294 



THE ROOT 



FIG. 49. A, three cells of the piliferous layer at the beginning of 

 the root-hair region in longitudinal section, showing the early 

 stages of the development of root hairs. Note the accumulation 

 of cytoplasm and the presence of the large nucleus (which passes 

 into the hair) at the point of origin of the root hair. B, diagram 

 of single endodermal cell, showing the cutinised band (black) 

 round the radial and horizontal walls. The tangential walls 

 (front and back) are supposed to be transparent. C, diagram 

 of portion of endodermis, supposed to be seen from within the 

 cylinder, showing the way the cells are fitted together and 

 " cemented " by the cutinised bands. D, small portion of edge 

 of cylinder in an old and large root, showing " horse-shoe " 

 thickening of endodermis, and " passage cell " opposite proto- 

 xylem. E, diagram of cross-section of vascular cylinder in 

 which secondary thickening has begun, the secondary tissue 

 at present formed only opposite the primary phloems. X t , 

 primary xylem ; X 3 , secondary xylem ; PI, primary phlom ; C, 

 cambium. 



