6 BOTANICAL GAZETTE [JANUARY 
definite time, alternately toward the convex and concave sides. 
They always bent farther toward the concave side, and the con 
clusion was drawn that the membranes were more extensible 
on the convex side. 
Noll sometimes found a shortening of the concave side of 
curving stems of Hippuris of 6-10 per cent. He quotes some 
figures from Sachs which show a shortening of 10-25 per cent. 
in nodes of maize (48, p. 528). He does not believe in the 
migration of protoplasm required by Wortmann’s theory, nor did 
the latter offer sufficient proof of it. In plasmolyzing experi- 
ments Noll discovered at the beginning of plasmolysis a slight 
increase of the curve, followed only later by the straightening 
which de Vries had found. This increase of the curve on 
plasmolyzing does not take place after the curve is complete. 
Kohl confirmed these observations (33, p. 70). Ina later paper 
(50, p. 84) Noll admits that his theory of the curvature does not 
fully explain this plus curve, 
Both Wortmann and Noll seemed to start out with the hope 
of showing that the factors which produce the curvature operate 
on only one side of the curving organ, Wortmann finding them 
on the concave, Noll on the convex side. Both men, however, 
were compelled either by their own discoveries or by their critics 
to admit at least secondary changes on the other side. 
In 1894 another theory of the mechanism of growth-curva- 
tures was presented by Kohl (33) Kt may be called the con- 
traction theory, and is Supposed to account for the shortening of 
the concave side of curving organs, which had been reported 
previously by a number of observers. Asa Starting point, Kohl 
sooner than in those on the 
concave. Healso found by microscopic examination that there 
bits 
ie 
ss a Re ae 
