88 



MERISTEMATIO SYSTEM 



Hanstein's classification is, of course, by no means universally 

 applicable : for, as a matter of fact, the structure of growing-points 

 with several initials varies within comparatively wide limits. More- 

 over, quite a number of cases are now known among Angiospermous 

 shoots and roots, in which Hanstein's three primary histogens cannot 

 he recognised. In particular, it is not uncommon for the apical dome 



Fio. 18. 



Longitudinal section through the root-apex of Briophorum vaginatum [CypkbackabJ. 



:, the oldest cells of the root-cap, which are sloughing off ; c, calvptrogen ; e, proto- 

 derm ; r, cortex (periblem) ; p, central cylinder (plerome). Protoderm and cortex 

 arise from the same initial. 



of a shoot to be invested by a perfectly distinct dermatogen, while 

 at the same time the plerome and periblem arise from a common initial, 

 and hence can in no sense be regarded as two separate histogenetic 

 layers. In many roots, again, the dermatogen and periblem obviously 

 have a common genetic origin. The adjoining figure, for example 

 (Fig. 18), which represents a radial longitudinal section through 

 the root-apex of Eriophorum vaginatum, shows quite clearly that, in 

 this case, periblem and dermatogen are derived from one and the 



