ANATOMY OF TISSUES. 45 
are primarily derived from, and renewed by the cluster of original 
meristematic cells. The latter Hanstein named ztial cells (or 
Urmeristem) while to the three groups he gave the name primary 
meristems. Each of these meristems was supposed to have its 
FIG. 21. 
Long section through the tip of stem of Hippuris vulgaris. d dermatogen. per periblem. 
pl plerome. bb'b" beginnings of leaves. x 400. —(dAccd. to Warming.) 
own initial cell or cells, necessitating a certain order of ar- 
rangement in this little group of original meristematic cells. 
(Fig. 21.) 
The dermatogen group consists of the external layer of 
meristematic cells, which remains a single layer, all subsequent 
divisions occurring by walls at right angles to the surface. The 
plerome forms a central cylinder in whose cells longitudinal divi- 
sions predominate. The periblem forms a zone, or hollow cyl- 
inder, around the plerome extending to the dermatogen, and its 
cells are marked by frequent and irregular transverse walls. 
The plant now consists of lasting tissue and two kinds of 
meristems, initial and primary. All the cells are still somewhat 
similar in form and size, and their axes, while by no means equal 
