202 The Anatomy of Plants BOOK n 



manently distinct as histogens. The same layer at one 

 time gives rise to cortex, and at another to the outer 

 region of the plerome. His general conclusions were that 

 the embryonic or merismatic tissue of the apex grows as 

 a whole, and that Hanstein's histogenetic layers have no 

 constant existence. 



The writers preceding our period established the fact 

 that in the plants we now know as Bryophyta and Pteri- 

 dophyta, the meristem is very generally, though not 

 always, dependent on the existence and regular segmenta- 

 tion of an apical cell. The details of the manner and 

 order of the segmentation and of the growth of the apical 

 cell were carefully investigated in 1867 by Naegeli and 

 Leitgeb. Still earlier Hofmeister showed that the segments 

 of the apical cell of the stems of Ferns undergo directly many 

 and repeated divisions, both in directions parallel to the prin- 

 cipal walls, and in the radial and tangential planes. By these 

 the growing meristem is cut up into many layers and rows 

 of cells, which are arranged similarly to the segments, but in 

 which the boundaries of the separate segments can be traced 

 only with difficulty. A permanent layer of dermatogen 

 becomes clear only after numerous tangential divisions, 

 and in no case is there a sharp boundary between periblem 

 and plerome. 



De Bary pointed out in 1877 that the divisions of the 

 segments come to form masses of meristem consisting of 

 several or many cells, and that the first products of their 

 division form common initial cells for the future layers 

 of meristem, which correspond in their arrangement to 

 dermatogen, periblem, and plerome of the Angiosperms. 

 These layers become defined particularly by tangential 

 divisions, which cut off concentric strata parallel to the 

 surface, and after becoming recognizable undergo a similar 

 development in many cases to that of the similar layers of 

 corresponding members of the Angiosperms. 



