1905] SNOW—DEVELOPMENT OF ROOT HAIRS 37 
examine a root grown in air, we find the cells shorter and thicker, 
but not equally so, the outer ones, in all but a few cases to be con- 
sidered later, being longer and thinner than those near the central 
cylinder, showing that the former are stretching more strongly than 
the latter. Fig. 6 represents a section of such a root, grown in the 
same experiment as that from which jig. 5 was taken. Tested with 
KNO, solution, the outer cells of an air root were plasmolyzed in 
o.2 N, while the inner cells showed no shrinking, thus indicating that 
the latter had more concentrated cell-sap and less water. In water 
roots the epidermal cells were plasmolyzed in o.2 N and the cortical 
cells shrank, but the protoplasm did not leave the walls. PFEFFER 
(66, p. 301) reports the turgor of the cortex cells of corn roots in air 
to be greater than that of the epidermis. In the air roots the epi- 
dermal cells seem to have more water, and to be able to stretch more 
than the inner ones. This can take place to a certain extent, but 
the inner cells cannot keep pace with them, but hold back the epi- 
dermal cells from their full elongation, and the growth energy finds 
an outlet in the direction of least resistance, i. e. the free outer wall. 
A similar occurrence is noted when Spirogyra is held in a plaster cast 
(PFEFFER 66, pp. 240, 385), or when Stichococcus is made fast at 
the ends (KLERCKER 30, pp. 94-5). 
This bulging takes place in corn near the tip of the root, while 
the cells are isodiametric, and nearly the whole wall curves at first 
(fig. 7), but with the continued stretching of the cell this primary 
bulge becomes a papilla. The lagging behind of the inner cells of 
the cortex during the elongation period allows this papilla to become 
a hair. It seems then that hairs represent the ratio between the 
capacity of the epidermal cells to elongate and their ability to do 
so. If the capacity be the greater, the hairs will be produced; if 
equal to or less than the ability to elongate, no hairs will be developed. 
This would limit the statement of ScHWARz (75, p- 155)—‘‘bei dem 
Maximum der Wachsthumsgeschwindigkeit und unter den giinstig- 
sten Bedingungen bildet die Wurzel die zahlreichsten Haar”—to the 
epidermal cells. 
Testing this explanation in the different cases reported here, I 
suppose first, in the corn roots growing with diminished oxygen 
supply, that the growth of the epidermal cells is decreased. The 
