35^ 



THE QUARTERLY REVIEW OF BIOLOGY 



hairs. Leavitt (46) reported in 1904 

 that in the grasses every cell may produce 

 a hair. In Alalia (45), a water fern, he 

 finds that root hairs arise only from short 

 cells. This latter observation was also 

 made by Savageau (68) upon Naias and 

 by Kraemer (41), Van Tieghem (82.) and 

 Juel (37). Schwarz (73) and Miss Snow 

 (75), however, think that there is no 

 significance in the size of the mother 

 cells. Schwarz gives the diameter of root 

 hairs as varying from 0.011 mm. in corn 

 to 0.050 mm. in Trianea. He estimated 

 the number per square millimeter of root 

 surface to be 10.9 in Trianea and 4x5 in 

 corn, and the number per millimeter of 

 root length to be 94 in Trianea and 4386 in 

 Scindapsis. 



Haberlandt (Z9) in 1887 was the first to 

 show definitely that the root hair grows 

 in length by the formation of additional 

 wall only at the apex, although Nageli 

 in 1846 had included plant hairs along 

 with pollen tubes, fungous hyphae, and 

 algal filaments, as structures with apical 

 growth. 



As to the exact method of cell wall 

 elongation there developed some differ- 

 ence of opinion. Wortman (88) in 1889 

 contended that it was by the addition of 

 new layers on the inside of the wall at the 

 tip, and a stretching of the wall simul- 

 taneously by increased turgor of the 

 protoplast, keeping the wall at a constant 

 thickness, but increasing progressively 

 the volume of the cell. Zacharias (89) in 

 1891 obtained strong evidence that this is 

 not the method, but that new wall 

 material is inserted into the wall at inter- 

 vals along the dome shaped tip, thus 

 extending the wall, and that turgor of the 

 cell is not the active factor in cell enlarge- 

 ment. This idea has recently been sup- 

 ported by the work of Ursprung and Blum 

 (81) in 1911. 

 The most convincing evidence that the 



root hair does grow at the tip only is 

 given by the experiments of Reinhardt (65) 

 in 1891. He placed minute particles of 

 red lead on the tip of the hair and watched 

 their change of position as the hair grew. 

 He found that they might become sub- 

 divided on the dome-shaped tip and in- 

 variably moved off of the dome as the 

 hair grew, coming to rest on the side of 

 the hair at the base of the dome, and 

 remaining in contact with this part of the 

 wall, while there was progressively more 

 lateral wall added from the dome at the 

 end. 



Stiehr (77) in 1903 found several lines 

 of evidence which indicate that the wall of 

 the hair at the tip is different in composi- 

 tion from that of the rest of the hair. He 

 found that it is more readily stretched and 

 broken, as is shown by the fact that if the 

 root hair is caused to burst the rupture 

 occurs almost invariably at the tip. He 

 also noted the form of the tip when parti- 

 cles were appressed to it, and describes it 

 as appearing in this respect like a stick of 

 warm sealing wax. 



On the basis of his experiments on the 

 curvature of root hairs in response to cer- 

 tain stimuli, as described below, Seidel 

 (74) concluded in 1914 that the wall is 

 more plastic at the apex of the root hair 

 than at other points . This would perhaps 

 be expected if the apex is the place where 

 new wall material is being deposited. It 

 is also in harmony with the work of 

 Ziegenspek (90) in 19x0, in which he 

 found that the cell wall at the tip is of 

 different chemical composition from that 

 along the sides. He decided that it 

 consists of a substance, called amyloid, 

 which is a transitional carbohydrate, 

 having some of the properties of starch. 

 For instance, it turns blue with iodine, 

 as does starch; whereas for cellulose to 

 give this reaction acid must be added. 

 However it is not entirely certain that 



