443 
Curvature of Roots. 
curvature takes place when the root-cap has been removed, 
and that in other ways the erroneousness of Detlefsen’s 
explanation has been fully shown. 
Wiesner 1 reviewed and criticized Darwin’s work shortly 
after its appearance, and some three years later published the 
results of further investigations of his own 2 . 
The experimental part of the latter work is devoted chiefly 
to a study of the behaviour of radicles grown in moist air and 
in water, and to the results of plasmolysis. As already stated, 
Wiesner showed that Darwin was mistaken in referring the 
curvature to simple contact, and his experiments in this 
direction are conclusive. From others, which it is not 
practicable here to review at length, Wiesner reasons that the 
removal of the root-tip causes a change (afterwards more 
specifically defined) in the cells lying above it,* which results 
in their membranes becoming more extensible. If, now, an 
intact root is injured on one side of the tip, the cells lying on 
that side just above the injured tissue show this increased 
£ ductility ’ of the cell-membrane and corresponding rapid 
growth resulting in curvature. His conclusion, which applies 
also to geotropic curvature, is that ‘ the irritation-hypothesis 
set up by Darwin, according to which the growth-movements 
of the root proceed from the assumed irritable tip, is proven 
to be untenable.’ * 
From a careful review of Wiesner s experiments I am quite 
unable to determine how they support the purely hypothetical 
explanation which he gives. The assumption is that when 
a root is wounded on one side of the apex, the food-materials 
that would naturally go to the injured cells stop in those 
lying above them, and there effect a change in the cell- 
membranes by which they become more extensible. But 
this supposition can have weight only when the still active 
discussion of the mechanics of growth and curvature has 
resulted in more definite knowledge than we now possess : 
1 Das Bewegungsvermogen der Pflanzen. Wien, 1881. 
2 Untersuchungen iiber die Wachsthumsbewegungen der Wurzeln, Sitzb. d. K. 
Akad. d. Wissensch. Wien, LXXXIX Bd., 1884. 
H h 2 
