46 



THE STRUCTURE OF ECONOMIC PLANTS 



which is one cell layer in thickness. The lateral members of this 

 intermediate zone divide periclinally, and the derivatives of the 

 inner cells function as a periblem producing the cortical tissue, 

 while the outer daughter cells become the dermatogen and form the 

 epidermis. This is the type found in many monocotyledons in- 

 cluding Zea and Triticum. (Fig. 15.) Treub (11) has described 

 a modification of type (i) that applies to monocotyledons in which 



a definite calyptrogen is not 

 diff"erentiated. In this case, 

 the root cap, epidermis, and 

 cortex arise from a common 

 group of initials two cell 

 layers in thickness, and 

 within this zone is a sharply 

 defined plerome. In this 

 modified type (i) are mem- 

 bers of the Liliaceae, includ- 

 ing the genus Allium. 



In type (3), the plerome 

 and periblem are sharply 

 defined; and outside the lat- 

 ter is a common initial layer 

 which produces the root cap 

 and epidermis. The cells of 



Fig. 16. Longisection of the apex of the root ^uj^ i.^^- nerDetuate the his- 



of Linum showing the character of the terminal ^"^^ ^^Y^^ perpetuate tne niS 



meristem: a and b, root cap and epidermis re- tOgen by anticlinal divisionS, 



spectively derived from the calyptrogen-der- ^^^ periclinal divisionS at 

 matogen layer; c, endodermis; a, stele, derived ^ 



from the plerome; e, pericycle limiting the itS apeX form SUCCeSsive 



stele; /, cortex, derived from the two-layered j^yetS of the rOOt Cap, the 



periblem; g, epidermis. (After Janczewski, Ann. . ^ 111 c 



Set. Nat.) innermost cells always func- 



tioning as the common in- 

 itial layer. The cells of the common layer, which form the 

 proximal margin, undergo a final periclinal division; and 

 the inner daughter cells function as a dermatogen producing 

 the epidermis by anticlinal divisions, while the outer ones are 

 added to the root cap. Thus, according to Janczewski (6), the 

 dermatogen and root cap are derived from the calyptrogen 

 layer; but Crooks (3) and others, recognizing the dual character 

 of the histogen, have designated it as the dermafogen-calyptrogen 

 layer. In most cases reported as representing this type, the 



