Hooker.—Hydrotropism m Hoots of Ltipinus albus . 281 
for one hour to a saturated atmosphere averaged 5-25 mm. for five hours in 
moist sawdust. 
These results show that an increase in the osmotic pressure does 
not stimulate growth directly. Although the vital factor may not be 
explained so simply, it may yet be stimulated to action by the difference in 
osmotic pressure. As long as one side of the root has a higher osmotic 
pressure, there must be a flow of water across the root brought about 
by diffusion from cell to cell. In each cell there would exist a difference 
in the osmotic pressure of its two sides, as long as the hydrotropic stimulus 
lasted. This disturbance of the equilibrium within the cell may be the 
direct stimulus perceived by the cell as a unit, which produces the differen¬ 
tial growth of the opposite sides of the root. The means by which this 
reaction is carried out are just as mysterious as they are in phototropism 
and geotropism. It was shown that the root-tip is more sensitive than the 
rest of the root. This may be connected with the absence of the vacuole in 
the embryonic cells of the growing point. Their presence would facilitate 
the establishment of an equilibrium and diminish the effect of a difference 
in osmotic pressure. 
Miss Eckerson ( 11 ) has found that, when roots bend after exposure to 
a difference of temperature on their opposite sides, the cells of the concave 
side are more permeable than those of the convex side. From this she 
concludes that heat affects the permeability directly, and that the con¬ 
sequent turgor change offers a mechanical explanation of the curvature. 
Pfeffer 2 has shown, however, that temperature can never exercise any 
marked direct effect on turgidity. Marked alteration taking place in either 
the osmotic pressure or in the diosmotic properties of the protoplast must 
be a reaction on the part of the cell to a stimulus, since such changes are 
regulated by the vital activity of the organism. Moreover, there is an 
exact parallel between the condition found by Miss Eckerson and that 
resulting from exposure to a hydrotropic stimulus. The difference of 
permeability would occasion a flow of water across the root from the con¬ 
cave to the convex side, and a disturbance of the equilibrium within the 
cells would thus be effected in exactly the same way as by the difference 
of osmotic pressure in hydrotropically stimulated roots. Therefore the 
resulting bending would be a reaction to a stimulus identical with that 
occasioning hydrotropic reactions, and not a mechanical curvature resulting 
from differences of turgidity. Consequently, so-called thermotropic reactions 
are largely due to the vital factor discussed in this paper. On account of 
the environmental conditions to which roots are exposed in thermotropic 
experiments, the mechanical factor mentioned above cannot play a part. 
Since it seemed probable that hydrotropism was in the last analysis 
osmotropism, the following experiment was made. Two solutions of 1*15 
1 Pfeffer’s Physiology of Plants, vol. i, p. 138. Second English edition. Oxford, 1914. 
