152 



BOTANY 



PART I 



loses its meristematic character, and becomes transformed into the 

 differentiated body of the plant. As a general rule, in plants with an 

 epidermis, primary cortex, and central cylinder, the epidermis is 

 developed from the dermatogen; the primary cortex from the 



Fig. 166. — Median longitudinal section of the apex of a root of the Barley, Hordeum vidgare. 1;, 

 Calyptrogen ; d, dermatogen ; c, its thickened wall ; pr, periblem ; pi, plerome ; en, endodermis ; 

 i, intercellular air-space in process of formation ; a, cell row destined to form a vessel ; r, 

 exfoliated cells of the root-cap. (x 180.) 



periblem ; the central cylinder from the plerome. This differentiation 

 of the tissues does not take place in all cases ; and, in fact, does not 

 extend to the embryonic tissue, the peculiar cell arrangement of which 

 is due rather to physical causes. The vascular bundles must pass 

 through the periblem in order to reach the leaves. The periblem is 

 therefore capable of producing, not only the primary cortex, but 



