196 The Anatomy of Plants bookii 



ticular system of tissue. They constitute in fact histo- 

 genetic layers, comparable to the germinal layers of the 

 animal embryo. Hanstein named them dermatogen, peri- 

 blem, and plerome. 



This hypothesis proved of considerable value in exciting 

 research. It possesses for us still a great scientific interest, 

 showing us that even then the idea of a single plan of 

 development throughout living organisms was welcomed 

 among biologists. 



We find that much of the anatomical work of the next 

 ten years was based upon Hanstein's hypothesis, and many 

 memoirs were published supporting and extending his 

 views. The most important contribution to them was 

 made by Janczewski in 1874; it was especially noteworthy, 

 as besides confirming Hanstein on the general distribution 

 of histogenetic layers, he gave descriptions of several varia- 

 tions of their arrangement in the root, which enabled him 

 to classify them, or to arrange them in four differently 

 differentiated groups, as under : — 



1. The three layers quite distinct, with the addition of 

 a fourth giving rise to the tissue of the root-cap. This 

 layer, covering the dermatogen and soon disappearing, 

 Janczewski was the first to observe ; he termed it the calyp- 

 trogen. This complete differentiation is found to exist in 

 only two aquatic Monocotyledons, Hydrocharis and 

 Pistia. 



2. Dermatogen and periblem arising from a common 

 group of merismatic cells, only one layer of cells thick. 

 The plerome and the calyptrogen layer quite distinct. This 

 type is the commonest type in the Monocotyledons. 



3. Plerome and periblem sharply defined, but a common 

 initial layer for dermatogen and root-cap. The periblem 

 in some cases one cell, in others two cells in depth. This 

 is the prevalent type among the Dicotyledons. 



4. A common merismatic zone extending over the apex, 



