38 BOTANICAL GAZETTE [yULY 
inner cells on the other hand may obtain oxygen from the aerial parts, 
and thus with less moisture be able to keep pace with the epidermal 
cells growing with more moisture and less oxygen. In ordinary air 
the moisture and the oxygen reach the epidermal cells more abundantly 
than the inner ones, consequently the numerator of the fraction is 
increased as well as the denominator decreased, and hairs are devel- 
oped. Upon the upper side of a corn root growing along the surface 
of water abundant hairs were developed, while the under side 
remained smooth. The difference in length between the epidermal 
cells and those of the cortex on the haired side was 20m, and on the 
smooth side 64. Kraus’s tables (37, p. 254) dealing with the lengths 
of epidermal and cortical cells in relation to hair production are not 
very complete, and it seems useless to attempt to harmonize the 
results with those here reported. 
KRABBE (33, Pp. 491) reports the inner cells of pith to be less 
turgescent than the outer ones when placed in water at 1-2°C, on 
account of the resistance to the passage of water offered by the proto- 
plasts. According to VAN RYSSELBERGHE (70, p- 103) the influence 
of temperature is exhibited not in the total amount of water taken up, 
but in the rapidity of its passage. In warm water, therefore, the water 
reaches the inner cells and allows them to elongate sufficiently rapidks 
to keep pace with the epidermis, which is thus allowed to elongate 
to its full capacity and shows no hairs. __ 
In the zone of hairs on seedlings in water cultures the available 
energy and the temporary retardation of growth (evidenced by the 
short outer and still shorter inner cells, and by the curling of many 
roots) combine to produce hairs. Also the presence of food may 
act as a stimulus to cause the cells to divide rapidly and form a thick 
_ Toot, whose inner cells do not get sufficient water, or oxygen, or 
both, to allow them to elongate as rapidly as the outer ones. Later, 
in the case of corn, the plumule elongates and probably supplies 
the inner cells with more oxygen. These are therefore better able 
to elongate, they are carried further from the food supply, 
is less active, the roots grow more slende 
inner cells increases, still greater elonga 
epidermal cells are allowed to stretch to t 
modation to a decrease of oxygen is m 
division 
tion takes place, and the 
heir full capacity. Accom- 
entioned by PFEFFER (65, 
ae 
oe Bea eae de 
