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THE QUARTERLY REVIEW OF BIOLOGY 



from the cell proper into that hair of the 

 duplex which is growing more rapidly, 

 and should migrate from a slow growing 

 hair to a more rapidly growing one may, 

 perhaps, also be associated with proto- 

 plasmic streaming. The nucleus is known 

 to move in other cells having protoplas- 

 mic movement, such as leaf cells of Elodea 

 and Vallisneria. Whether it is carried 

 along passively, or whether it is activated 

 by the same principle that activates the 

 plastids and cytoplasm is not yet known. 

 Sokolowa, after a careful study of nuclear 

 position and protoplasmic streaming, 

 decided that they are interrelated. He 

 thinks that there is an exchange between 

 the nucleus and the stream. 



Protoplasmic streaming in root hairs 

 has been observed for many years. It is 

 especially evident in the large root hairs of 

 such aquatics as Hydrocharis, Trianea, and 

 Limnobium. Here it is so rapid that under 

 high magnification it takes on the appear- 

 ance of the surging of the surf. Reinhardt 

 (65) noted that upon immersing hairs to 

 coat them with red lead, growth ceased. 

 He also noted that protoplasmic streaming 

 stopped. After a time both the streaming 

 and the growth were resumed approxi- 

 mately simultaneously, which indicates, 

 as suggested above, that streaming does 

 facilitate, at least, if not condition, root 

 hair elongation. In the small root hairs 

 of most seedlings protoplasmic streaming 

 is not usually apparent. Mrs. Farr (2.5) 

 has reported it to be conspicuous in oats, 

 but it is not evident in collards, rice, and 

 similar root hairs, owing possibly to its 

 absence, but also perhaps to the greater 

 degree of refraction of the more highly 

 curved surface of the smaller hairs, or to 

 the absence of particles in the protoplasm. 

 That protoplasmic streaming does occur 

 in these hairs is indicated by the rather 

 rapid change in the arrangement of the 

 vacuoles. 



The vacuolar system of root hairs has 

 not been extensively studied. Mer first 

 (1880) referred to them. Miss Addoms 

 (1) describes the development of a root 

 hair as regards its vacuoles as follows: 



In a very young root hair the protoplasm is dense 

 and almost devoid of vacuoles. As the root hair 

 grows, the protoplasm becomes less dense, vacuoles 

 form and enlarge, and the cell is apparently at the 

 height of its usefulness as an absorbing organ. The 

 vacuoles continue to enlarge and begin to coalesce, 

 and the protoplasm is crowded more and more toward 

 the outside of the cell, so that finally it is but a thin 

 film separating the cell sap from the cell wall, and the 

 root hair is of little value to the plant. 



Her interpretation of the efficiency with 

 which root hairs function as organs of 

 absorption, based upon the intracellular 

 organization, is in harmony with the 

 experimental evidence of Popesco (64), 

 which indicates that the older hairs, that 1 

 is, the longer ones, do not function in : 

 absorption. 



Strugger (78) finds in barley, as does the ;■ 

 writer (zi) in collards, that the most I 

 typical arrangement of vacuoles consists in 1 

 a terminal vacuole between the proto- 

 plasm surrounding the nucleus and that : 

 in the dome-shaped tip (fig. 2. (a)), and a 1 

 basal vacuole, which is an extension of the : 

 vacuole of the cell proper up into the root : 

 hair to the vicinity of the nucleus. Strug- ■ 

 ger finds, however, that if the hydrogen 1 

 ion concentration of the medium is in- • 

 creased slightly, for instance, from pH 

 6.6 to 6.3, the apical vacuole disappears. 

 If it is increased still more, that is to 6.2., 

 many small vacuoles appear. If changed 

 to pH 6.x secondary vacuoles disappear. 

 A change now to slightly greater acidity, 

 pH 6.0, gives the normal condition again. 

 By increasing the acidity farther the cycle 

 is repeated once, reaching a lethal acidity 

 at pH 5.6. He finds a similar bimodal 

 curve for streaming with a maximum rate 

 at pH 6.15. This indicates a remarkably 



