BETA VULGARIS 



2.63 



lateral roots are slender and differ from the fleshy, primary root 

 chiefly in having a larger proportion of xylem elements, especially 

 vessels, and in the absence of any considerable amount of ray 

 parenchyma cells. Artschwager (i) has also observed in this 

 connection that the primary xylem strand of the lateral roots is 

 occasionally "triarch instead of diarch, which is always the case 

 in the taproot." 



Secondary Thickening. — The great increase in the size of 

 the axis is due to the activity of primary and secondary cambiums. 

 The cells of the interstitial parenchyma begin to elongate axially 

 and divide tangentially, forming the primary cambium which cuts 

 off secondary vascular elements that are centripetally placed in 

 relation to the primary 

 phloem groups and abut 

 the metaxylem. (Fig. 

 119.) In some instances, 

 there may be a layer of 

 interstitial parenchyma 

 remaining between the 

 primary xylem and the 

 first-formed secondary xy- 

 lem. The production of 



secondary xylem and p,,, ^^^ ^ portion of stele of primary root in 



phloem proceeds simul- transection showing early stages in secondary thicken- 



1 A tVi ' '"S • ^"^ cambium ; en, endodermis ; mx, metaxylem ; 



taneOUSly ana tnere is ^^^^ pericyde ; ph, phloem ; px, protoxylem ; xy z, 



also a progressive lateral secondary xylem. 



extension of the two in- 

 itial regions of cambial activity toward the protoxylem points, 

 so that finally the primary cambium forms a complete cylinder 

 involving sectors of the pericycle outside the protoxylem. The 

 cambium in these sectors does not produce vascular elements, but 

 cuts off successive layers of parenchymatous cells so that two 

 wedge-shaped xylem rays are formed. These rays may be re- 

 garded as pericyclic in cases where the production of the ray 

 parenchyma is the result of a general division of the pericyclic 

 cells, compensating for the growth of the adjacent tissue, rather 

 than the product of a definite cambial layer that cuts off paren- 

 chymatous cells reciprocally. 



The development of the secondary tissues of the stele results 

 in modifications of the primary tissues lying outside this region. 



