96 
Effect of Tension on Plants. 
strength to be observed; this would suggest that there was no develop- 
ment in the opposite side, which is an observed fact. Perhaps an arti- 
ficial thickening of the tissues on both sides would cause an increase in 
the tensile strength. 
As has already been shown, one-sided thickening may be produced by 
different causes. In the one case, this occurs through mechanical bend- 
ing when gravitational influences have been eliminated, as upon the 
klinostat, as Elfving has already shown. Again, it occurs when geotro- 
pic curving is prevented by tension or mechanical obstruction; in this 
case, on the upper side. The same result is attained when a helio tropic 
curving is prevented and also when the object was immovably fixed in a 
plaster of Paris cast and subject only to the action of gravitation. It is 
possible that the prevention of any tropistic curving will produce like 
results. 
In view of the fact that longitudinal stress, without curving, has no 
effect, we may infer that the tension of mechanical bending can not of 
itself act as a stimulant through which cell thickenings on the convex 
side are produced. This does not exclude the possibility, however, that 
differences in tension between the opposed flanks may act as a stimulus. 
Irritations are also produced by differences of light supply which can not 
be measured by the results of the action of diffuse light. Likewise, it is 
also possible that in some way affecting the supply of food products, or 
other like changes may act as stimuli. 
On the other hand, one-sided thickening of the cell walls are pro- 
duced by gravitation, and indeed, most noticeably when no geotropic 
curving occurs. If the latter takes place, there follows a reduction of 
the one-sided hypertrophy, which appears, at least to a certain extent, 
upon the upper or concave side, while the cell walls of the under or con- 
vex side* may undergo a certain reduction. 
Although in the case of the negatively geotropic curving portion, a 
mechanical tension is exercised upon the antagonistic upper side by pre- 
vention of curving in the lower, which strives constantly to increase in 
length, it must, from the same considerations which were developed in 
the discussion of the mechanical curving, appear doubtful if tension is 
the active agent. Directly supporting this view is the fact that in well 
grown objects, in which any such strain on the part of the lower side 
must be at a minimum, a thickening occurred on the upper, and further 
that this effect was also produced ii^ stems which had been enclosed in 
plaster of Paris casts where all stress from the under side was excluded. 
A satisfactory causal explanation is, for the time being, as with other 
irritation processes, impossible. This is likewise true for the epitrophic 
and hypertrophic stimulations of growth in many plants which are due 
to the stimulus of gravitation. 
