IN VEGETABLE BIOLOGY. 237 
about stasis. Two leaves, similar in all respects apparently, may 
be illuminated under precisely the same conditions ; yet in the 
one rotation may still be in progress some time after its cessation 
in the other. Now it is admitted by every one that rotation is 
merely a sort of exaggeration of photolysis; consequently the 
phototactic doctrine previously advanced in this memoir with re- 
ference to the latter will apply in every way to rotation which, 
when in progress, proclaims that the apostrophizing factors have 
triumphed over the epistrophizing ones. What are the factors 
favouring apostrophe and rotation apart from the form and 
ehlorophyll-contents of the cell? Obviously (1) the strength of 
illumination, because the more to the right of the positive 
critical point the plant is set in the photrum, the more rapidly 
will rotation ensue; and (2) any lessening of the capacity of the 
protoplasm to resist the motile agency of light upon it. If, 
therefore, illumination be identical in the two Elodea-leaves, an 
increase in the time of recovery from rotation must depend upon 
difference in protoplasmic tone; and it is clear that the higher 
the tone, the less is the advantage gained by the apostro- 
phizing forces over those which make for epistrophe ; so that, 
as the motile effects of light soak out in darkness, stasis—or 
rather a condition near stasis signalized only by the slight 
movement whereby chlorophyll is epistrophized—must, other 
things equal, sooner supervene the higher the tone of the pro- 
toplasm. 
In fact, the protoplasm varies in tone to a very great degree. 
As an instance, the cells in the neighbourhood of discoloured 
spots of Elodea-leaves are iustructive. The spots referred to are 
small discolorations affecting one or a few cells, the cell-sap, and 
apparently the protoplasm, of which are turned dark brown, red, 
or purple, the chlorophyll grains remaining in epistrophe, where- 
from they cannot be moved by light. It would appear that these 
cells, which are of common occurrence, have been injured by 
small aquatic creatures or in other ways. The facts about to be 
described are so universal that they can have but one expla- 
nation. If an Zlodea-leaf with one or more discoloured spots 
be cut off a plant growing in low light and set in good diffused 
illumination, or in the sun’s rays, the cells in the neighbourhood 
of a spot will have their grains in apostrophe at a time when 
those of remoter cells will still be, for the most part, upon the 
superficial wall; and before the running together of the grains 
