1900} MECHANISM OF ROOT CURVATURE 25 
4. The stimulus can also be transmitted transversely, or at 
least tangentially in the cortex. This follows from the fact that. 
the curve was formed when only a small part of the cortex was 
left either on the concave or convex side. The stimulus, after 
passing the cut on one side, still influenced the cells on the 
opposite side to aid the curve.3 This is a fact which every theory 
of curvature must take into consideration. 
5. The experiments in table II disprove Wiesner’s theory of 
the traumatropic curvature of roots. He says decapitation of 
the root tip accelerates growth in the region of the curve by 
making the walls more ductile. Injury to one side of the tip 
increases the ductility of the membranes on that side of the 
root; hence it grows faster than the other and the curve results 
(78, p. 268). If Wiesner’s view were correct the gash on the 
same side of the root as the brand would prevent any curve above 
the gash; since neither the stimulus nor any tensile strain could 
be transmitted across the cut. As the root does curve above the 
gash Wiesner’s view cannot be correct. The process is more 
complicated than he supposed. 
6. These results seem to show that both sides are concerned 
in the formation of the curvature, certainly to the extent of con- 
ducting the stimulus, and if to that extent probably both sides 
are also concerned in the changes in the cells that produce the 
curve directly. This point will be discussed more fully later. 
TISSUE TENSIONS. 
It has long been known that the pith of dicotyledonous stems 
grows faster than the cortex, and that as a consequence there is 
considerable tension between these tissues. The pith is under 
positive, the cortex under negative tension, so that when the stem 
is split in the growing part the two halves bend outward, the 
cortical side becoming concave, the pith side convex. In the 
normal stem the tensions between the pith and the cortex of any 
3 Czapek (7) has recently shown, by the same method that I have sige that the 
stimulus can be transmitted in a transverse direction in roots. A résumé of my own 
experiments was first published in the ohitoaertieagd of the American asain for 
the Advancement of Science for 1897. 
