ee ee ee 
1900] MECHANISM OF ROOT CURVATURE 13 
the correctness of MacDougal’s conclusion, I am forced by 
a careful examination of his results to the view that roots geotropi- 
cally stimulated shorten their cells on the concave side. This 
is shown even without taking his mechanically bent roots as the 
basis of comparison. Roots curved geotropically three hours 
through 60° have convex cells 43. 3° of the scale in length, 
concave cells 29.9°. Roots curved geotropically twenty hours 
through 105° have convex cells 48.05°, concave 24.7° of scale in 
length. Obviously, as the curve progresses, the cells of the con- 
cave side shorten 5.2° of the scale, the convex cells lengthen 
4.7°. Iam unable to explain the difference in the length of 
cells in the ‘‘normal” roots and those bent mechanically, as 
reported by MacDougal. So far as the paper makes it clear, 
they were supposed to be measured in corresponding regions. 
If the difference is due to individual variation it still vitiates 
MacDougal’s conclusions. If it is due to the fact that the cells 
measured were not in corresponding regions, then the measure- 
ments in ‘‘normal”’ roots do not correspond to those in roots 
curved geotropically, for the author states distinctly (30, p. 346) 
that ‘“The region of curvature artificially produced coincided 
with that of geotropically excited roots.” 
HISTORICAL SUMMARY. 
In summing up the historical part of the question under con- 
sideration it is easily seen that no theory of the mechanism of 
the curvature yet advanced is satisfactory for all cases, and that 
the very different mechanical relations of tissues in different 
organs capable of curving make it extremely improbable that 
a single simple explanation will apply to all growth-curvatures. 
The causes of curvature as conceived in the different theories 
may be classified somewhat artificially as follows : 
1. Difference in rate of growth on opposite sides of the organ. 
Sachs, Pfeffer (?). 
2. Activity of protoplasm. 
a. By increased formation of osmotic substances on the 
concave side, accompanied by contraction. Kohl, Loeb. 
