BETA VULGARIS 



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concentric rings, outside of which there may be several narrower 

 ones; but the inner rings are not equal in width, each being nar- 

 rower than the one centrad to it. This indicates that, in ring 

 formation, the activation of the successively formed secondary 

 cambiums is centrifugally progressive; and that several of them 

 are functional at one time. 

 (Fig. 131.) 



Because of this mode of de- 

 velopment, it is possible to 

 determine the ontogeny of one 

 ring by studying successive 

 rings in a centripetal direc- 

 tion. The outermost one, 

 lying immediately within the 

 periderm, is entirely meriste- 

 matic, consisting of cambial 

 cells derived from the peri- 

 cycle, parenchyma, and undif- 

 ferentiated vascular elements. 

 The first of these to differ- 

 entiate are sieve tubes and 

 companion cells which may 

 generally be found in the 

 second youngest ring. (Fig. 

 131, J5.) Following this, 

 there is a differentiation of 

 a large number of phloem 

 parenchyma cells which by 

 their later divisions and en- 

 largement separate the sieve 

 tubes and companion cells 

 from the cambium ring that 

 produced them. Ultimately, 

 these first-formed phloem ele- 

 ments are obliterated and crushed. (Fig. 131, F.) In most cases, 

 the xylem elements are arranged in narrow radial bands and are 

 separated from each other tangentially by zones of ray parenchyma. 

 (Fig. 133.) In general, the production of phloem cells precedes 

 the differentiation of xylem cells in any given ring; but^ occasion- 

 ally, the production of vascular elements-is reciprocal. 



Fig. 132.. Ontogeny of phloem: A, most 

 peripheral ring ; B, second ring ; C, third ring ; 

 D, fourth ring; E, fifth ring; F, fifth ring but 

 from a different part of the beet. The phloem 

 shows a very marked degree of development 

 compared with the xylem. The outermost 

 phloem groups in F are already obliterated : 

 ca 2., secondary cambium , obi fh, obliterated 

 phloem; phi,, tertiary phloem; xj 3, tertiary 

 xylem. (After Artschwager, Jour. Agr. Rw.) 



