198 The Anatomy of Plants BOOK n 



division of the dermatogen, the inner layer resulting there- 

 from giving rise to it, while the outer forms the root-cap. 

 He gave up the calyptrogen as a definite histogen. 



Passing from these higher plants, it was pointed out by 

 Strasburger in 1872 that the differentiation of the meri- 

 stem at the growing-point of the root of the Gymnosperms 

 differs in several important particulars from the types 

 then known in the Angiosperms. There is a distinct 

 and well-defined plerome-cylinder showing a small initial 

 group of cells at its apex, and surrounding it a kind 

 of mantle, consisting of many approximately regular and 

 concentric layers of cells, each layer being quite continuous 

 round the apex and having apparently its own group of 

 initial cells, which divide only by radial or anticlinal 

 walls. No layer of dermatogen or calyptrogen is differ- 

 entiated, and the root-cap consists of only the more 

 loosely arranged cells of the outer part of the periblem. 



The meristem at the apex of the stem in this group 

 varies a good deal. In the Araucarieae the three layers 

 are traceable to the apex ; in the Abietineae and in 

 Cycas the meristem is not differentiated so far up, a 

 common initial group at the apex separating into three 

 layers some distance further down. Strasburger was able 

 to trace these two types elsewhere in the Gymnosperms, 

 but with no particular regularity. He also confirmed 

 Cramer's conclusions as to the similarity of structure of 

 the Gymnosperms and the Lycopodiaceae as far as the 

 apical meristem is concerned. 



The theory of differentiated histogens corresponding to 

 germinal layers was subjected to severe criticism by De 

 Bary in 1877. He said : J 



' To the question whether in all cases only definite zones 

 of meristem give rise to definite sorts of tissue, the most 



1 Comparative Anatomy of the Phanerogams and Ferns, Eng. ecL, 



p. 22. 



