48 BOTANICAL GAZETTE [JANUARY 
1. The transmission of the stimulus from the sensitive tip to ‘: 
that part of the root which curves takes place in the cortex. It 
may take place in the cortex of either the side that becomes 
concave or that which becomes convex, and is not restricted to 
the side on which the root tip is wounded. 
While it is not absolutely demonstrated that transmission 
cannot take place in the axial cylinder tiie evidence is strongly 
against such an assumption. 
2. The stimulus may be transmitted in the transverse or tan- 
gential direction as well as in the longitudinal direction.‘ 
3. The mechanism of root curvature, as in curvature of stems, 
is closely connected with the tissue tensions existing in the 
organ previously to stimulation. 
4. The ordinary tissue tension of roots is the reverse of that 
in stems. That is, the outer cortical parenchyma is under posi- 
tive tension, the central cylinder under negative tension. Three 
lines of evidence show this to be true; first, the rupturing of the 
central tissue in rapidly growing roots ; second, the curving in 
of split roots with the cut face concave ; third, the difference in 
length of the cortex and axial cylinder when separated from 
each other. It is not uncommon in Vicia faba roots for this dif- 
ference to be as great as 10 per cent. of the length of the 
separated tissues, when the separation is for 1° back from 
the tip. 
5. Stimulation causes an increase of the tension between the 
axial cylinder and the cortex on the side which becomes convex, 
and decreases or reverses the tension between the axial cylinder 
and the cortex on the side that becomes concave. The shorten- 
ing of the concave side may be merely relative or there may be 
an absolute shortening as has been shown by Kohl and myself. 
That the shortening is not due merely to compression by the 
more rapid extension of the cortex on the convex side is proven 
by the behavior of split roots that are laid in horizontal position 
‘A recent article by Ewart (20) shows that in the climbing hooks of various trop- 
ical plants the stimulus is transmitted in a transverse or tangential direction from the 
concave side, which alone is sensitive to contact, pressure, and injury, to the cambium 
which is incited to increased growth. 
