HUGO VON MOHL 113 



The period of which I am speaking, viz., from 1830 

 to 1850, was one of great activity in anatomical and 

 histological research, but, among the numerous workers 

 whose pubUcations we meet with during these years, two 

 stand out head and shoulders above all the rest, Von Mohl 

 and NaegeH. As we have already seen, these two men 

 took a prominent part in estabhshing the great generaUsa- 

 tion known as the " cell theory " and in the discovery of 

 protoplasm. Over and above this, however, their con- 

 tributions to general histology were important and 

 varied, and to some of these I must now refer. 



Von Mohl's earliest work was concerned with the 

 development of tissue elements from cells, showing how 

 the primary wall becomes modified both in external form 

 and also as a result of secondary thickening in various 

 ways, contradicting Meyen's view that annular, reticulate, 

 and other types of vessels were derived from spiral. 

 Von Mohl demonstrated also that pits were thinner 

 places on the wall between secondary thickenings, and 

 neither elevations nor pores. An essay on the Anatomy 

 of Palms did much to clear up the architecture of the 

 Monocotyledonous stem, and, in a paper pubUshed in 1838, 

 we obtain at last a correct account of the structure and 

 mode of development of stomata. A few years later 

 Von Mohl took up Moldenhawer's interpretation of the 

 vascular system and confirmed his idea that the vascular 

 bundle was a compound structure composed of xylem 

 and phloem. He traced the course of the bundles in the 

 stem both of Monocotyledons and of Dicotyledons, and 

 showed how the first bundles of the stem were bundles 

 derived from the leaves — '* leaf traces " as we term them, 

 after Hanstein. He also studied the structure of the 

 epidermis, and demonstrated the real nature of the 

 cuticle, which had often in previous years been confused 

 with the epidermis itself. He then turned his attention 

 to the cork and worked out its origin and development, 

 pointing out how, by successively deeper formations of 



