1904] BENNETT: ARE ROOTS AEROTROPIC 259 
majority of roots growing straight or, if curved, the curves were 
not directed by the presence of the gases. 
3. When the roots of seedlings were grown in a thin layer of 
gelatin, between different gases, as air and CO, or air and hydro- 
gen, or when grown between and enclosed by blocks of gelatin, 
one saturated with air and the other with CO, or hydrogen, no 
curves of constant direction were shown. 
4. The preparation of experiments similar to those of Molisch, 
where roots were supported close to narrow slits opening into 
gas chambers from which gases were constantly diffusing, brought 
to light the important truth that curves were produced, gener- 
ally toward the gas chambers, no matter whether gases (oxygen, 
hydrogen, or CO,) were diffusing from the chambers, or no 
gases whatever were diffusing and a like pressure of air sur- 
rounded the roots on all sides. The observations led to the 
discovery that these curves were purely hydrotropic. 
5. Finally, from a careful consideration of the results of 
experiments set forth in this paper, with the large number of 
individuals of representative species, and the variety of methods 
of experimentation employed, both under nearly natural and 
under artificial conditions, one is driven to the conclusion that, 
at least so far as the representative land plants here used are 
concerned, definite direction curvatures are not induced in roots 
by the one-sided access of such gases as oxygen, hydrogen, or 
carbon dioxid, and their roots are therefore not aerotropic. 
Thus far the evidence is decidedly against a belief in the 
aerotropism of roots. 
UNIVERSITY OF MICHIGAN. 
