GOSSYPIUM 



42-3 



Secondary Thickening of the Root. — Secondary thickening 

 begins early in ontogeny and the first cambial activity occurs 

 in the zone of fundamental parenchyma lying between the central 

 metaxylem vessels and the primary phloem. The secondary xylem 

 consists of large vessels, smaller conductive elements, fibers and 

 parenchyma. The parenchyma forms rays that are one to three 

 or more cells in width, and the thin-walled ray-cells are radially 

 elongated. The large ves- 

 sels are laid down in 

 radial rows, but become 

 somewhat irregular in ar- 

 rangement owing to the 

 increase in their size. Fre- 

 quently, two or more of 

 these large vessels are clus- 

 tered together, but the ma- 

 jor portion of the xylem 

 consists of smaller vascular 

 elements, fibers, and the '^^^^>^^^^^^^>'^^^~r^^ TrTOMSSrr'Tf^-?' 



thin-walled parenchyma -'-<):Ai;i^cJXK?v">K^tAA^^#KTv^ 



which surrounds the large 



vessels. 



Outside of the cambial 

 zone, the secondary phloem r~A ywyWrSKYyrKh~\ ^--par 



has a characteristic layered / -y v^ VXY'Y'^N-^ ^ csp 



appearance which results 



from the differentiation of p.G. 116. Transection of young pentarch root .- co, 



zones of small, thick- cortex; csp, Casparian strip; en, endodermis ; ep, 



^11 J *»U1„ ™ i:u ^i- ^ epidermis; mx, metaxylem; par, parencliyma; pel, 



walled phloem fibers that ^ • 1 ' t ui ^1 



^ pencycle ; ph, phloem; px, protoxylem. 



alternate with regions of 



sieve tubes and companion cells. In some instances, the fibers 

 occur in compact groups rather than in layers. The wedge- 

 shaped segments of fibers and sieve tubes are separated from each 

 other by funnel-shaped zones of ray cells. In most cases, these 

 are secondary phloem rays that are continuous centripetally with 

 the secondary xylem rays; but in less frequent instances where 

 they are not produced by cambial activity, they may be regarded 

 as being pericyclic in origin. As the root increases in size, the 

 cortical and epidermal cells are stretched and ultimately disinte- 

 grate; but the pericycle remains active during the secondary 



---ma; 



