4o8 



THE STRUCTURE OF ECONOMIC PLANTS 



pared with a longevity of approximately 30 days when they remain 

 on the plant axis. The roots usually originate adjacent to each of 

 the three veins of the cotyledon and as many as three roots may 

 develop near one vein. They push through the adaxial portion of 



r-tnes 



y--sto 



Fig. 2.08. A, longisection through median vein of cotyledon showing origin of adven- 

 titous root on adaxial side of bundle; B, transection of cotyledon showing development of 

 adventitious root; C, transection of cotyledon showing later stage in development of adven- 

 titious root : /w.r, mesophyll ; /^A, phloem; j/o, stoma; //f/?, upper epidermis ; .vy i, primary 

 xylem; xj z, secondary xyleni. (After Crooks.) 



the mesophyll of the cotyledon; but may emerge from the cut sur- 

 face 3 to 4 mm. distant. The primordia are initiated by an activa- 

 tion of a layer of parenchymatous cells surrounding the vascular 

 bundle. (Fig. xo8, B.) The meristematic region usually extends 

 a millimeter or more along the bundle, producing a dome-shaped 

 group of cells on the adaxial side of the vein, and the subsequent 



