CELL MEASUREMENT 
205 
size for division, in any strict sense. Observation, unsupported by 
measurements in the case of the present material, but borne out by 
studies of epidermal cells in the case of the genus Oenothera,^ leads to 
the conclusion that cells which attain an excessive length are usually 
very slender, and vice versa. On this ground it is possible to explain 
the great range of variation in the length attained by different cells 
before division. It is quite correct, however, to speak of specific 
mean length for division, which is simply the mean of the lengths at 
which division takes place in a large number of cells, and this is the 
constant which we have determined with some approximation to 
accuracy as 0.140 mm. in the stem epidermis of Phaseolus multiflorus. 
It appears that the specific mean length for division is the same in 
both light and darkness. The mean length of undivided primary cells 
is indeed greater in the etiolated internode, but this fact is readily 
explained. In the light only 76 out of 1,000 primary cells taken at 
random were both undivided and longer than 0.140 mm.: 583 were 
undivided. In the dark, however, there were only 176 undivided 
primaries in 1,000, and a relatively larger number of them, 103, ex- 
ceeded the putative specific mean length, 0.140 mm. The extension 
of a considerable number of cells to somewhat beyond the specific 
mean length would be expected to bring about the division of most of 
the cells in the lower part of the range within which division takes 
place, and to leave only the cells which, because of slenderness or some 
other cause, come within the extreme upper part of the range. The 
small number of undivided primaries in the material grown in the dark 
suggests that most of those that remain must have passed the specific 
mean length. It is therefore not surprising that their mean length is 
0.149 mm., whereas in every other instance the mean length of undi- 
vided cells, whether primaries or secondaries, was found to be well 
below 0.140 mm. 
The simplest assumption with regard to the effect of light is that 
it retards extension of the cells, and that as an indirect result there are 
fewer secondary divisions, since relatively fewer primary cells enter 
the range of length within which division takes place. With regard 
to the cell divisions in the primary meristem, it is clear that more of 
them take place in the dark than in the light, but there is no evidence 
with regard to the cause. 
^ Tupper, W. W. and Bartlett, H. H. The relation of mutational characters 
to cell size. Genetics 3: 93-106. I9i§. 
