94 
Effect of Tension on Plants. 
(Eig. 8). It is worthy of mention that when this cell division occurs, 
the starch sheath which surrounds the bast bundle disappears. 
If the thread be fastened close to the cotyledons and the plant be bent 
towards one side only, there follows always a curving of the younger 
upper part in a direction opposite to the mechanical bending. This is 
apparently a geotropic curving, since at every beat the plant is subjected 
to gravitational influences. When the plant is bent in both directions 
there follows a thickening of tissues on both sides, but no difference 
could be seen in the flanks, all portions being alike affected. 
Knight, 10 an English investigator, working in 1801-02, fastened young 
apple trees in such manner that the wind could bend them only in the 
H.-S. direction. After about two years years he found that the diameter 
of the wood in the H.-S. direction to that of the E.-W. was as 13 to 11. 
However, Hartig, * 11 observed an eccentric thickening of the yearly 
rings of pines which had been subjected to a constant West wind. The 
cells were also thicker walled on the East side. 
On the other hand, Wiesner has shown that on the side limbs of plants 
gravitation sometimes causes an epitrophy, sometimes a hypertrophy. 
XI. In order to observe the effect of one sided light, young plants 
were placed in a dark chamber into which light was .admitted through a 
narrow slit. As is known to all, plants in such position will bend towards 
the source of light. If this heliotropic curving be prevented, e. g., by 
weighting the upper end with just enough force to prevent the curving 
and not to exercise any great degree of tension, hypertrophy of the 
tissues on the side exposed to the light will be found to occur. When 
one takes into account the partial etiolation that must result in this 
case the thickening is as noteworthy as in the other cases. 
Here, however, there is no geotropic force at work, although a degree 
of tension exists among the tissues as is shown from the fact that when 
the plant is released there follows an instant curving toward the source 
of light. 
DISCUSSION OF THE RESULTS. 
From the facts given it follows that simple tension applied in the line 
of growth will not produce alterations in the tensile strength of plants 
nor any changes in their tissues. A general increase of the strength of 
the plant occurred during the time of investigation as a natural result 
of thickening of cell walls or of growth in cross section, or of both factors 
combined. 
10 Knight, Phil. Trans., 1803, II, p. 280; 1811, p. 217. 
11 Hartig, Wachtumsunter suchungen an Fichten. Forstl. Naturwiss. Zeit- 
schr. z. Haft., 1896. 
