IN VEGETABLE BIOLOGY. 217 
at the positive end the epistrophic interval was somewhat cur- 
tailed. 
One word of caution is necessary here. The above-drawn epi- 
strophic intervals were found in November and December last ; 
it is probable that some difference in their length and position 
will be noted if experiments be conducted in summer. I am the 
more inclined to think this from observing during the past summer 
that it was not necessary to put the leaves of Funaria hygro- 
metrica into actual sunlight to apostrophize the grains; if the 
sky was clear the change took place in the shade close to the 
rays of sunlight streaming into the room. 
Hitherto it has seeried, in the absence of any statement to the 
contrary, that the protoplasm of all cells of a plant is affected to 
the same extent by light, the grains epistrophizing from negative 
apostrophe and positively apostrophizing from epistrophe at 
certain fixed grades of illumination. But this is by no means 
always the case. If an Elodea-leaf be mounted in water and 
exposed on a glass slide to rather poor diffused light, after a little 
time all or most of the chlorophyll grains in the larger cells 
(those placed upon the leaf's upper face) will be found upon the 
side-walls ; while in the lower layer of smaller cells, provided that 
the illumination be not too high, only a slight (if any) tendency 
to apostrophe will show itself. The effect of the inertia of the 
grains upon their transference in space is alluded to on p. 224: 
with reference to this point, it may be pointed out that the 
grains of the lower are considerably smaller than those of the 
upper cell-layer, à circumstance favourable to rapidity of move- 
ment; but it must not be forgotten that the lower cells, being 
smaller than their overlying neighbours, contain less protoplasm, 
and there is every reasan to believe that the gain on the score of 
inertia is approximately balanced by the loss in respect of proto- 
plasmie momentum, and, as a corollary, that the difference in the 
epistrophic interval of the upper and lower cells is, toa large 
extent, the result of a difference in the response to the stimulus 
exerted by light upon their protoplasm ; and this view seems still 
more reasonable when it is remembered that resistance to move- 
ment cannot be greater in the lower than it is in the upper 
cells, provided that the form of the cells and the relation between 
their size aud the size and number of their grains be approxi- 
mately the same; and this seems to be the case. | 
Diagram 3 is a graphic representation of these facts. Imme- 
