1905] SNOW—DEVELOPMENT OF ROOT HAIRS 41 
cases occurred in which the central cylinder was torn apart at regular — 
intervals by the stretching cortex, the epidermis bearing no hairs. 
The food supply seemed not enough to give the cells of the central 
cylinder sufficient strength to retard the stretching of the outer cells. 
No change in turgor is needed to explain the appearance of root 
hairs, for according to PFEFFER (647, p. 29) there is no change when 
growth is accelerated by a rise of temperature or by absence of light, or 
when growth is retarded by lack of oxygen or (66, p. 296) by pressure. 
In the first three cases hairs disappeared or were diminished, while 
in the last they appeared. 
An interesting relation was noticed between the epidermal and 
the hypodermal cells of some corn roots. In roots growing in the 
air and producing hair, the nuclei of the hypodermal cells were 
usually larger than those of the epidermal or cortical cells (jig. 9). 
This may indicate that the hypodermal cells were passing food to 
the outer cells, the starting of the lateral growth thus initiating a 
movement of material in that direction. SAUVAGEAU (73, Pp. 171) 
reports small hypodermal cells under the piliferous cells in Zostera. 
This demand for food by the outer layer would decrease the supply 
in the central cylinder and may account for the inverse relation 
between root hairs and lateral roots, noted by LESAGE (42, p. 110), 
COSTANTIN (9, p. 149), MER (52, p. 666; 53, p. 1278), SACHS (71, 
p- 589), e¢ al. In Eichhornia the lateral roots extend nearly to the 
tip, but there are no root hairs. This activity of the central cylinder, 
contrasted with that of the epidermis, is in harmony with the results 
of the experiments here reported. 
In spite of the structural and functional similarity which often 
exists between root hairs and rhizoids, it does not seem appropriate 
to consider them together. In the first place, they are not morpho- 
logically similar, rhizoids being of gametophytic origin and root hairs 
developing from the sporophyte. The fact that rhizoids arise usually 
from a rather small gametophyte, all the cells of which retain in large 
measure their primitive condition, may account for the irritability 
they display toward geotropic, phototropic, and thigmotropic stimuli. 
Root hairs, on the other hand, are developed on a highly differentiated 
organ of a highly differentiated sporophyte, and are not thus sen- 
Sitive, a difference pointed out by HABERLANDT (22, pp. 194-5)- 
