SECONDARY THICKENING OF ROOTS 



131 



A transverse section of any unthickened Dicotyledonous root 

 will show a narrow band of two or three laj'ers of parench^-matous 

 cells between each phloem-group and the adjacent xylem. Prior 

 to the commencement of secondar}- growth tangential division- 

 walls arise in these cells, leading to the production of a cambium 

 (Fig. 27, ca.,p. 66) 

 like that of the 

 stem. In this way 

 there originate as 

 many cambial 

 strips (Fig. 63, A, 

 B, C.) as there are 

 groups of phloem, 

 and, by the active 

 division of the 

 former, secondary' 

 xylem is produced 

 on the inside and 

 secondary phloem 

 on the outside. 

 Differentiation of 

 the cambium is at 

 first confined to 

 the inner surface 

 of each phloem- 

 group, but by slow 

 degrees it extends 

 along the sides of 

 the xjdem-arms 

 till finally, by the 

 development of 

 tangential divi- 

 sion-waUs in the 

 cells of the pericycle opposite the protoxjdems, a complete 

 lobed cylinder of cambium is established. 0\\ing to the late 

 development of the cambium opposite the protoxylem-groups, 

 secondary thickening is at first more extensive in the bays, so 

 that the outline of the cambium, at first lobed, gradually 

 becomes circular. 



Fig, 



63. — Diagrams showing secondary thickening 

 in the root. A, a tetrarch, and B, a diarch 

 root, before thickening, showing the position 

 of the cambium (C). C, a tetrarch root after 

 secondary thickening has been going on for 

 some time. Co., cork ; Cor., cortex ; En., 

 endodermis; PA., phloem ; P.;., primary ray ; 

 P.xy., protoxylem ; Xy., primary xylem ; 

 2Xy., secondary xjdem, 



