a 
1900] MECHANISM OF ROOT CURVATURE 7 
was a larger amount of copper-reducing substance in the cells of 
the concave side. Unable to accept either Wortmann’s or Noll’s 
views as to the cause of the curvature, Kohl elaborated the theory 
that the curvature is caused by an active contraction on the con- 
cave side, with a corresponding passive stretching on the convex 
side. The possibility of this contraction depends upon the fact, 
which can be mathematically demonstrated, that an oblong cell 
may increase in volume, and at the same time shorten its long 
diameter. Of course, the transverse diameter must increase, 
and the cell become more nearly spherical. The force that 
produces this shortening is the increased turgor of the cell, . 
which, beyond a certain point, makes the cell shorter and broader, 
or, as Kohl expresses it, barrel shaped. It is at once seen that 
a condition necessary to the contraction of an organ in this man- 
ner is that the cells must lie with their long diameter parallel to 
the long axis of the organ. This condition Kohl himself seems 
to have overlooked, and it will help us to determine whether his 
theory is applicable to given cases of curvature. 
In discussing the theoretical possibility of producing a con- 
traction by increased turgor Kohl attempts to show that the 
conditions are favorable to it in stems where there are large 
intercellular spaces. Here we have what he calls alternately 
single and double membranes, the latter where two cells are in 
direct contact, the former where the cells border on intercellular 
spaces. To make this more clear, he compares this condition to 
an India rubber cell (33, p. 8) on the outside of which thicker 
strips have been fastened longitudinally. Now, if the India 
rubber cell is filled with water under pressure, it will stretch 
more in the transverse than in the longitudinal direction, because 
of the thicker strips in the latter direction. Here again Kohl 
overlooked a condition which must exist in organs with inter- 
cellular spaces, namely, that where the membrane is double the 
stretching force is doubled also, for a cell on each side of the 
membrane is stretching it in a longitudinal direction. Kohl prob- 
ably got his idea of the contraction theory from some experi- 
ments of de Vries (74) in which he found that many roots, 
