CELL MEASUREMENT 
203 
thus correcting for the total effect of Hght.) As already explained, 
the choice of an extremely long normal internode obviated any possi- 
bility of failure to detect the cell number factor in the elongation due 
to etiolation, in case any such factor existed. The available evidence 
indicates that under relatively constant environmental conditions, 
variation in internode length is correlated with the number rather 
than with the size of cells. {Teste Kraus; results with long and short 
internodes of Philadelphus.) If it had been grown undef normal con- 
ditions, therefore, the etiolated internode from which our measure- 
ments were made might have been expected to produce 27 percent 
fewer cells than the normal one with which it was compared, yet in 
the dark it actually produced 38 percent more, if we base the com- 
parison upon primary cells, including both undivided and divided, or 
78 percent more, if we base the comparison upon undivided primary 
and secondary cells, taken at random. There can remain no doubt, 
therefore, that the effect of light, directly or indirectly, is to retard 
cell division. 
If a correction is made for the difference in the position of the two 
plants in the range of variation, the number of primary meristematic 
divisions in darkness shows an increase of 88 percent over the number 
in the light, accounting for 34 percent of the total increase in length, 
leaving 66 percent to be accounted for by increased extension of the 
cell or group of cells derived from each division. In case primary and 
secondary cells are not distinguished, it appears that the number of 
cells in etiolated internodes is greater by 142 percent than in the normal 
ones, and that 55 percent of the increase in length is due to the cell 
number factor, and only 45 percent to the cell size factor. Since Kraus 's 
conclusions were based upon cells taken at random, as in the latter 
case, the discrepancy between his results for Phaseolus vulgaris and 
ours for P. multiflorus requires a further word of explanation. He 
found the entire increase in length to be due to the cell size factor, but 
that his material was not strictly comparable is indicated by the fact 
that he did not determine the fluctuating variation of the plants 
which he used. Consequently he could neither select comparable 
internodes in the first place, nor correct for their deviation from com- 
parability. In our work we have determined the range of variation 
for both normal and etiolated seedlings, and have assumed that, within 
the limit of experimental error, the cell number factor and the cell 
size factor have the same relative weight in bringing about the elonga- 
