1900] MECHANISM OF ROOT CURVATURE < 
measurement of cells, that in geotropically curved roots the con- 
vex side grows only a little faster than the normal root, while 
the strong curve is produced by the much reduced growth of 
the under side (62, p. 463). In another paper (63) he used 
stems of several dicotyledonous plants of different families, and, 
after separating different tissues of stems curved geotropically, 
he found that the concave cortex grows less and the convex 
cortex more than the corresponding parts in normal stems. 
Also the pith grows faster than cortex, and the relation of pith 
to cortex is such that in curved stems the tension is increased 
on the concave side and diminished on the convex side. 
An interesting paper by Ciesielski (5) furnishes some valuable 
experiments in connection with root curvature, though his explana- 
tion of the curve is of no value. He was the first to assert that 
the removal of the root tip prevented the root from curving as 
usual when it was placed in a horizontal position. He cut off the 
cortex of roots on one side down to the axial strand, and when 
this side was turned downward with the root laid in a horizontal 
position the curve formed more rapidly and its radius was smaller 
than in the normal root under the same conditions. If sucha 
root is laid with the cut face upward, in the majority of cases it 
curves downward likewise, though more slowly. Sometimes it 
curves upward at first and only afterward does it curve downward 
(5,p- 19). The significance of these experiments will be discussed 
later. 
Kraus (35) investigated juice pressed out from the separated 
convex and concave halves of curving stems, and found, contrary 
to the theory of de Vries, that osmotic substances diminished | 
both relatively and absolutely on the convex side during the prog- 
ress of the curve. Hence the curve cannot be produced by an 
increase of such substances on that side. 
Charles Darwin (9, 10) gave a new impetus to investigation 
of curvatures produced by stimuli. His conclusion that the root 
tip performs a function similar to that of the brain in lower 
animals (10, p- 573) called out a great number of articles both 
for and against it, and the controversy only ended in 1894 when 
