lio THE STRUCTURE OF ECONOMIC PLANTS 



an epidermal initial and the inner one a cortical initial. Subse- 

 quent anticlinal divisions of the epidermal initials produce the 

 epidermis of the root, while divisions of the cortical initials in all 

 three planes result in the formation of several parenchymatous 

 layers of the cortex. The calyptrogen lies immediately outside the 



'^Jv^-v^V^ 



Fig. 46. Median longisection through the tip of the primary root of corn showing the 



histogens. (See also Fig. 15.) 



periblem-dermatogen layer and by successive periclinal divisions 

 produces the multi-layered root cap. Some anticlinal divisions 

 of root cap cells may follow the initial periclinal divisions. 



In the development of the stelar tissue, the protoxylem is dif- 

 ferentiated early in ontogeny, consisting of slender elongated ele- 

 ments with spiral or annular thickenings. The vessel segments 

 of the metaxylem increase in diameter very rapidly and may be 

 broader than they are long, but final maturation is not completed 



