absorbed) water moves in tubes in the walls of the plants without the cooperation of living cells and not within the cell cavities. In 1865 Sachs proved that chlorophyll was not generally diffused in all the tissues of a plant but instead was confined to special bodies within the cell, later named chloroplasts. In 1862 and 1864 he proved that the starch present in the chloroplasts results from the absorption of carbon dioxide, and he established that starch is the first visible product of photosynthesis.

Sachs also studied the formation of growth rings in trees, the importance of tissue tension in promoting organ growth, and the influence of light and gravity in determining the growth rings and symmetry of plants. For this study he invented the clinostat, which measures the effects of such external agents as light and gravity on the movement of growing plants.

Many of Sachs's own investigations can be found in Lehrbuch der Botanik (1868; “Textbook of Botany”), which is also a summary of the botanical knowledge of the period. His Geschichte der Botanik vom 16. Jahrhundert bis 1860 (1875; History of Botany 1530–1860) remains an indispensable guide to the history of botany and to the first stages in the emergence of plant physiology as a separate discipline. Sachs was also influential in establishing the importance of experimentation as a means of gaining biological knowledge.