IN VEGETABLE BIOLOGY. 241 
tion have some, at least, of their cells at the section-edge with 
rotating protoplasm, even if the plant furnishing them has been 
growing in low light, and the light by the aid of which the section 
has been made has been only sufficient to just see by. A large 
number of experiments were made upon this point; but the 
amount of rotation which declared itself after a given interval 
varied to so extraordinary a degree, that I was for along time 
completely baffled for an explanation. In one case the edge of 
asection would show only faint signs of rotation, which in another 
from a neighbouring leaf was well established the whole length 
of the section, while a third would be in an intermediate con- 
dition. It was only after grasping the idea that negative rota- 
tion, like negative apostrophe, is the expression of the preponder- 
ance of the unexhausted or potential effects of light over the 
tonicity of the protoplasm, that the mystery appeared to be 
solved. Rotation at the section-edge was then seen to result in 
precisely the same way as that around’ dead cells, the operation 
of sectioning destroying the cells at the edge, which then act the 
part of discoloured cells as depressors of the vitality of their 
neighbours’ protoplasm. The variations in the amount of rota- 
tion were now seen to be due either to initial difference in the 
quality of the protoplasm in the cells abutting on the section, or, 
what amounts to the same thing, to the presence of one or more 
dead cells along or near the sectioning line. 
But it has been said that a leaf may be sectioned without rota- 
tion being set up in the cells at the section-edge; and if the 
doctrine here advanced be sound, it is obvious that this might 
well happen. For suppose the potential effect of light to be 
equal in two cases, it might be surmised, and surmise would 
be borne out by experiment, that rotation will first be estab- 
lished in that case where the protoplasm is of poorer quality ; 
and in the other it might happen that rotation will not appear at 
all, because the protoplasm would be so highly toned as to be 
able to resist the tendency of sectioning to bring it down to the 
Tequired point. Successful resistance to rotation upon section 
should, then, be met with in uninjured leaves immediately after 
their removal from healthy plants growing under the most favour- 
able conditions; in fact, these are the only leaves which have been 
found capable of resistance. Thus, in one experiment, a leaf was 
cut off a vigorous plant removed a few hours before from a large 
pond, and kept in low light meanwhile. This was st 10.40 a.m.: 
