For the purpose of these observations he looked about in every case for the most suitable objects for examination, a point to which former phytotomists, with the exception of Moldenhawer, had paid little attention; he then studied these objects thoroughly, and thus prepared the way for the examination of others, which presented greater difficulties. Every monograph of this kind was a nucleus, round which a larger number of observations might afterwards gather. In a long series of such solid productions he treated conclusively all the more important questions of phytotomy. Von Mohl’s extraordinary carefulness was not however able to guard him, calm observer though he was, from some serious mistakes, at least in his earlier years, such as those which occur in his first theory of intercellular substance (1836), and in his earliest views on the nature of the cell-membrane of the pollen-grain (1834). These and some other errors on the part of a gifted and truly inductive enquirer are instructive, since they show that observation without any groundwork of theory is psychologically impossible; it is a delusion to suppose that an observer can take the phenomena into himself as photographic paper takes the picture; the sense-perception encounters views already formed by the observer, preconceived opinions with which the perception involuntarily associates itself. The only means of escaping errors thus produced lies in having a distinct consciousness of these prepossessions, testing their logical applicability and distinctly defining them. When von Mohl laid down his theory of intercellular substance, there evidently floated before his mind indistinct, half-conscious ideas of the kind that Wolff and Mirbel entertained of the structure of the vegetable cell; and as he considered the cell-membrane of the pollen-grain to consist of a cell-layer, he summarised its obscure structural relations under the then very obscure conception of the cell. As a true investigator of nature, who adheres always and firmly to the results of further observation, and endeavours to clear his ideas by their aid, conceding only a relative value to every view, von Mohl soon escaped from these errors, and himself supplied proofs of the incorrectness of his former opinion. The number of really erroneous statements in his works is wonderfully small considering the very large number of investigations in which he engaged.