44 2 Spalding. — On the Traumatropic 
others. Thus it was found that deflection followed when 
silver nitrate, shellac-solution, thin pieces of glass, copper 
sulphate, and caustic potash were applied laterally to the tip 
of the radicle, and that the same result followed when the 
radicle was wounded at this point by cutting or by the 
application of a hot glass rod. It is assumed by Detlefsen 
that in all these cases the treatment results in injury or death 
to the cells touched, whether by cutting off the necessary 
supply of oxygen, as when bits of glass are attached, or by 
direct killing through the agency of heat or a poison. The 
curving of the root as the result of such injury he holds to be 
strictly mechanical, ‘ The mode of curvature is always the 
same, whether the root-cap alone is wounded, or a larger or 
smaller part of the punctum vegetationis is destroyed. There- 
fore the cause of the curvature can only be injury to the 
root-cap. 5 The curvature is explained as due to changes of 
tension. The tissues of the root-cap are stretched over those 
lying beneath, and when the root-cap is cut through, or is 
otherwise injured, the tissues just beneath extend more 
rapidly than those on the opposite side, hence the resulting 
convexity of the side operated upon. 
But as the root-cap extends a considerable distance from 
the apex in the roots he employed (often 5 mm. or more in 
Vicia Faba ), injury above the tip should produce curvature in 
the same direction, while as a matter of fact the reverse is the 
case. The difficulty was perceived by Detlefsen, but not 
cleared up. He states that he did not succeed in producing 
a curvature by branding more than 1 to 1*5 mm. from the 
apex, but did accomplish this by making fine transverse 
incisions, although in the case of three oak-roots, the only 
ones reported specifically, ‘ the curvature was not very 
pronounced.’ Yet it is upon such evidence that the case was 
decided by Detlefsen, and that Sachs, accepting the proof as 
final, based the criticisms of Darwin’s work that have been 
given such wide publicity 1 . It is enough at this point to say 
that it has now been proven experimentally that traumatropic 
1 Vorlesungen iiber Pflanzenphysiologie, 1882, pp. 843, 879, 880. 
