THE EPIDERMAL CELLS OF ROOTS 
CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY 221 
EpitH 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. 
MotsturE.—PERSECKE (16), working with Zea mais and 
Pisum 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. 
Licut.—The effect of light upon cells in general was investi- 
gated by Kraus (14), who found that darkness increased the 
length of cells. Drvaux (2) found the same and that this favored 
Botanical Gazette, vol. 62] [488 
