BETA VULGARIS 



x65 



"When the cambium initial undergoes the first division, the outer 

 of the two daughter cells becomes the initial of a new supernumerary 

 cambium while the inner daughter cell divides further and produces 

 xylem, phloem and medullary ray tissue. This process is repeated 

 until all supernumerary cambiums have been formed. However, there 

 is no uniform method governing the formation of the supernumerary 

 cambiums. Often sections of two supernumerary cambiums originate 

 simultaneously, one from an inner, the other from a more peripheral 

 phloem parenchyma cell. Since most of the supernumerary cambiums 

 of the beet are initiated in quick 



succession, a beet no thicker y^x'X:^7S-iXJL?JViS<>..J^~^r=^ en 



than a pencil contains practi- :^^^^^^^^^^^^^^r^i>-— -pc7 



cally all annular zones of growth ^^^^^^^^^^^^^^^V^~ 'P^ 3 



developing simultaneously." ^^^^^^^^^^^^^^^^^- — ca 2 



An alternative explanation 

 of the mechanism of tertiary 

 thickening accounts for the 

 origin of the secondary cam- 

 biums as a result of the con- 

 tinued activity of the pericycle. 

 Early in ontogeny, the pericycle 

 becomes an actively dividing 

 multi-layered zone which keeps 

 pace with the enlargement of 

 the axis. It seems probable 



that the secondary cambiums p^^ ^^^ Transection of portion of stele 



arise in fairly rapid succession of primary root showing primary xylem, 



from the pericyclic parenchyma; ^^^""'^^[y ^hickening and initiation of ter- 



ir ■' r J ^ nary thickening : ca, primary cambium ; 



but there may be some time ca-L, secondary cambium; en, endodermis; 



elapsing between the formation M pericycle; ^h i., secondary phloem; 



f. . . ,. ^>!) 3, tertiary phloem; />«.vj, primary xylem; 



of the successive cambiums, ^^i, secondary xylem; x> 3, tertiary xylem. 



During this interval, those al- 

 ready formed function actively in producing the tissues of their 

 respective rings while the remaining pericyclic tissue is increased 

 by continued radial and tangential divisions. This would har- 

 monize with the fact that the innermost rings are the widest ones. 

 (Fig. 118.) In either case, the pericycle perpetuates itself as the 

 outer zone of the axis and produces a phellogen which forms 

 cork and phellodermal cells. 



Regardless of what explanation is advanced for the mode 

 and sequence of origin of the secondary cambiums, there is 



