i68 



THE STRUCTURE OF ECONOMIC PLANTS 



The manner in which the vascular elements are distributed 

 indicates that the secondary cambiums do not occur as complete 

 rings, but consist of discrete sectors derived from the pericycle. 

 The parenchymatous cells which intervene between the radial rows 

 of vascular elements are of pericyclic origin and may be regarded 

 as pericyclic rays. Each cambium produces a zone of paren- 

 chymatous cells centripetally before the tertiary xylem elements 

 are differentiated; and further growth and division of these 

 cells results in a radial or, occasionally, a tangential displacement 



of the xylem elements. 

 mtpar j£ ^.j^^ successive second- 

 ary cambiums are formed 

 from a persistent peri- 

 cyclic zone, the develop- 

 ment of the interzonal 

 parenchyma can be ex- 

 plained as the result of 

 the proliferation of the 

 increments of the pericycle 

 which remain centrad to 

 each secondary cambium. 

 To this zone of pericyclic 

 parenchyma are added the 

 products of the activity of 

 ,,,,,,. the secondary cambiums 



Fig. 133. Sector of large bundle from fourth ring : ■' 



ca 1, secondary cambium; int par, interzonal paren- On either Side OI It. ihe 

 chyma; obi ph 3, obliterated tertiary phloem; pel ra, cambium within produceS 



phloem parenchyma which 



constitutes the inner zone 



of the parenchymatous band; and, in this region, the obliterated 



remains of crushed sieve tubes and companion cells occur. The 



cambium external to the parenchymatous band contributes an outer 



zone of xylem parenchyma in which there are scattered vessels. 



On the basis of this interpretation, the interzonal parenchyma 



is made up of three regions: an outer zone of xylem parenchyma; 



an intermediate one of pericyclic parenchyma; and an inner region 



of phloem parenchyma. A somewhat different explanation is 



given by Artschwager (i) in which he states that 



"the interzonal parenchyma in its entirety is made up of three regions: 

 an outer zone containing scattered xylem cells, a central purely paren- 



- -int par 



pericyclic ray; ph t,, tertiary phloem; xjy 3, tertiary 

 xylem. (After Artschwager, Jour. Agr. Kes.') 



