- 



THE EPIDERMAL CELLS OF ROOTS 

 CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY 221 



Edith Adelaide Roberts 



(with seventeen figures) 

 Introduction 



The epidermal cells of roots have received more or less atten- 

 tion, especially those epidermal cells which form root hairs. The 

 lines along which investigations have been conducted may be 

 grouped readily under investigations made when the root is in 

 an air medium and those made when the root is in a liquid medium. 

 The factors in the air medium which have been discussed are 

 moisture, light, temperature, contact, length of cells, mode of 

 succession, position of nucleus, osmotic pressure, membranes, and 

 food supply. The factors in the liquid medium discussed are 

 calcium nitrate, potassium nitrate, salt, and bog conditions. 



Moisture. — Persecke (16), working with Zea mats and 

 Pisurn sativum, states that the root hair development depends 

 upon the amount of air and water in the interstices of the soil. 

 Schwarz (20), using the same forms, comes to the conclusion 

 that there is a minimum of moisture at which the hair formation 

 begins, an optimum in which the best development is obtained, 

 and a maximum where the hair development nearly or entirely 

 ceases. Pfeffer (18) and others attribute more importance to 

 moisture than to light as a factor in hair development. The con- 

 clusion from these observations is that moisture is a factor in the 

 determination of the formation or non-formation of an epidermal 

 cell of a root into an outgrowth called a root hair, but how or why 

 this is so receives no consideration, nor do the limits of the amount 

 of moisture necessary to become a limiting factor. 



Light. — The effect of light upon cells in general was investi- 

 gated by Kraus (14), who found that darkness increased the 

 length of cells. Devaux (2) found the same and that this favored 

 Botanical Gazette, vol. 62] [488 



