1900] MECHANISM OF ROOT CURVATURE 53 
Loeb seeks to explain the contraction of the concave side in 
curving plant organs by the same process as the contraction of 
animal muscle. He states that in an active muscle there is an 
increase of osmotic substances, and that a weighted contracting 
muscle takes up more water than a resting one. He points to 
the observations of Kraus as proof that osmotic substances are 
increased in the cells of the concave side of curving plant 
organs. He offers no proof or evidence, however, that these 
cells take up more water than the cells of the convex side. 
Indeed all the evidence is against it. My weighing experiments 
with the concave and convex halves of curving roots prove 
beyond doubt that the convex side contains more water than the 
concave side. Hence the contraction in plant cells cannot be 
the same as in animal cells... Pulvini also show a loss of water 
in the cells of the contracting side. Loeb does not consider 
sufficientiy the contracting power of the elastic membranes of 
turgid plant cells, which may contract with a decrease of osmotic 
pressure. 
When we consider the varied mechanical conditions which 
are found in such plant organs as fungus hyphae, grass nodes, 
dicotyledonous stems, and roots, we should hardly expect, even 
@ priori, that a single explanation of curvature would apply to 
all plant organs. How much less should we expect that a single 
explanation would apply not only to all plant curvatures but also 
to contractions of animal muscles, and to free movements of 
both plants and animals, as Loeb would have us believe. The 
€xperimental evidence is against this view. 
Pfeffer has shown that in grass nodes the curve is brought 
about by an active growth of the collenchyma bundles of the 
convex side. Even here a certain amount of potential energy, 
which exists previously to stimulation in the form of the positive 
tension of the parenchyma, becomes available for growth of the 
collenchyma. In their growth the walls of these collenchyma 
cells undergo those changes which Noll says indicate plastic 
Stretching and greater ductility of the membranes. That the 
walls are more ductile may be admitted and the ductility still be 
