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306 this state of things was most distinctly felt when it became necessary to compare the structure of different classes of plants, Cryptogams, Conifers, Monocotyledons and Dicotyledons, and to establish their true differences and actual agreements. How little phytotomy had advanced in this respect is shown plainly in the account of tissues given by Meyen in his 'Neues System' in 1837. To von Mohl belongs the merit of having perceived at an early period in his scientific career, and more clearly than his contemporaries, the value of a natural and sufficient discrimination of the various forms of tissue, and the necessity of obtaining a correct view of their relative disposition ; he thus showed the way to an understanding of the general structure of the higher plants, and rendered it possible to make a scientific comparison of the structure of different classes of plants.

Von Mohl, like Moldenhawer long before, showed from the first a correct apprehension of the peculiar character of the vascular bundles as compared with other masses of tissue. He, too, examined them first in Monocotyledons, and gave an account of them in his treatise on the structure of Palms (1831), and also in his later essays on the stems of Tree-ferns, Cycads, and Conifers and on the peculiar form of stem in Isoetes and Tamus elephantipes, to be found in his 'Vermischte Schriften' of 1845. His just conception of them as special systems composed of various forms of tissue has made his account clear and intelligible, and his whole treatment of the subject appears new in comparison with that of every previous writer except Moldenhawer. If these labours of von Mohl are surpassed in value by later studies of the history of development, they served for the time as a nucleus for further investigations, especially into the nature of stems. It contributed in a high degree to a correct insight into the structure of the stem, that von Mohl, agreeing in this with Moldenhawer, distinguished the portion belonging to the wood from the portion belonging to the bast in the vascular bundles, and