He recognized under the name of primordial utricle the protoplasmic lining of the vacuolated cell, and first described the behaviour of the protoplasm in cell division. These and other observations led to the overthrow of Schleiden's theory of origin of cells by free-cell-formation. His contributions to knowledge of the cell-wall were no less remarkable; he held the view now generally adopted of growth of cell-wall by apposition. He first explained the true nature of pits, and showed the cellular origin of vessels and of fibrous cells; he was, in fact, the true founder of the cell theory. Clearly the author of such researches was the man to collect into one volume the theory of cell-formation, and this he did in his treatise Die vegetabilische Zelle (1851), a short work translated into English (Ray Society, 1852).