ROOT HAIRS AND GROWTH 



355 



mined. But whether either or neither 

 of these alternatives is true, it is evident 

 that the older portions of the cell wall are 

 composed, with one exception, of material 

 derived only from the root proper and not 

 ordinarily of material from the outside. 

 There is thus need of rapid transportation 

 in the root hair if rapid elongation is to 

 be effected. 



The one constituent of the cell wall of 

 the hair which may be, and probably for 

 the most part is, derived from the exterior 

 is calcium. The fact that root hairs, as 

 is shown below, will not grow at all with- 

 out the presence of calcium in the external 

 medium indicates that it is used directly 

 from this source for cell wall formation. 

 But it is hardly conceivable that the 

 calcium could enter alone. Theoretically 

 it is possible for it to enter by exchange 

 with potassium or some other cation, and 

 doubtless some potassium and perhaps 

 magnesium is liberated from the root in 

 this way. But there is so much calcium 

 absorbed, and furthermore so much carbon 

 dioxide liberated from the root, that 

 potassium liberation is not the only thing 

 which happens at the surface of the root 

 hair. There must be the absorption of 

 some anions accompanying the calcium 

 which are not received by exchange with 

 potassium and such ions, and to compen- 

 sate for the carbon dioxide which is liber- 

 ated. From a calcium nitrate solution, 

 for instance, nitrate ions are thus absorbed. 

 They are not used apparently in the forma- 

 tion of the cell wall, so that they must be 

 conducted into the interior of the root. 

 But nitrate ions cannot travel alone. 

 Neither can they be accompanied by the 

 calcium ions with which they enter, for 

 the calcium, as we have already seen, is 

 deposited as calcium pectate in the cell 

 wall, or as calcium oxalate in the vacuoles. 

 Apparently the thing that happens is that 

 in addition to carbohydrate being trans- 



ported outward in the root hair, there is 

 also a quantity of potassium oxalate and 

 perhaps likewise potassium pectate, which 

 meet the calcium nitrate entering the cell 

 and undergo exchange with it so that the 

 calcium pectate or calcium oxalate remains 

 in the hair and the potassium and ni- 

 rate ions travel back into the cell proper. 

 There is thus a migration of carbohydrate 

 molecules and ions of organic potassium or 

 magnesium salts outward and of ions of 

 inorganic potassium or magnesium salts 

 inward. Thus we see that the root hair 

 is a region of considerable physical and 

 chemical activity, and that the streaming 

 of the protoplasm greatly facilitates 

 these changes. 



As to the effect of the presence of the 

 nucleus in a root hair upon the streaming, 

 it is evident that its location part way 

 along the hair, will modify markedly the 

 paths of streaming. It usually lies, as 

 shown below, surrounded by cytoplasm 

 between two vacuoles. Its size is nearly 

 such as to completely partition the interior 

 of the root hair. There would thus be a 

 tendency for the material to stream out to 

 the cytoplasm surrounding the nucleus 

 and then be diverted back along the oppo- 

 site wall to the cell proper again without 

 passing to the dome-shaped tip at all. 

 Thus free movement of materials from the 

 cell proper to the tip and back may be 

 interrupted partially or almost completely. 

 Unfortunately in the hairs in which the 

 nuclear migrations and cessation of 

 growth have been observed, as noted 

 above, streaming of cytoplasm cannot be 

 seen, so that confirmation of this interpre- 

 tation by direct observation has not as 

 yet been made. It is thus seen that there 

 is at least a possible explanation as to 

 how the nucleus may operate as a passive 

 causal agent in the retardation or cessation 

 of growth of these root hairs. 



The reason why the nucleus should move 



