114 HISTORY OF BOTANY 



phellogen, bark was produced, and how the isolated 

 portions of the cortex scaled off from time to time. His 

 best known work is, perhaps, The Vegetable Cell, first 

 pubUshed in 1851, and translated at a later date into 

 Enghsh. In this volume he summarises the principal 

 work that had been carried out on the cell by himself 

 and by others, and for many years it formed the reference 

 textbook on all questions connected with the subject of 

 cytology as it was then understood. 



The work of Payen must not be passed over without 

 notice. To him we owe our earliest knowledge of the 

 chemistry of the cell wall, for he showed that every cell 

 wall was at first composed of cellulose, and that^s age 

 increased the cellulose might become altered by the 

 addition of " incrusting bodies " which, however, were 

 removable by the aid of certain chemical reagents, thus 

 paving the w^ay for the broader generaUsations of cutina- 

 tion, suberisation, minerahsation, Ugnification, and so 

 on, that we meet with at a later date. 



NaegeH began his work by an investigation of the 

 Thallophyta and was among the first to study the 

 morphology and anatomy of Algae with a view to their 

 natural classification. He did a considerable amount of 

 work on the growing apex of the stem and root, and 

 traced the development of all permanent tissues to the 

 segments of an apical cell in the lower plants and of an 

 apical meristem in the higher forms. He also confirmed 

 Unger's views on cell division in these apical regions, and 

 exposed the fallacy of " free cell formation " as advanced 

 by Schleiden. 



During the years 1847-49 Naegeli worked out the 

 taxonomy of some groups of Algae, and made many 

 additions to our knowledge of their structure and life 

 history. He studied the development of vascular bundles 

 from the procambial stage and, together with Hanstein 

 and Sanio, succeeded in explaining the mode of secondary 

 thickening in the stems of Dicotyledons, a point in which 



