48 THE STRUCTURE OF ECONOMIC PLANTS 



and maturation and up to the point where the primary tissues are 

 mature. On the basis of such an examination, it is evident that 

 the differentiation and maturation of the several types of cells do 

 not proceed with equal rapidity. Various stages of differentiation 

 of the xylem may be observed at a given level owing to the fact that 

 the development of the primary xylem is a progressive process in 

 which the protoxylem elements are the first to mature, followed 

 later by those of the metaxylem. Similarly, there is a progressive 

 differentiation and maturation of the primary phloem; and the 

 appearance of well-defined sieve tubes and companion cells follows 

 the initial differentiation of slender parenchymatous protophloem 

 elements. Like the stelar elements, the endodermal tissue under- 

 goes a succession of stages leading to maturation, passing through 

 primary, secondary, and tertiary phases of wall thickening as 

 described in the preceding chapter. The degree of epidermal dif- 

 ferentiation may be gauged by the origin of the root hairs, which 

 reach a maximum production in the region of maturation. Many 

 subsequently die and disintegrate as the root hair zone is extended 

 or additional ones develop. 



Secondary Thickening. — In many roots, especially those 

 which are fleshy, increase in the diameter of the axis is accom- 

 plished by secondary thickening. This is usually initiated upon 

 the completion of the maturation of the primary stelar tissues, but 

 may begin before this process is entirely completed. The first 

 stages of secondary thickening occur in the zone of fundamental 

 parenchyma which lies between the primary xylem and phloem 

 strands. At points centrad to the primary phloem strands, the 

 parenchymatous cells begin to function as a cambium and divide 

 tangentially and radially. The derived cells differentiate, matur- 

 ing as secondary phloem elements centrifugally, and secondary 

 xylem elements centripetally. 



From the points of initial cambial activity, there is a progressive 

 tangential development; and the zones of cambium are extended 

 laterally until they reach the points where the protoxylem strands 

 abut the pericycle. In this way, a continuous meristematic cylin- 

 der is formed, including the sectors of the pericycle from which the 

 broad parenchymatous rays are derived. (Fig. i8.) The manner 

 in which this occurs results in a rounding out of the xylem portion 

 of the stele. The first secondary xylem elements derived from the 

 cambium are laid down in the angles between the protoxylem 



