THE PRIMARY MERISTEM AND THE APICAL CELL. 
that even at the first commencement of an organ no one predominating apical cell 
can be recognised (Fig. 113). After Sanio ^ had investigated these processes in 
Phanerogams, Hanstein ^ studied them in a more general and detailed manner, and 
has recently shown that even in the embyro of Phanerogams the first divisions take 
place in such a manner as to negative from the first the existence of an apical cell ; 
while, on the other hand, a differentiation into an outer layer and an inner nucleus 
of tissue early manifests itself ^ 
The outermost layer of the primary meristem which covers the growing point 
together with its apex is the immediate continuation of the epidermis of the older 
part which lies further backwards ; it may therefore be termed the Primordial 
Epidermis ; Hanstein has however already applied to it the name Dcrmatogen. 
It is distinguished by the circumstance that divisions occur in it exclusively at right 
angles to the surface ; it is only at 
a subsequent period that tangential 
divisions also sometimes occur, when 
the epidermis becomes divided into 
several layers. 
Beneath the Primordial Epidermis 
one or more layers are generally 
found which also cover the apex 
continuously, and out of which the 
cortex originates further backwards 
from the apex (Fig. 1 22, r r, p. 163); 
they represent therefore the Pri- 
mordial Cortex ; Hanstein calls this 
layer of the primary meristem the 
Periblem. Enclosed and overarched 
by this is a nucleus of tissue, 
which may be followed out as an 
immediate continuation of the fibro-vascular bundles, and of the pith enclosed 
or traversed by them. The layer of tissue in which the first fibro-vascular 
bundles originate, termed by Sanio the Thickening-ring, thus corresponds to the 
outer layer of this inner tissue-nucleus (which Hanstein terms Plerome), when a 
pith is formed*. If no pith is formed, as in many roots and some stems {e.g. 
Hippuris, Anacharis, &c.), the whole of the plerome is developed into procambium, 
and this into an axial fibro-vascular cylinder, which is then traversed by two or more 
vascular bundles and bast-bundles. 
The origin of the root-cap in Phanerogams may be considered, according to 
the recent investigations of Hanstein and Reinke, simply as a luxuriant growth of 
the primordial epidermis or dermatogen, localised at the apex in such a manner that 
FIG. 113.— Lonsfitudiiial section through the apical region of tlie stem 
of an embryo of Phaseohes 7)! nltißoriis ; ss apex ; parts of the two 
first leaves ; /c k their axillary buds. 
^ Sanio, in Bot. Zeitg. 1865, p. 184 et seq. 
^ Hanstein, Die Scheitelzellgruppe im Vegetationspunct der Phanerogamen. Bonn 1868. 
^ Hanstein, Monatsber. der niederrh. Gesell., July 5, iSip. For further details see the general 
* Compare however Russow, I.e., pp. 177, 183, fi-om which it appears questionable whether this 
first differentiation of the primary meristem invariably takes place in this way. 
