256 BOTANICAL GAZETTE [APRIL 
have diffused rapidly enough through the gelatin to cause curves 
had the roots been aerotropic; and if CO, diffuses rapidly, 
hydrogen and air would diffuse even more rapidly. 
Beyond doubt, in these experiments with the gelatin cultures, 
and especially in the last-described series, the roots have been 
met on opposite sides by different gases, and if they were aero- 
tropic have been given the opportunity to show such reaction. 
In spite of this, under all these conditions the majority of the 
roots have remained straight, and the main result has been sim- 
ply retardation of growth due to the diminished oxygen pressure. 
It is certainly difficult to conceive of any more favorable condi- 
tions under which the roots might have been placed to exhibit 
aerotropism. 
5. Experiments similar to those of Molisch. 
There remains only one more task, namely to determine if 
possible the cause of the curves which Molisch observed and 
termed aerotropic. To this end an apparatus was made similar 
to that used by him and described in the first part of this paper. 
It consisted of a rectangular jar containing folds of moist filter 
paper, having an opening on one side through which moist gases 
were led from a gasometer; and having a plate of cork infiltrated 
with paraffin sealed into the open end. In the plate were cut 
six vertical slits 2°™ long by 1 to 2™" in width, and fastened to 
the upper edge of the closure was a ledge of cork in which was 
cut a longitudinal slit 5™™ wide. Small corks, through which 
projected the roots held firmly in their support with moist cotton, 
rested upon this ledge so that the roots passing through the slit 
in the ledge hung directly before, and within 2 to 4™™ of the 
slits in the cork plate closing the gas chamber. 
An apparatus prepared in this way was placed in a large cov- 
ered aquarium lined with and also containing many folds of 
moist filter paper supported on wooden bars, so as to add to the 
evaporating surface. The aquarium was covered with opaque 
black paper and the whole wrapped in black cloth. The gas, led 
in through an opening in the cover of the aquarium from the 
gasometer, was passed through a wash bottle containing water 
