306 Examination of the M attired Framework [BOOKII. 



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 Dicoty- 

 ledons, 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 under- 

 standing 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 'Ver- 

 mischte 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 pre- 

 vious writer except Moldenhawer. If these labours of von Mohl 

 are surpassed in value by later studies of the history of deve- 

 lopment, they served for the time as a nucleus for further 

 investigations, especially into the nature of stems. It con- 

 tributed 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 



