ROOT HAIRS AND GROWTH 



359 



5.8 per cent sodium chloride, whereas 

 ordinary plants growing in cultivated 

 soils cannot resist more than 1.5 per cent. 



REACTIONS OF ROOT HAIRS TO EXTERNAL 

 CONDITIONS 



Root hairs are found to respond to exter- 

 nal conditions in various ways. In the 

 first place certain conditions will permit 

 their development, while they will not 

 arise at all under others. Under some 

 conditions they burst; under others their 

 protoplasm is coagulated. They also are 

 found to curve in response to certain 

 stimuli, and to change their form by 

 varying their diameter or direction of 

 growth. Finally they grow at different 

 rates under different conditions. 



Pfeffer (63) was the first to observe the 

 bursting of root hairs by hypotonic 

 solutions. 



Zacharias (89) in 1891 found that the 

 cell wall might burst and growth be 

 continued by the formation of a new inner 

 membrane. Klemm (39) studied the 

 bursting and contraction of cells under 

 various conditions, including observations 

 on the root hairs of Trianea. He found 

 that the protoplasm does not contract 

 with high temperature, but in young 

 hairs it does contract at low tempera- 

 tures. He found that low concentrations 

 of acids have an explosive effect on these 

 hairs. Stiehr (77) observed bursting in 

 root hairs of timothy when placed in 

 0.15 per cent to 1 per cent solutions of 

 several common salts. 



Klemm (39) observed coagulation of 

 protoplasm of hairs of Trianea in 0.1 M 

 nitric acid or oxalic acid. Miss Addoms 

 (1) has made the most extensive study of 

 various substances in bringing about the 

 coagulation of the protoplasm of root 

 hairs. She attributes it in part to high 

 degree of acidity of the solutions, ranging 

 as they did from pH 3.94 to pH 3.47, or to 



high concentration of the salts, using o. 1 M 

 solutions of potassium, sodium, calcium, 

 magnesium, zinc, and aluminum salts, 

 aluminates and cyanides. She has also 

 found that ultraviolet light may bring 

 about the coagulation of root hairs. 



The effect of light upon root hairs has 

 been considered, especially in connection 

 with the question of the proper conditions 

 for their production. Schwarz found no 

 effect (73). Constantin found that root 

 hairs develop more in darkness than in 

 light. Devaux (1888) found that roots 

 in water grew faster in the dark, but that 

 they bore few or no hairs, whereas roots 

 in the light grew less rapidly, and bore 

 many hairs. Later (1891) he found that in 

 some grasses, as Lolium, there was a daily 

 periodicity of root hair length in light, 

 so that each day's production formed a 

 cone or spindle shaped mass of root hairs. 

 This showed that not only are more root 

 hairs produced in light, but they attain a 

 greater length. Went's experiments were 

 not convincing. Pethyridge (6x) on the 

 other hand reports that light retards root 

 hairs of wheat and oats in water cultures. 

 Miss Snow (75) found little difference in 

 the number or length of root hairs in light 

 or darkness. Seidel (74) reports that only 

 in intense light is root hair growth in- 

 hibited. Jeffs (36) made a definite study 

 carefully measuring the elongation of root 

 hairs in saturated air using different in- 

 tensities of illumination. He found no 

 effect of light so long as temperature is 

 kept constant. It thus seems that much 

 of the earlier work indicating a definite 

 effect of light on root hair production and 

 growth is not supported by more recent 

 and more detailed observations. 



The first definite study of the effect of 

 temperature on root hair production was 

 made by Miss Snow (75). She found that 

 wheat produces numerous hairs between 

 4.5 and n.5°C., fewer between 16.0 and 



