X] 



ROOTS. 



345 



further from the centre of the stele. The structure is therefore 

 that of a typical tetrach root. In the example represented in 



px~ 



Fig. 91. A. Diagrammatic sketch of a transverse section of a young root of 

 Calamites. x, xylem; /, lacuna. After Hick. 



B. Central cylinder (stele) of root, px, protoxylem; ph, phloem; x'-. 

 secondary xylem ; I, phloeoterma. x 75. After Williamson and Scott. 



the figure secondary thickening has begun, and the cambial 

 cells internal to each phloem group have given rise to a few 

 radially disposed tracheids, of. Beyond the phloem there are 

 two layers of parenchyma representing, as regards position, a 

 pericycle and an endodermis. In the ordinary pericycle and 

 endodermis of the roots of most plants the cells of the two 

 layers are on alternate radii, but in the Calamite root, as in 



