242 BOTANICAL GAZETTE [APRIL 
The roots of Pisum sativum and Zea Mays were subjected 
to the application of various gases diffusing or streaming in 
mass from the slits, and the reactions of the roots were termed 
aerotropic. Only those responses to the application of oxygen, 
hydrogen, carbon dioxid, and nitrogen are considered in my 
paper, as the other gases used by Molisch were poisonous, and 
might produce chemotropic or traumatropic reactions. 
The striking similarity in the results of Molisch’s experiments 
—namely, that regardless of the species of seedling or the gas 
used, whether it were a stream of pure oxygen, of oxygen ofa 
lower pressure than the normal, of nitrogen, or of carbon dioxid, 
the majority of the roots in every case made an initial curve 
toward the source of the gas and later turned away—looks 
suspicious. His own explanation of these phenomena is that the 
curves both toward and away from the gas chamber were due to 
the presence of an optimum pressure of the gas for longitudinal 
growth. If this were true, it would seem to make the response 
simply a mechanical one. 
Several features of Molisch’s work are subject to criticism. 
To thrust needles through the cotyledons, as he did, is objection- 
able for several reasons. To take seedlings directly from the saw- 
dust and support them before the slits is objectionable, since 
injurious substances often occurring in sawdust effect curvatures in 
roots, which do not show until some time after the seedlings have 
been set up. The gases which streamed out of the slits were not 
moist, except in the experiments with CO, and possibly pure 
oxygen. This condition would tend to bring about hydrotropic 
curves. Too few seedlings were used to establish certain con- 
clusions. The data of the experiments lack details as to growth 
of seedlings, degree of curvature, and other minor points. 
Lastly, the position of the root at the conclusion of the experi- 
ment is accepted for record, regardless of the numerous curves, 
positive and negative, which this root made during the continuance 
of the experiment. Since the paper by Molisch nothing on this 
subject has appeared. 
The present investigation of the supposed aerotropism of 
roots was conducted inthe Botanical Laboratory of the University 
