The origin of the cell was unknown in Mohl’s time. Schwann had regarded cell growth as a kind of crystallization, beginning with the deposit of a nucleus about a granule in the intercellular substance – the “cytoblastema”, as Schleiden called it. But Mohl, as early as 1835, had called attention to the formation of new vegetable cells through the division of a pre-existing cell. Ehrenberg, another high authority of the time, contended that no such division occurs, and the matter was still in dispute when Schleiden came forward with his discovery of “free cell-formation” within the parent cell, and this for a long time diverted attention from the process of division which Mohl had described. All manner of schemes of cell-formation were put forward during the ensuing years by a multitude of observers, and gained currency notwithstanding Mohl’s reiterated contention that there are really but two ways in which the formation of new cells takes place – namely, “first, through division of older cells; secondly, through the formation of secondary cells lying free in the cavity of a cell.”
But gradually the researches of such accurate observers as Unger, Nageli, Kolliker, Reichart, and Remak tended to confirm Mohl’s opinion that cells spring only from cells, and finally Rudolf Virchow brought the matter to demonstration about 1860. His Omnis cellula e cellula became from that time one of the accepted facts of biology.
Mohl’s early investigations on the structure of palms, cycads, and tree ferns permanently laid the foundation of all later knowledge of this subject. His later anatomical work was chiefly on the stems of dicotyledons and gymnosperms. He first explained the formation and origin of different types of bark, and corrected errors relating to lenticels. Following his early demonstration of the origin of stomata (1838), Mohl wrote a classical paper on their opening and closing (1850). He received many honors during his lifetime, and was elected foreign fellow of the Royal Society of London in 1868.