98 



HISTOLOGY AND PHYSIOLOGY. 



divide into two cells, wliich, 

 in turn, divide, and so oii in- 

 definitely (Fig. 218). 



124. In the higher plants, 

 the punctum vegetationis has 

 no apical cell. Here the pri- 

 mary meristem consists of 

 cells which are very small 

 and numerous. The outer- 

 most layer of cells is continu- 

 ous, with the epidermis of the 

 older portion further back. 

 In fact, it produces it. Hence it is called the primordial 

 epidermis (Fig. 219, D), or dermatogen (Gr. derma, 

 skin ; genein, to be produced). Beneath the dermatogen 

 are generally found several continuous layers out of which 

 the cortex originates. These are called the primordial cor- 

 tex (Fig. 219, Per), or periblem (Gr. periblema, cloak). 

 The nucleus of tissue, enclosed by the periblem_, and out of 

 which the fibro-vascular bundles and pith are produced 

 (Fig. 219, Pier), is called the plerome {Gr. pleroma, a fill- 

 ing up). A short distance back from the apex of the 

 punetum vegetationis the leaves 

 (and buds) take their origin as 

 exogenous structures, that 

 is, from the external cell-layer, 

 or dermatogen. 



125. The root, like the stem, 

 is furnished with a pimetum vege- 



Fig. 219. The growing point, or punctum veffetaiionis , in longitudinal section; 

 D, dermatogen ; Fer, periblem ; Pier, plerome. Fig. 220. Diagram of longi- 

 tudinal section through the root-tip, showing the apical cells i^Ap) and the root-cap 



