ROOT HAIRS AND GROWTH 



361 



Micheels (57a) found root hair elongation 

 more rapid in nitrate than in chloride. 

 Hansteen (31), Coupin (5), Kuster (44), 

 Trelease (79), and the writer (14), all find 

 that calcium is apparently necessary for 

 root hair production. Hansteen has been 

 the most ardent and insistent advocate of 

 this view in repeated papers (31, 31). 

 Kisser (38) has, however, demonstrated 

 it in the most convincing manner. He 

 supported seedlings in a chamber in such 

 a way that no contact of the seedling was 

 made with substances containing calcium. 

 For this purpose he found it necessary to 

 use special calcium-free cloth and glass. 

 In this case the root grew, but no root 

 hairs were formed. If, however, the 

 slightest trace of calcium came in contact 

 with the root, root hairs emerged. Han- 

 steen (31) found that the calcium must be 

 absorbed from the exterior in order for 

 root hairs to appear. He grew seedlings 

 with one root in a calcium solution and 

 the other in a solution of some other salt, 

 and found root hairs only on the former. 

 Only one reference (49) is found in the 

 literature to root hairs developing in 

 solutions of single salts other than those 

 of calcium, and in this case it is not un- 

 likely that impurities may have been 

 present. Both Kisser and Hansteen found 

 that magnesium, potassium, and pure 

 water suppress development of root hairs. 

 The writer has confirmed these findings 

 for pure water. The significant feature 

 with regard to calcium in relation to root 

 hairs seems to be that it is utilized directly 

 in cell wall formation, and is in fact, the 

 only constituent of the cell wall which is 

 not synthesized within the plant. 



Sachs (67) was the first to attempt an 

 explanation of the curving of root hairs 

 around soil particles. He attributed it 

 to thigmotropism, that is, a response to 

 contact with soil particles. Schwarz 

 (73) concurred in this interpretation and 



noted that such curvatures do not occur 

 in water or in saturated air, where no 

 solid bodies come in contact with root 

 hairs. Jost and Pfeffer, the leading plant 

 physiologists of the last part of the last 

 century, accepted this interpretation also. 

 No further studies were, however, made 

 until very recently, when by a careful and 

 thorough investigation Kurt Seidel (74) 

 found that this interpretation is incorrect, 

 and that the curving of root hairs in soil 

 is a chemotropism, and not a thigmo- 

 tropism. 



Seidel noted that root hairs of lamb's 

 quarters, Cheno-podium, curve toward cast- 

 off pieces of the root cap of the same plant. 

 He then used various parts of this and 

 other plants, fresh and decomposed, also 

 various organic and inorganic compounds, 

 and finally purified quartz crystals alone. 

 He found no response at all to the latter. 

 He did find that Cheno-podium gives a 

 positive curvature toward all soluble 

 phosphates, while salts of other simple 

 cations, sodium, iron, potassium, calcium, 

 magnesium, ammonium, etc., gave no 

 effect. Neither oxygen nor carbon diox- 

 ide brings it about; neither does acidity or 

 alkalinity. The minimum threshold con- 

 centration for potassium acid phosphate is 

 0.00x5 to 0.0040 per cent. The maximum 

 was about z per cent, in which case 33 

 per cent of the hairs reacted, and the 

 others appeared injured. Seidel demon- 

 strated by a number of ingenious experi- 

 ments that the normal direction of root 

 hairs at right angles to the root is not 

 a matter of mutual repulsion or attraction. 

 When he fitted a glass sleeve over the root, 

 for instance, the hairs grew out normally 

 above and below it without curving. He 

 also observed that the root hairs on the 

 concave side of a curved root are straight 

 and cross each other, showing no mutual 

 interaction. 



With regard to other species, he found 



