592 Wisconsin Academy of Sciences, Arts, and Letters. 
white : and the red light seemed to be the least efficient of all 
in producing accuracy of aim. In fact, there seemed to be 
almost no definiteness of aim whatever in the cultures subjected 
to the red light. So far as we can find, no explanation has 
yet been suggested for the different efficiencies of lights of 
different colors in cases of this sort. 
The question as to whether the sporangiophore of Pilobolus 
is aimed high in order to make allowance for the distance the 
sporange must travel to reach the light, appears to be settled 
by the data given in Table VII. The sporangiophore is 
aimed straight toward the light, making no allowance for dis¬ 
tance. The sporange follows the path of a projectile, and the 
distance that it fails below the source of illumination increases 
with an increase in the distance it must travel. The curve 
(Figure 11) which represents the results of our observations 
upon this point, shows the average path, of a large number of 
sporanges. Any individual sporange, as just pointed out, is 
fired straight toward the opening, and its path is that of a 
projectile, since gravity tends to pull it downward as is the 
case with any body thrown horizontally or approximately so. 
Different sporanges, as has been shown, vary in the accuracy 
with which they are aimed and in the force of their discharge; 
and these individual differences necessarily influence the di¬ 
rection of the curve. Consequently, the curve (Figure 11) 
which shows their average path does not represent the path 
of any single sporange and is therefore not a parabola. 
Hathansohn (14) in testing Talbot’s law for plants, found 
the point between a constant and an intermittent light at which 
the two had equal power to induce a heliotropic reaction in 
seedlings of Brassica. It is noteworthy that at this point, 
where the difference between the two lights (as tested by the 
human eye) was less than one per cent, each seedling reacted 
accurately to one light only. About half of the seedlings ex¬ 
perimented with, bent toward one light, the other half toward 
the other light. In other words, one light in each case pro¬ 
duced a full and complete reaction and the other had no 
visible effect. Which of the two lights would prevail could 
