218 MR. 8. LE M. MOORE’S STUDIES 
diately below the epistrophic interval of the healthy lower cells 
of a leaf (which, it will be observed, is developed much further to 
the right than the upper cells’ interval) is drawn the greatly 
curtailed interval of certain upper-layer cells in the neighbour- 
hood of the small discoloured spots so frequently to be found in 
JElodea-leaves; this curtailment shows, if the argument just con- 
cluded be sound, that injury to the leaf lowers in a marked degree 
the capacity of the protoplasm of still apparently healthy cells in 
the neighbourhood of the wound to withstand the motile effects 
of light. Underneath this is shown the interval of the upper, 
and below that of the lower cell-layer of small leaves growing 
upon plants which, after having lain for some days in a 2-per-cent. 
solution of ferrous sulphate, had, after washing and transference 
1. Elodea canadensis, upper cell-layer of green leaf. 
2. Ditto, lower cell-layer of green leaf. 
3. Ditto, upper cells of green leaf near discoloured spots. 
4. Ditto, upper cell-layer of the leaf of an etiolated plant growing for some 
time in very low light in confined space at the back of a room. 
5. Ditto, lower cell-layer of etiolated plant. ` 
to water, been placed for several weeks in very poor light, and 
had suffered etiolation in consequence. Between these two there 
is an extraordinary difference, for while the protoplasm of the 
lower layer shows not much more sensitiveness to light than is 
usually met with in healthy green leaves, the plant must be kept 
within a very short range of the lowest illuminations if the grains 
of the upper-layer cells are not to be driven into apostrophe. 
It might be thought that the greater sensitiveness of the latter 
cells’ protoplasm is due to its more exposed position with respect to 
the impact of light; this is, however, a mistake; side by side with 
every leaf used in the construction of the intervals of the present 
diagram was placed a similar leaf taken from the same whorl, and 
