14 BOTANICAL GAZETTE [JULY 
millimeter was calculated by multiplying the average length by the 
average width by the average number of hairs per square millimeter 
by 7. Scrutiny of the results in the sunflower shows that for 
equally long roots the increase of surface varies, but that there is a 
slight predominance in the average increase of plants in dark (14.8) 
over those in light (14.02), and that this is entirely due to the greater 
average number of hairs per square millimeter (395 as against 373). 
The individual measurements for white mustard and radish show 
a like fluctuation of increase, hut this time with a predominance 
in the average of light over darkness.? This is probably due to the 
fact that these are small seeds with little reserve food, and soon begin 
photosynthetic work in the light, while the plants in darkness have 
no such advantage. No evident difference in the length of hairs 
was observed in dark and light, as was noted by BENECKE (5, pp. 28, 
29) for rhizoids of Lunularia. 
B. Older roots. 
Under ordinary conditions corn plants one or two weeks old, 
with roots growing through the bottom of the pots, did not produce 
hairs in water, whether illuminated or not. DEvaux (10), on the 
other hand, found that light favored hair development on the roots 
of corn two months old growing in water. These plants, however, 
had been subjected to the rather severe operation of having all the 
roots cut off to one centimeter from the base, after which they were 
plunged into water. Upon repeating the experiment it was found 
that the plants in a day or so became yellow and unhealthy. In 
light five apparently healthy adventitious roots developed, and 
produced several isolated patches of hair, usually at the same time 
on all the roots, generally covered with a film of bacteria. In the 
darkened jar only three apparently healthy roots and two diseased 
ones were developed. No hairs or bacteria films appeared, although 
the odor of the culture betrayed greater decomposition than in the 
illuminated jar. Too many factors are involved to make the experi- 
ment, in its present form at least, of much value. 
? Thus, mustard showed average increase, dark 41.33, light 44.11; and the radish, 
dark 29.44, light 32.09. Here also number of hairs, 321 to 344 and 345 to 357, accounts 
for the increase. 
iA ata oy 2a OP eh 1 a ens a a 
