ROOT HAIRS AND GROWTH 



353 



in this case the amyloid is to be regarded 

 as an intermediate carbohydrate between 

 the soluble form in the cell sap and the 

 cellulose of the fully developed cell wall 

 along the sides of the hair. Miss Roberts 

 (66), it is true, in 191 6 reported that there 

 was an inner layer of cellulose along the 

 sides of the root hair, the outer layer of 

 the wall being of calcium pectate. How- 

 ever, in 19x1 Miss Howe (35) published 

 a further investigation of the composition 

 of the cell wall of root hairs in which she 

 found no cellulose at all. In this way the 

 root hair wall is strikingly different from 

 that of partition walls and other external 

 walls of the root proper. She also found 

 that the outer layer of the lateral walls of 

 the root hairs is of calcium pectate or 

 pectose, but that the inner layer is of cal- 

 lose, which Miss Roberts found only at 

 the tip of the root hairs of some species. 

 Considerable attention has been given 

 the question of the location of the nucleus 

 and its possible relation to root hair 

 elongation. Mer (53) first reported the 

 nucleus as moving along in the hair be- 

 hind its tip. Haberlandt (30) found that 

 in many plants it lies along the outer wall 

 of the superficial cell, near the location of 

 the bulge which developes into the root 

 hair. He pointed out that in wheat and 

 Commelina, while the nucleus does not lie 

 against the wall, it is connected with it 

 by numerous plasmatic threads. He also 

 found that the nucleus, except in Hydro- 

 charis, moves into the hair after it has 

 attained a short length, and that it remains 

 at a rather constant distance behind the 

 tip. This distance varies for different 

 plants. In peas it is about 13 microns and 

 in pumpkin about 130 microns. He con- 

 cluded from his study that the nucleus is 

 an active agent in the controlling of the 

 growth of root hairs. More recently 

 Haberlandt's student, Windel (87), has 

 added evidence to support his contention. 



He found that in mustard root hairs, the 

 growth is at first apical, then basal, the 

 nucleus moving accordingly. However, 

 Kuster (44) in 1907 published his study 

 upon the same subject, from which he 

 concluded that there is no relation be- 

 tween the position of the nucleus and the 

 growth of the hair. He believes it to be a 

 mere coincidence that the nucleus should 

 in some cases lie near to the point of origin 

 of the root hair and attaches no signifi- 

 cance to its presence in the hair. Miss 

 Roberts (66) confirms this view and finds 

 great variation in the position of the 

 nucleus in hairs which are apparently 

 behaving alike in their development. 



Ziegenspek (91) has very recently found 

 that the root hair of Hydrocharis does not 

 grow at the tip, but that the amyloid is 

 deposited intercalarly near the base, and 

 suggests as the explanation of Haber- 

 landt's observations that the nucleus in 

 this plant does not move into the hair. 

 Besides Hydrocharis Kuster noted seven 

 other genera of aquatics in which the nu- 

 cleus lies always in the cell proper, and 

 three genera in which it may lie in the 

 middle or lower part. 



By studying the same hair for a period of 

 time under normal living conditions the 

 writer has obtained evidence that the 

 nucleus not only may have no active part 

 in stimulating or controlling the growth 

 of root hairs but apparently may even 

 passively retard elongation, or bring about 

 the cessation of growth. It is, for in- 

 stance, found that the nucleus may enter 

 a hair and remain there for a short time 

 during which the hair does not grow so 

 rapidly; and then after it retreats to the 

 cell proper again, the hair may resume 

 its original rate of elongation. But the 

 more convincing evidence is obtained from 

 a study of so-called duplex hairs, that is 

 in cases where two hairs emerge from a 

 common base which is formed from a bulge 





