10 BOTANICAL GAZETTE [JANUARY 
As for the increased extensibility of the membranes on the Con- 
vex side by which Noll accounts for stimulation-curvatures, it 
undoubtedly exists, but in the collenchyma, and not in the 
parenchyma. The change in the quality of the membrane is 
inferred by Noll both from a difference in the optical properties 
of the membranes, and from their decreased thickness and 
greater superficial area. However it is almost beyond doubt 
that in grass nodes this change in the quality of the membranes 
is a phenomenon of growth and is nota simple process of stretch- 
ing, either plastically or elastically (56, pp. 405, 416). 
In 1897 J. Loeb published a paper (36) in which he accounted 
for geotropic and heliotropic curvatures by a contraction of the 
concave side, not in the sense of Kohl’s contraction, but in the 
sense in which the term is used in animal physiology, The 
contraction in plants is exactly the same as the contraction of 
muscles. Loeb thinks his theory is of universal application. 
It not only explains all the stimulation-curvatures of plants both 
unicellular and multicellular, but also those of animals (marine 
worms and hydroid polyps) and even puts phototactic and 
chemotactic movements of motile organisms on the same basis 
as heliotropic and geotropic curvatures. Loeb’s plea for a simple 
explanation which will apply to all cases of movements and 
curves caused by stimuli has been anticipated and answered by 
Pfeffer. He says (56, p. 413): ‘The demand which is always 
recurring in relation to movements of curvature that the mechanics 
of the process be everywhere identical is only to be understood 
from the natural inclination to simplifying schemes, or from 4 
narrow-minded view of living nature.” Nature makes use of many 
means to accomplish similar ends. In its application to particular 
cases Loeb’s theory is as inadequate to account for the curvature 
of grass stems as those of Wortmann and Kohl, and for the Same 
reason, namely, it could not develop sufficient energy to accomplish 
the result, as Pfeffer’s experiments prove (56). In the case of 
movements such as those performed by the pulvini of Mimosa 
and by sensitive stamens, water is given out into the intercellular 
spaces when the contraction takes place, and the shortened sidé 
