Chap. ii The Differentiation of the Tissues 217 



at that time held to be a special feature of the root. 

 One of its functions was described by Van Tieghem in 

 1872, when he spoke of it as ' membrane rhizogene \ The 

 development of the lateral roots from it was described in 

 much detail in 1874 by Janczewski. 



The existence of this special layer in the stem was first 

 shown by Falkenberg in 1876, when he described the 

 ■ Aussenscheide ' occurring in the rhizomes of certain Mono- 

 cotyledons, and claimed that it corresponded to the peri- 

 cambium in roots. In 1882 Mangin confirmed Falkenberg 

 and showed that adventitious roots arise from the layer 

 he described. 



A more complete identification of the pericycle in stems 

 was made by Van Tieghem in 1882 ; he pointed out not 

 only its individuality as a layer, but showed of what varia- 

 tion of extent and of composition it is capable. Two years 

 later, in 1884, one of his pupils, Morot, published the 

 results of very comprehensive and painstaking researches 

 upon it, and made the most complete exposition of its 

 individuality that appeared during the century. Its uni- 

 versality as the seat of origination of lateral roots in the 

 Phanerogams was demonstrated by Van Tieghem and Dou- 

 liot three years later. 



Many details of the structure and composition of the 

 vascular bundles marked the period under review, but they 

 were little more than modifications and extensions of what 

 was known still earlier. It is impossible to write of them 

 at length, for though a more complete knowledge of their 

 elements was gradually gained, the process was a slow and 

 continuous one extending over many years. The occur- 

 rence of phloem on the axial side of the woody strand was 

 perhaps the most striking discovery of this nature. It was 

 investigated by Vesque (1875), J. E. Weiss (1880), Petersen 

 (1882), Solereder (1885), and Herail (1885), and was shown 

 to occur in three forms, as a constituent part of the bicol- 



