LINUM USITATISSIMUM 407 



tissue which extend outward from each protoxylem strand, and the 

 roots arise in two rows, each of which lies in the vertical plane of 

 the cotyledons. In some cases, the meristematic activity extends 

 from the outermost layer of the pericycle centrad to the parenchym- 

 atous cells adjacent to the primary xylem. (Fig. 107, A.^ Most 

 of the primary xylem elements are crushed and disintegrate as this 

 activity proceeds, and the pith parenchyma may be involved in the 

 process of adventitious root formation. Cell divisions occur in all 

 planes, and a dome-shaped root primordium is formed which 

 pushes outward into the cortical region. The endodermis becomes 

 active, undergoing radial cell divisions, keeping pace with the 

 subsequent growth of the underlying cells. (Fig. 107, 5.) It 

 maintains itself as a single layer, and persists over the root cap of 

 the adventitious root until it has grown i or x mm. into the soil. 



No organized histogens can be observed until about the time 

 that the root emerges from the cortex, when the cell layer adjacent 

 to the endodermis begins to divide periclinally, and functions as a 

 calyptrogen. (Fig. ioy, D.) The periblem and plerome are 

 clearly defined by the time the root has grown 2. or 3 mm. in length, 

 and the dermatogen is derived from the calyptrogen which func- 

 tions as a dual layer in the manner described for the primary root. 

 (Fig. to-/, £.) Coincident with root formation, cell divisions fre- 

 quently occur throughout the medullary region of the hypocotyl, 

 forming a continuous cylinder of parenchymatous cells which may 

 remain active for a month or more after the new roots develop. 

 (Fig. i07, F.) This activity of the pith is more pronounced when 

 two adventitious roots develop at the same level, and some tra- 

 cheids may differentiate in the meristematic region. 



Less frequently, adventitious roots may form as a result of gen- 

 eral cell activity in the pericycle and phloem parenchyma, in which 

 case they are not in the same vertical plane as the cotyledon. 

 (Fig. X07, C.) Crooks has shown that adventitious roots may also 

 arise from severed cotyledons when they are placed on the surface 

 of moist soil. About 90 per cent of the cotyledons taken from 

 plants 10 days old developed from three to six adventitious roots, 

 and a few cotyledons from xo-day-old plants also produced roots 

 under similar conditions. The cotyledons develop an extensive 

 root system and enlarge somewhat, being thicker than those which 

 are not removed from the plant axis; but they do not produce 

 adventitious buds, although they live for 60 days or more as com- 



