THE PRIMARY MERISTEM AND THE APICAL CELL. 



147 



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 Dermatogen. 

 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. 122, rr, 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, Sic), 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. — Longitudinal section through the apical region of the stem 

 of an embryo of Phaseolus inultiflorus ; ss apex ; fb parts of the two 

 first leaves ; k k their axillary buds. 



^ Sanio, in Bot. Zeitg. 1865, p. 184 et seq. 



2 Hanstein, Die Scheitelzellgruppe im Vegetationspunct der Phanerogamen. Bonn 1868. 



3 Hanstein, Monatsber. der niederrh. Gesell., July 5, 1859. For further details see the general 

 characteristics of Phanerogams in Book H. 



* Compare however Russow, I.e., pp. 177, 183, from which it appears questionable whether this 

 first differentiation of the primary meristem invariably takes place in this way. 



L 2 



