IN VEGETABLE BIOLOGY. 215 
slides, each leaf being covered with a piece of thick glass to keep 
it extended in the plane of the slide. The slides were then set 
out in the photrum at short intervals. After remaining in these 
positions for three hours, during which time the sun had been 
shining brightly, care having been taken to keep the specimens 
well moistened with cold water, the slides were successively in- 
spected, commencing at the positive end, and as soon as a leaf 
was found with grains showing no signs of positive apostrophe, 
the distance between the place occupied by it and the positive 
end of the photrum was measured. The length of the photrum 
on the reduced scheme being three inches, it only remained to 
divide the above distance by forty-eight and mark off the quotient 
on the diagram. The point at the extreme right end of the 
epistrophic interval it is proposed to call the “ positive critical 
point,” because the light at this point in the photrum is not 
quite strong enough to apostrophize the chlorophyll. It some- 
times happened that, while one specimen showed signs of apo- 
strophe, the grains of its successor were wholly in epistrophe, in 
which event the critical point was fixed halfway between the 
two; and no appreciable error could ensue from this, seeing that 
if error there were, the scale would represent it diminished 
forty-eight times. 
The determination of the point at which epistrophe passes into 
negative apostrophe (which may be named the “ negative critical 
point") is a much more difficult matter. In the case of Chry- 
santhemum, it was found by placing in various positions in the 
photrum scales from a cut shoot set overnight in darkness. It 
has already been seen that the grains of Chrysanthemum-scales 
move from negative apostrophe into epistrophe in from three to 
four hours; consequently in three hours the movement will be 
nearly completed. Acting upon this idea, the specimens were 
examined after the above interval, beginning at the extreme right 
end, and as soon as one was reached which had none of its grains 
in epistrophe, the position occupied by it was noted as the nega- 
tive critical point required. This method would have been appli- 
cable to Lemna trisulca, for we know that it can recover in thirty- 
five minutes from the large amount of apostrophe induced by six 
and a half hours’ withdrawal of light; only here, in order to avoid 
ambiguity, it would have been necessary to decide the negative 
eritical point from the failure, after the three hours, of any of 
the grains to pass into epistrophe. Instead of this, the negative 
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