DIV. I 



MORPHOLOGY 



133 



the meristematic cells enlarge greatly as they are transformed into 

 permanent tissue, a marked elongation of the root accompanying these 

 processes. By this growth in length, which begins close behind the 



157. Median longitudinal section of the apex of a root of the Barley, Hordeum vulgare. 

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

 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. After STRASBURGER.) 



apex and in subterranean roots is limited to a zone only 5-10 mm. 

 long, the root becomes a cylindrical colourless structure. 



In the shortness of the zone of elongation subterranean roots contrast with aerial 

 shoots. In aerial roots this zone may be many centimetres in length. Its short- 

 ness in subterranean roots is evidently connected with the conditions of their life. 



At some distance from the root-tip, about the region where growth 



