194 WILBER BROTHERTON, JR., AND H. H. BARTLETT 
Dougal's work (he gives few measurements, but the figures are drawn 
to scale) seems to bear out Kraus's conclusion that the elongation of 
an etiolated stem is due to increase in both number and size of cells. 
Kraus's data for Phaseolus, however, were in conflict with his own 
general conclusion. He found that etiolated internodes of Phaseolus 
vulgaris L., although elongated, had fewer cells than normal inter- 
nodes, seemingly indicating that the entire increase in length was due 
to cell size. Our work leads us to believe that in this case there was 
an unusually extreme error due to failure to make cell measurements 
from plants of comparable position in the range of fluctuating variation. 
In our experiments seeds of the scarlet runner bean {Phaseolus 
multiflorus Willd.) were grown in complete darkness and in light. 
The length of the epicotyls of 80 etiolated and 92 normal plants was 
measured ; the symmetrical frequency distributions gave the following 
constants : 
Range of Variation 
M 
0- 
cv 
Grown in darkness 
168-517 mm. 
305 
71.4 
234 
30-141 mm. 
85 
16.9 
19.9 
Thus, the etiolated stems were 3.6 times as long as those grown in the 
light, and had only a slightly greater coefficient of variation. 
An extremely long normal epicotyl (141 mm.) was taken for cell 
measurements. For our purposes it was more favorable than one of 
modal length, in that the epidermis was surely made up of almost the 
maximum number of cells for a normal epicotyl. A rough check on 
this statement is afforded by multiplying the length of the normal 
epicotyl (141 mm.) by the factor 3.6, the ratio between the mean 
length of normal and etiolated stems. The result is 508 mm., in 
sufficiently close agreement with the actual maximum length (517 mm.) 
for an epicotyl grown in darkness. If, therefore, an etiolated epicotyl 
of less extreme position in the variation curve should contain more 
cells, it would prove conclusively that the effect of light is to retard 
cell division as well as to diminish cell size, and that in Phaseolus, as 
in other plants, the etiolated internodes have more cells than the 
normal. The etiolated epicotyl chosen for cell measurements was 
372 mm. long; i. e., it was exceeded in length by 20 percent of the vari- 
ates, and corresponded to a normal epicotyl 133 mm. long. 
