CHAP, in.] of Cell- membrane in Plants. 309 



at present possess. Especially important were his researches 

 into the origination and true form of stomata (1838 and 

 1856), and into the cuticle and its relation to the epidermis 

 (1842 and 1845). He brought entirely new facts to light by 

 his study of the development of cork and the outer bark in 

 1836 ; these tissues had scarcely been examined with care 

 till then, and their formation and relation to the epidermis and 

 the cortical tissue were quite unknown. In the latter treatise, 

 one of his best, the difference between the suberous periderm 

 and the true epidermis was first shown, the various forms of 

 the periderm were described, and the remarkable fact esta- 

 blished that the scaling of the bark was due to the formation 

 of fine laminae of cork, which, penetrating gradually into the 

 substance of the cortex, withdraw more and more of it from its 

 connection with the rest of the living tissue, and as they die off 

 form by their accumulation a rugged crust, which is the outer 

 bark surrounding most thick-stemmed trees. The investiga- 

 tion was so thorough and comprehensive, that later observers, 

 Sanio especially in 1860, could only add to it some more 

 delicate features in the history of the process. In the same 

 year appeared his enquiry into the lenticels, where von Mohl 

 however overlooked what Unger discovered at the same time 

 ('Flora,' 1836), namely, that these forms arise beneath the 

 stomata; but he at once corrected Unger's hazardous sup- 

 position that the lenticels are similar forms to the heaps of 

 gemmae on the leaves of the Jungermannieae. Unger, for his 

 part, was not long in adopting von Mohl's explanation of the 

 lenticels as local cork-formations. 



Since von Mohl thus distinctly brought out the special character 

 of the vascular bundles and of the different forms of epidermal 

 tissues, it must excite surprise that he, like former phytotomists, 

 did not find himself under the necessity of framing some con- 

 ception of the rest of the tissue-masses in their peculiar grouping 

 as a whole, as a special system, and of classifying and suitably 

 naming the different forms that compose them, though his 



