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). Mohl's early investigations on the structure of palms, of cycads, and of tree ferns permanently laid the foundation of all later knowledge of this subject: so also his work on Isoetes (1840). His later anatomical work was chiefly on the stems of dicotyledons and gymnosperms; in his observations on cork and bark he first explained the formation and origin of different types of bark, and corrected errors relating to lenticels. Following on his early demonstration of the origin of stomata (1838), he wrote a classical paper on their opening and closing (1850). In 1843 he started the weekly Botanische Zeitung in conjunction with Schlechtendal, which he edited jointly till his death. He was never a great writer of comprehensive works; no textbook exists in his name, and it would indeed appear from his withdrawal from co-operation in Hofmeister's Handbuch that he had a distaste for such efforts. In 1850, he was elected a foreign member of the Royal Swedish Academy of Sciences. In his latter years his productive activity fell off, doubtless through failing health, and he died suddenly at Tübingen on 1 April 1872. References Proc. Roy. Soc., xxiii. 1; Allgemeine Deutsche Biographie, xxii. 55. 1805 births 1872 deaths Scientists from Stuttgart People from the Kingdom of Württemberg 19th-century German botanists Bryologists Members of the Royal Swedish Academy of Sciences Members of the French Academy of Sciences Foreign Members of the Royal Society von Mohl family