THE PRIMARY MERISTEM AND THE APICAL CELL. 



127 



vascular bundles arise, 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 shoots {e. g. Hippuris, Udora, &c.), the whole of the plerome is 

 developed into procambium, and this into an axial fibro-vascular cylinder, in which 

 two or more vascular bundles and bast-bundles are then formed. 



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 (the dermatogen) localised at the apex, in such a manner 

 that the part of the dermatogen which covers the apex of the root divides periodi- 

 cally by tangential walls. Thus the dermatogen splits at the apex into tw'o layers 

 of cells, the outermost of which developes into a (many-celled) cap the Root-cap, 



Fig. 104.— Longitudinal section of the apical region in the young root of the sunflower (after Reinke) ; h h the root-cap ; 

 * b (figured dark) tlie dermatogen ; // the plerome ; its inner (dark) layer it it the pcricambium ; between jr and b lies tlie peri- 

 blcm ; I jthe primary mother-cells, the origin of the periblcm and plerome. 



while the inner layer at first again performs the functions of the dermatogen, until a 

 new splitting of the layer at the apex causes the formation of a new stratum, which 

 again, on its part, as in Cryptogams, becomes separated by tangential divisions into 

 several layers, as is exemplified in Fig. 104. (On the origin of secondary roots 

 from the pericambium. Fig. 104, tttt, cf. sect. 23.) 



According to the description here given, which can only serve as a preparation to 

 the beginner in a few examples for what follows, it might almost appear as if the pro- 

 cesses in the pwictum 'vegetationis of Phanerogams were essentially different from those 

 in Cryptogams, a hypothesis w^hich I however do not accept. On the one hand the 

 careful investigations of Nageli and Leitgeb in Lycopodiacese (/. c.) on this point prove 

 that in this family the significance of the apical cell in the production of the primary 

 meristem is different from that in other Cryptogams, and approximates to that which 

 occurs in Phanerogams ; and on the other hand the apical cell of Cryptogams may, 

 equally with the apical cell-group of Phanerogams, be considered the starting-point of 

 the first differentiation of the layers of tissue. 



