Julius von Sachs (German: [zaks]; 2 October 1832 – 29 May 1897) was a German botanist from Breslau, Prussian Silesia. He is considered the founder of experimental plant physiology and co-founder of modern water culture. Julius von Sachs and Wilhelm Knop are monumental figures in the history of botany by first demonstrating the importance of water culture for the study of plant nutrition and modern plant physiology in the 19th century.

Sachs was born at Breslau on 2 October 1832. His father, Christian Gottlieb Sachs, was an engraver by trade, and father taught son delineation and accuracy of line and color. From earliest boyhood Julius was fascinated with plants, making collections of them on many field excursions with his father. He gave much of his time between the ages of thirteen and sixteen to drawing and painting the flowers, fungi, and other specimens which he collected. At the Gymnasium from 1845 to 1850 he was most interested in the natural sciences, collecting skulls, writing a monograph on the crayfish. His natural science teacher, one Krober, showed a singular lack of foresight when he solemnly warned young Sachs against devoting himself to the natural sciences.

When he was sixteen years old, his father died, and in the next year both his mother and a brother died of cholera. Suddenly without financial support, he was fortunate to be taken into the family of Jan Evangelista Purkyně who had accepted a professorship at the University of Prague. Sachs was admitted to the university in 1851.

Sachs famously labored long hours in the laboratory for Purkyně, and then long hours for himself each day after his work in the laboratory was finished. After the lab, he could devote himself entirely to establishing how plants grow.

In 1856 Sachs graduated as doctor of philosophy, and then adopted a botanical career, establishing himself as Privatdozent for plant physiology. In 1859 he was appointed physiological assistant to the Agricultural Academy of Tharandt (now part of the Technical University of Dresden) at Julius Adolph Stöckhardt; and in 1862, he was called to be director of the Polytechnic at Chemnitz, but was almost immediately transferred to the Agricultural Academy at Poppelsdorf (now part of the University of Bonn), where he remained until 1867, when he was nominated professor of botany in the University of Freiburg.

In 1868 he accepted the chair of botany in the University of Würzburg, which he continued to occupy (in spite of calls from more prestigious German universities) until his death.

Sachs achieved distinction as an investigator, a writer and a teacher; his name will ever be especially associated with the great development of plant physiology which marked the latter half of the 19th century, though there is scarcely a branch of botany to which he did not materially contribute. His earlier papers, scattered through the volumes of botanical journals and of the publications of learned societies (a collected edition was published in 1892–93), are of great and varied interest. Prominent among them is the series of "Keimungsgeschichten," which laid the foundation of our knowledge of microchemical methods, as also of the morphological and physiological details of germination. Then there is his resuscitation of the method of "water culture", and the application of it to the investigation of the problems of nutrition.

Most important are his experiments, developing the concept of photosynthesis, that the starch-grains, found in leaf chloroplasts, depend on sunlight. A leaf that has been in sunlight, then bleached white and stained with iodine turns black, proving its starch content, whereas a leaf from the same plant that has been out of the sun will remain white. A demonstration of this experiment is shown in the second episode of BBC Four's "Botany: A Blooming History" presented by Timothy Walker.^{[citation needed]}