LINUM USITATISSIMUM 



383 



opment and regeneration in the flax seedling, Crooks (7) has con- 

 firmed and elaborated Janczewski's work, and his detailed account 

 of the root ontogeny is quoted. 



"The calyptrogen-dermatogen layer of cells overlying the periblem 

 multiplies by periclinal divisions around the tip and forms a root cap 

 of regular, radial rows of cells. In the lateral portion of the same 

 layer the epidermis is formed by anticlinal divisions. Laterally, 

 where the divisions are anticlinal the histogen is strictly a dermatogen, 

 while the layer at the tip is a calyptrogen. The root cap at the tip 

 is very regular because the cell divisions in the calyptrogen are periclinal 

 only. Occasionally, some of the 

 cells in the lateral portion of the 

 root cap divide anticlinally and 

 distort the radial rows. This 

 occurs only in lateral portions of 

 the root cap and not at the tip, 

 where the rows of cells are regular. 



"The cortex is derived from the 

 periblem, which consists of two 

 layers of cells overlying the 

 plerome. The outermost layer of 

 the periblem divides only anti- 

 clinally and forms the outer layer 

 of the cortex, the hypodermis. 

 The inner layer divides in all 

 planes and forms the remainder of 

 the cortex, which at maturity is 

 about 6-9 cells in thickness. (Fig. 

 194, /4.) 



' 'The stele is differentiated from 

 a small group of plerome cells 

 which divide in all planes at the 

 tip, while divisions at a higher 

 level are chiefly in a transverse 

 plane. The outer layer of this 

 group produces the pericycle, which can be distinguished rather early 

 by the relatively dense cytoplasm and larger size of the cells. The first 

 evidence of differentiation of the vascular system appears about 0.4 mm. 

 above the tip of the plerome. Two primary phloem ducts are first dif- 

 ferentiated opposite each other by the elongation and breakdown of a 

 single row of cells lying next to the pericycle, and alternate with the 

 two protoxylem points which are differentiated later. Figure 195 shows 

 how these ducts are formed and stretched by the elongation of neigh- 

 boring cells. These ducts collapse after becoming greatly stretched 

 but usually persist until the protoxylem elements are well differentiated. 

 Elongation of the root often causes the protoxylem to collapse. The 



Fig. 195. A, B, and C, longisections of 

 portions of primary root showing forma- 

 tion of primary phloem duct at 0.61, i.o, 

 and 1.5 mm. from tip respectively. (After 

 Crooks.) 



