574 THE STRUCTURE OF ECONOMIC PLANTS 



thickening has begun owing to tangential enlargement and radial 

 cell division. Lateral roots arise from sectors of pericyclic cells 

 on each side of the protoxylem points so that they come out at 

 oblique angles to the xylem strand between the protoxylem point 

 and the adjacent primary phloem. For this reason, the number of 

 rows of lateral roots is double the number of protoxylem points. 



Adventitious roots originate in the parenchyma of the phloem 

 and pericycle of the stem. The root primordium is usually located 

 on the same radius as a medullary ray, and centripetal differentia- 

 tion of vascular tissue involves ray parenchyma and some phloem 

 parenchyma laterally. As the growing point of the adventitious 

 root increases in diameter, its lateral extension forces the peri- 

 cyclic fibers apart; and there is a displacement of the cortical 

 cells in addition to a resorption of tissue as the root penetrates the 

 cortex and epidermis. The rupture of the epidermis results in the 

 formation of wound tissue, and a phellogen produces cork cells 

 adjacent to the basal portion of the adventitious root. 



The Stem. — The stem has a dissected amphiphloic siphonostele 

 in which the bicollateral bundles are at first separated by broad 

 medullary rays. Later in ontogeny, the fascicular and inter- 

 fascicular cambium form a continuous cylinder, and a solid zone 

 of secondary xylem is produced. The inner phloem is not re- 

 stricted to points directly centrad to the primary xylem of the 

 bicollateral bundles, but forms a ring of scattered phloem strands 

 that are separated from each other and from the xylem by paren- 

 chymatous cells. The outer phloem strands also form a dis- 

 continuous ring at first, but this zone becomes more or less 

 continuous as a result of the development of secondary phloem. 

 The secondary xylem formed by the fascicular cambium consists 

 of large vessels and wood parenchyma, while the interfascicular 

 cambium produces chiefly connective or conjunctive tissue. Cen- 

 trad to the inner phloem are groups of inner pericyclic fibers that 

 are less numerous than those of the outer pericyclic region and 

 parenchymatous cells separate the fiber groups. The endodermis 

 can be recognized by the presence of starch, and Casparian strips 

 may develop under certain conditions. 



Immediately inside the epidermis is a zone of chlorenchyma 

 and centrad to it there is a band of collenchyma. The collen- 

 chymatous region is more strongly developed and the cells are 

 thicker angled in the more vegetative axes. Anderson (i) has 



