CHAP, ii The Differentiation of the Tissues 225 



The curious plant Welwitschia or Tumboa was des- 

 cribed by Hooker in 1862, having been found for the 

 first time in 1860 by Welwitsch in South Africa. Its 

 flowers were carefully investigated by McNab in 1873. The 

 anatomy and histology of the plant were the subjects of 

 a careful research by Bower in 1881, but its comparative 

 inaccessibility rendered investigation difficult, and little 

 more was learned about it till the appearance of Pearson's 

 researches early in the present century. 



The general embryology of the Gnetaceae was investigated 

 by Bower in 1882, but many points remained undetermined 

 at the end of the century. 



Other writers on the anatomy of the Gymnosperms were 

 von Mohl, Geyler, Dippel, Rossmann, and especially Van 

 Tieghem, by all of whom substantial contributions were 

 made to knowledge. The paper of Van Tieghem in 1872 

 on the secretory system of the Conifers is especially note- 

 worthy. The development of the resin ducts was described 

 by Kreuz in 1877. 



GREEN 



