1916] ROBERTS— EPIDERMAL CELLS OF ROOTS 480 



the development of root hairs on corn. On the other hand, Ewart 

 (5), in the formation of root hairs on the root tendrils of Vanilla, 

 found darkness accelerating and light retarding their formation. 

 In this work, however, the moisture factor was not eliminated. 

 Pethybredge (17) found that light retarded the formation of hairs 

 upon oat and corn roots grown in water cultures. Schwarz (20) 

 found that light and darkness had no effect on the root hair forma- 

 tion. Snow (21) states that the effect of light and darkness, if 

 any, is indirect. Light, then, as a factor has been experimented 

 with to some extent with varying results, possibly owing to the 

 fact that it has been associated with other factors. 



Temperature. — No work on temperature as a single factor 

 has been carried on. Schwarz (20) found that a temperature of 

 2 7-28°C. did not overcome the inhibitory effect of water as the 

 roots grew smooth. Snow (21), working with high temperatures 

 plus moisture, found a decreased hair production brought about 

 by the increasing elongation of the internal cells. 



Contact. — The effect of contact has received some attention. 

 Schwarz (20) observed that water roots upon entering the sub- 

 stratum develop hairs, and that when the soil is saturated the hairs 

 on corn seedlings disappear, although soil particles are still present, 

 and suggests that it may be due to chemical stimuli or retardation 

 of growth, since it could not be due to contact. Pfeffer (18) 

 denies that contact is a factor, for he found that on climbing roots 

 hairs were produced on the side near the support where there is 

 the greater moisture. Snow (21) grew corn seedlings between 

 plates and found no hairs on the sides touching the plates, while 

 there were hairs on the other two sides; but again moisture may 

 be the limiting factor and not contact. 



Jtjel (11) associated short cells and root hairs, as also did Van 

 Tieghem (25) and Kraemer (13), and he finds that short cells 

 remain short if they do not form root hairs. Leavitt (15) finds 

 two types of potential root hair cells. The first is that in which 

 any cell of the outer layer may acquire the character of a root hair 

 (trichome) by putting out a hair. This is characteristic of the 

 dicotyledons, of some divisions of the monocotyledons, and of the 

 Filicales. The second type is that in which they originate as 



